Biochemistry Unit 5

(a) The major energy storage compound of animals is fats (except in muscles). Why would this be advantageous? (b) Why don't plants use fats/oils as their major energy storage compound?

(a) For mobile organisms—such as a migrating hummingbird—weight can be a critical factor, and packing the most energy into the least weight is decidedly advantageous. A 2.5-g hummingbird needs to add about 2 g of fat for migration energy, which would increase body weight by 80%. The equivalent amount of energy stored as glycogen would be about 5 g, which would increase its body weight by 200% the bird would never get off the ground! (b) For immobile plants, weight is not a critical factor, and it takes more energy to make fat or oil than it does to make starch. (The second law of thermodynamics would dictate that the energy obtained from oil would be less than that expended making oil. You can verify this numerically if you wish.) In the case of plant seeds, "compact" energy is beneficial, because the seed must be self-sufficient until enough growth has occurred to permit photosynthesis.

The functioning of a particular pathway often depends not only on control enzymes in that pathway but also on control enzymes of other pathways. What happens in the following pathways under the indicated conditions? Suggest what other pathway or pathways might be influenced. (a) High ATP or NADH concentration and the citric acid cycle. (b) High ATP concentration and glycolysis. (c) High NADPH concentration and the pentose phosphate pathway. (d) High fructose-2,6-bisphosphate concentration and gluconeogenesis.

(a) High ATP or NADH concentration and the citric acid cycle: isocitrate dehydrogenase (and the citric acid cycle) would be inhibited. The resulting pileup of acetyl-CoA (or citrate) would stimulate fatty acid and steroid synthesis, gluconeogenesis, and (in plants and some microorganisms) the glyoxylate cycle. (b) High ATP concentration and glycolysis: phosphofructokinase-1 (and glycolysis) would be inhibited. Glucose-6-phosphate would pile up, stimulating glycogen (or starch) synthesis, the oxidative pentose phosphate pathway, or glucose formation. (c) High NADPH concentration and the pentose phosphate pathway: the oxidative branch of the pentose phosphate pathway would be inhibited, thus making glucose-6-phosphate available for other purposes. These include glycolysis, glycogen synthesis, glucose synthesis, and the "reverse" pentose phosphate pathway (yielding only pentose phosphate). (d) High fructose-2,6-bisphosphate concentration and gluconeogenesis: fructose-2,6-bisphosphate inhibits fructose-1,6-bisphosphatase and activates phosphofructokinase-1. Gluconeogenesis would thus be inhibited and glycolysis would be stimulated, as would the reverse pentose phosphate pathway and the production of glycerol phosphate for lipids.

(a) Where in an earlier chapter have we encountered something comparable to the action of the acyl carrier protein (ACP) of fatty acid synthesis? (b) What is a critical feature of the action of the ACP?

(a) The lipoate "swinging arm" of the pyruvate dehydrogenase complex. (b) The "arm" or ACP carries the group to be acted on from one enzyme to another (avoiding a diffusion-limited process and also positioning key groups correctly). In the case of the ACP, the group to be acted on (β-carbon) is always the same distance from the ACP, regardless of the length of the growing fatty acid, and thus the critical group is always in proximity to the active sites of the several pertinent enzymes.

How does epinephrine affect the enzymes: (a) Glycogen phosphorylase (b) Glycogen synthase (c) Phosphofructokinase I

(a) Activation (b) Inhibition (c) Activation

How does glucagon affect the following enzymes: (a) Glycogen phosphorylase (b) Glycogen synthase (c) Phosphofructokinase I

(a) Activation (b) Inhibition (c) Inhibition

Will an amino acid be glucogenic or ketogenic if it is catabolized to the following molecules? (a) Phosphoenolpyruvate (b) α-Ketoglutarate (c) Succinyl-CoA (d) Acetyl-CoA (e) Oxaloacetate (f) Acetoacetate

(a) Glucogenic (b) Glucogenic (c) Glucogenic (d) Ketogenic (e) Glucogenic (f) Ketogenic

What is insulin's effect on the following? (a) Glycogen breakdown (b) Glycogen synthesis (c) Glycolysis (d) Fatty-acid synthesis (e) Fatty-acid storage

(a) Glycogen breakdown is decreased. (b) Glycogen synthesis is increased. (c) Glycolysis is increased. (d) Fatty-acid synthesis is increased. (e) Fatty-acid storage is increased.

How many acetyl-CoA molecules are involved in forming HMG-CoA during ketogenesis? 1 2 3 4

3

Why is a photosynthetic reaction center comparable to a battery?

A photosynthetic reaction center is analogous to a battery because its reactions produce a charge separation. The charge separation is comparable to the stored energy of the battery.

Why is it a good agricultural practice to plant a field with alfalfa every few years?

Alfalfa has nitrogen-fixing bacteria within its nodules to replenish the soil with nitrogen while decreasing the need for nitrogen based fertilizers.

Immature rats are fed all the essential amino acids but one. Six hours later they are fed the missing amino acid. The rats fail to grow. Explain this observation.

All amino acids must be present at the same time for protein synthesis to occur. Newly synthesized proteins are necessary for growth in the immature rats.

Calculate the net ATP yield from oleic acid. Hint: Remember the step that bypasses acyl-CoA dehydrogenase.

An 18-carbon saturated fatty acid yields 120 ATP. For a monounsaturated fatty acid, the double bond eliminates the step that produces FADH2, so there would be 1.5 ATP fewer for oleic acid, or 118.5 ATP total.

Which of the following participates in AMP synthesis from IMP? A) ATP B) GTP C) Asp D) A and C E) B and C

B and C (GTP and Asp)

The carrier molecule which transports fatty acids through the inner mitochondrial membrane is ATP. Carnitine. Coenzyme A. Lipoic Acid. none of these

Carnitine

List two hormones that work through the cAMP second messenger.

Epinephrine and glucagon are the two that were discussed the most in this book.

PSI and PSII couple oxidation of __________ to the reduction of NADP+. NADPH H2O O2 NADH

H2O

What is the metabolic importance of malonyl-CoA?

It is a molecule that commits itself to fatty acid synthesis. It is also a potent inhibitor of carnitine acyltransferase I, thereby shutting down β-oxidation.

How is carbamoyl-phosphate synthetase I (CPS-I) controlled?

It is controlled by a special effector molecule, N-acetylglutamate, which is itself controlled by levels of arginine.

Is it likely that any metabolic pathway can exist without control mechanisms?

It is most unlikely that a metabolic pathway could exist without control mechanisms. Many pathways require energy, so it is advantageous for an organism to shut down a pathway when its products are not needed. Even if a pathway does not require large amounts of energy, the many connections among pathways make it likely that control is established over the levels of important metabolites.

Is it reasonable to list standard reduction potentials for the reactions of photosynthesis? Why or why not?

It is quite reasonable to list reduction potentials for the electron-transfer reactions of photosynthesis. They are entirely analogous to the electron-transfer reactions in mitochondria.

How many cycles of β-oxidation are required to process a fatty acid with 17 carbons?

It would take seven cycles of β-oxidation to release 14 carbons as acetyl-CoA, with the last three being released as propionyl-CoA.

Biologically and nutritionally important trace elements tend to be metals. What is their likely biochemical function?

Metal ions play a role in the structure and function of proteins and some coenzymes. They tend to do so because they operate as Lewis acids.

Where in the cell does β-oxidation occur?

Mitochondrial matrix

Reduction reactions during fatty acid synthesis utilize A) FADH2 B) NADH C) NADPH D) FADH2 and NADH E) FADH2 and NADPH

NADPH

Is it possible to convert fatty acids to other lipids without acyl-CoA intermediates?

No, acetyl-CoA intermediates are essential in the conversion of fatty acids to other lipids.

The production of ATP in photosynthesis: is not linked to a proton gradient across a membrane does not involve membrane-bound proteins does not require the presence of light provides the energy for production of sugars occurs only in the dark

Provides the energy for production of sugars

A coenzyme frequently encountered in transamination reactions is A) tetrahydrofolate B) pyridoxal phosphate C) thiamine pyrophosphate D) biotin

Pyridoxal phosphate

The processing of one molecule of stearic acid (18 carbons) by β-oxidation

Requires 8 cycles of β-oxidation and produces 9 molecules of acetyl-CoA.

Why is rubisco likely to be the most abundant protein in nature?

Rubisco is the principal protein in chloroplasts in all green plants. This wide distribution makes it likely to be the most abundant protein in nature.

Why does the degradation of palmitic acid to eight molecules of acetyl-CoA require seven, rather than eight, rounds of the β-oxidation process?

Seven carbon-carbon bonds are broken in the course of β-oxidation.

Why do we refer to the conversion of six molecules of carbon dioxide (six carbon atoms) to one molecule of glucose (also six carbon atoms) as a net reaction?

Six molecules of carbon dioxide fixed in the Calvin cycle do not end up in the same glucose molecule. However, labeling experiments show that six carbon atoms are incorporated into sugars for every six carbon dioxide molecules that enter the Calvin cycle.

Outline the steps involved in the production of malonyl-CoA from acetyl-CoA.

Step 1: biotin is carboxylated using bicarbonate ion (HCO3-) as the source of the carboxyl group. Step 2: the carboxylated biotin is brought into proximity with enzyme-bound acetyl-CoA by a biotin carrier protein. Step 3: the carboxyl group is transferred to acetyl-CoA, forming malonyl-CoA.

To what extent can metabolic pathways be considered reversible? Why?

The effect of biochemical pathways can be reversed. Examples include glycolysis and gluconeogenesis, glycogen formation and synthesis, and the pentose phosphate pathway. The details are not completely reversible. An irreversible step in one pathway tends to be replaced with another reaction, catalyzed by another enzyme.

It is frequently said that camels store water in their humps for long desert journeys. How would you modify this statement on the basis of information in this chapter?

The humps of camels contain lipids that can be degraded as a source of metabolic water, rather than water as such.

How do the actions of the hypothalamus and the pituitary gland affect the workings of endocrine glands?

The hypothalamus secretes hormone-releasing factors. Under the influence of these factors, the pituitary secretes trophic hormones, which act on specific endocrine glands. Individual hormones are then released by the specific endocrine glands.

In chloroplasts, A) the light reactions take place in the thylakoid disks, whereas the dark reactions occur in the stroma B) the dark reactions take place in the thylakoid disks, whereas the light reactions occur in the stroma C) the light reactions take place in the thylakoid space, whereas the dark reactions occur in the stroma D) the dark reactions take place in the thylakoid space, whereas the light reactions occur in the stroma

The light reactions take place in the thylakoid disks, whereas the dark reactions occur in the stroma

The oxidation of water requires four electrons, but chlorophyll molecules can transfer only one electron at a time. Describe how these two statements can be reconciled.

The oxygen-evolving complex of photosystem II passes through a series of five oxidation states (designated as S0 through S4) in the transfer of four electrons in the process of evolving oxygen. One electron is passed from water to photosystem II for each quantum of light. In the process, the components of the reaction center go successively through oxidation states S1 through S4. The S4 decays spontaneously to the S0 state and, in the process, oxidizes two water molecules to one oxygen molecule. Four protons are released simultaneously.

What cofactors are involved in one-carbon transfer reactions of amino acid anabolism?

The principal ones are tetrahydrofolate and S-adenosylmethionine.

Which generates more ATP—the processing of the reduced electron equivalents formed during β-oxidation through the electron transport chain, or the processing of the acetyl-CoA generated from β-oxidation through the citric acid cycle and the electron transport chain?

The processing of the acetyl-CoA through the citric acid cycle and the electron transport chain produces more energy than the processing of the NADH and FADH2 produced during β-oxidation.

How can a proton gradient be created in cyclic photophosphorylation in photosystem I?

The proton gradient is created by the operation of the electron transport chain that links the two photosystems in non-cyclic photophosphorylation.

What is the second messenger for the insulin response?

The second messenger is a protein called the insulin receptor substrate, which is phosphorylated on a tyrosine by the insulin receptor kinase.

Compare and contrast the pathways of fatty acid breakdown and biosynthesis. What features do these two pathways have in common? How do they differ?

The two pathways have in common the involvement of acetyl-CoA and thioesters, and each round of breakdown or synthesis involves two-carbon units. The differences are many: malonyl-CoA is involved in biosynthesis, not in breakdown; thioesters involve CoA in breakdown and involve acyl carrier proteins in biosynthesis; biosynthesis occurs in the cytosol, but breakdown occurs in the mitochondrial matrix; breakdown is an oxidative process that requires NAD+ and FAD and produces ATP by electron transport and oxidative phosphorylation, whereas biosynthesis is a reductive process that requires NADPH and ATP.

Kwashiorkor is a protein-deficiency disease that occurs most commonly in small children, who characteristically have thin arms and legs and bloated, distended abdomens due to fluid imbalance. When such children are placed on adequate diets, they tend to lose weight at first. Explain this observation.

The weight loss is due to correction of the bloating caused by retention of liquids.

What is the relationship between α-ketoglutarate, glutamate, and glutamine in amino acid anabolism?

They are all interrelated. α-Ketoglutarate can be changed to glutamate via transamination or glutamate dehydrogenase. Glutamine synthetase makes glutamine out of glutamate.

Most calcium supplements have calcium carbonate as the main ingredient. Other supplements that have calcium citrate as the main ingredient are advertised as being more easily absorbed. Do you consider this a valid claim? Why?

This claim has a chemical basis. Calcium carbonate dissolves in stomach acid, releasing calcium ion in its usual hydrated form. Calcium citrate is likely to have the calcium ion bound to the citrate in a manner similar to iron in heme. Consequently, the charge of the calcium ion is effectively decreased. Calcium bound to citrate can pass a cell membrane more easily than a hydrated calcium ion.

Why is it advantageous for a metabolic pathway to have a large number of steps?

When a pathway has a number of steps, it is possible for energy changes to take place in steps of manageable size. It also allows for control of a pathway to be exercised at more points than would be the case if there were only a few steps.

Briefly describe the series of events that takes place when cAMP acts as a second messenger.

When a stimulatory hormone binds to its receptor on a cell surface, it stimulates the action of adenylate cyclase, mediated by the G protein. The cAMP that is produced elicits the desired effect on the cell by stimulating a kinase that phosphorylates a target enzyme.

What is the logic behind high levels of arginine positively regulating N-acetylglutamate synthase?

When arginine levels build up, it means that the urea cycle is going too slowly and not enough carbamoyl-phosphate is available to react with ornithine.

People who begin to lose weight often have a rapid weight loss in the first few days. Common knowledge says that this is "just" because of a loss of water from the body. Why might this be true?

When the body breaks down proteins to supply material for gluconeogenesis, the increased urea output results in greater urine production, which uses water stored in the body. Fat metabolism also produces much metabolic water.

Which group of small molecules best fit the boxes associated with the reaction shown? a) ATP b) ADP a) NAD+ b) NADH a) NADP+ b) NADPH a) FAD b) FADH2

a) NAD+ b) NADH

What is the net yield of ATP when palmitate (C16) is completely oxidized via the β-oxidation pathway? Express answer as whole number.

106 ATP 8 acetyl-CoA x 10 = 80 ATP 7 rounds x 4 = 28 ATP -2 ATP From seven cycles of β-oxidation: 8 acetyl-CoA, 7 FADH2, and 7 NADH From the processing of 8 acetyl-CoA in the citric acid cycle: 8 FADH2, 24 NADH, and 8 GTP From re-oxidation of all FADH2 and NADH: 22.5 ATP from 15 FADH2, 77.5 ATP from 31 NADH. From 8 GTP: 8 ATP. Subtotal: 108 ATP. A 2-ATP equivalent was used in the activation step. Grand total: 106 ATP

The reactions during the light phase of photosynthesis include the following: 1) Light absorption in photosystem I 2) Light absorption in photosystem II 3) Formation of O2 from water 4) Formation of NADPH The order of the reactions is as follows:

2 → 3 → 1 → 4

The reactions involved in β-oxidation of fatty acids include the following: 1) Cleavage of acetyl-CoA from the fatty acid 2) Hydration of a double bond 3) Formation of a C−C double bond 4) Oxidation of an alcohol The correct order of these reactions is:

3 → 2 → 4 → 1

The carbon skeleton used to make serine is: A) glutamate B) α-ketoglutarate C) oxaloacetate D) pyruvate E) 3-phosphoglycerate

3-Phosphoglycerate

Which of the following glycolytic intermediates is the common precursor to serine, cysteine and glycine? pyruvate dihydroxyacetone phosphate glyceraldehyde-3-phosphate 3-phosphoglycerate fructose-6-phosphate

3-Phosphoglycerate

How many photons will be involved in the transfer of 4 electrons from PSII to 2 NADP+?

8

People on high-protein diets are advised to drink lots of water. Why?

A high-protein diet leads to increased production of urea. Drinking more water increases the volume of urine, ensuring elimination of the urea from the body with less strain on the kidneys than if urea were at a higher concentration.

How are lipases activated hormonally?

A hormone signal activates adenylate cyclase, which makes cAMP. This activates protein kinases, which phosphorylate the lipases, thereby activating them.

What is the purpose of having ACP as a distinct activating group for fatty acid synthesis?

ACP is a molecule that earmarks acyl groups for fatty acid synthesis. It can be managed separately from acyl-CoA groups. Also, the ACP attaches to the acyl groups like a "swinging arm" that tethers it to the fatty acid synthase complex.

In fatty acid degradation, we encounter coenzyme A, mitochondrial matrix, trans double bonds, L-alcohols, β-oxidation, NAD+ and FAD, acetyl-CoA, and separate enzymes. What are the counterparts in fatty acid synthesis?

ACP, citrate, cytosol, trans double bonds, D-alcohols, β-reduction, NADPH, malonyl-CoA (except for one acetyl-CoA), and a multifunctional enzyme complex.

Fatty acids are activated for β-oxidation by the transfer of UMP from UTP UDP from UTP CMP from CTP AMP from ATP ADP from ATP

AMP from ATP

What are the two primary molecules that link anabolic and catabolic reactions?

ATP and NADPH are the two molecules that link the most pathways.

Can ATP production take place in chloroplasts in the absence of light? Give the reason for your answer.

ATP can be produced by chloroplasts in the absence of light if some way exists to form a proton gradient.

What is the role of citrate in the transport of acetyl groups from the mitochondrion to the cytosol?

Acetyl groups condense with oxaloacetate to form citrate, which can cross the mitochondrial membrane. Acetyl groups are regenerated in the cytosol by the reverse reaction.

ATP and NADPH are the two molecules that link the most pathways.

Acetyl-CoA, pyruvate, PEP, α-ketoglutarate, succinyl-CoA, oxaloacetate, and several sugar phosphates, such as glucose-6-phosphate and fructose-6-phosphate.

Outline the role of carnitine in the transport of acyl-CoA molecules into the mitochondrion. How many enzymes are involved? What are they called?

Acyl groups are esterified to carnitine to cross the inner mitochondrial membrane. There are transesterification reactions from the acyl-CoA to carnitine and from acylcarnitine to CoA.

What is the difference between the type of oxidation catalyzed by acyl-CoA dehydrogenase and that catalyzed by β-hydroxy-CoA dehydrogenase?

Acyl-CoA dehydrogenase removes hydrogens from adjacent carbons, creating a double bond and using FAD as coenzyme. β-Hydroxy-CoA dehydrogenase oxidizes an alcohol group to a ketone group and uses NAD+ as a coenzyme.

What is the metabolic purpose of linking a fatty acid to coenzyme A?

Acyl-CoAs are high-energy compounds. An acyl-CoA has sufficient energy to initiate the β-oxidation process. The CoA is also a tag indicating that the molecule is destined for oxidation.

Atoms from the amino acids _____ are utilized in purine biosynthesis. Asp, Gly, Gln Asp, Gly, Gln, Phe Asp, Gln, Phe Asn, Gly, Gln, Phe None of the above

Asp, Gly, Gln

Phenylketonuria is an inherited disease that results from the lack of the enzyme phenylalanine hydroxylase (PAH), which catalyzes the first step in the degradation of phenylalanine, as shown below. Excess phenylalanine accumulates in the blood and urine and has the effect of causing mental retardation if untreated. Why should patients with PKU avoid the artificial sweetener aspartame?

Aspartame is high in phenylalanine, which patients with PKU cannot break down given their deficiency of PAH.

Which amino acids in the urea cycle are the links to the citric acid cycle? Show how these links occur.

Aspartate and arginosuccinate are the amino acids that link the two pathways. Aspartate is made by transamination of OAA. The aspartate then combines with citrulline to form arginosuccinate, which then releases a fumarate to go back to the TCA cycle.

The nitrogen-fixing enzyme system, nitrogenase, is found exclusively in plants bacteria some viruses eukaryotic organisms mammals

Bacteria

A bacterial ATP synthase has 10 c subunits, and a chloroplast ATP synthase has 14 c subunits. Would you expect the bacterium or the chloroplast to have a higher P:O ratio? A) Bacterium B) Chloroplast

Bacterium Having more c subunits results in the translocation of more H+ to fully rotate the Fo component. Full rotation always generates 3 ATP - so we get more ATP per H+ translocated in the bacterium.

Metabolic cycles are rather common (Calvin cycle, citric acid cycle, urea cycle). Why are cycles so useful to organisms?

Because all the components of a cycle are regenerated, only small amounts ("catalytic quantities") are needed. This is important from an energy standpoint and, perhaps with some compounds, because of insolubility problems.

What are the benefits of cyclic electron flow in photosynthesis? A) Energy is recovered in NADPH synthesis. B) Energy is conserved in a proton gradient. C) It promotes ATP synthesis by chemiosmosis. D) Both B and C E) All of the above

Both B and C

How is the structure of chloroplasts similar to that of mitochondria? How does it differ?

Both chloroplasts and mitochondria have an inner and outer membrane. Both have their own DNA and ribosomes. Chloroplasts, however, have a third membrane, the thylakoid membrane.

What are the major similarities and differences between ATP synthesis in chloroplasts, as compared with mitochondria?

Both depend on a proton gradient, resulting from the flow of electrons. In chloroplasts, protons come from the splitting of water to produce oxygen. In mitochondria, protons come from the oxidation of NADH and ultimately consume oxygen and produce water.

How is ACP similar to coenzyme A? How is it different?

Both have a phosphopantetheine group at the active end. In coenzyme A, this group is attached to 2'-phospho-AMP; in ACP, it is attached to a serine residue of a protein.

What roles do thioredoxin and thioredoxin reductase play in the metabolism of nucleotides?

Both thioredoxin and thioredoxin reductase are proteins involved in the conversion of ribonucleotides to deoxyribonucleotides. Thioredoxin is an intermediate carrier of electrons and hydrogens, and thioredoxin reductase is the enzyme that catalyzes the process.

Is it fair to say that the synthesis of NADPH in chloroplasts is merely the reverse of NADH oxidation in mitochondria? Explain your answer.

By and large, the synthesis of NADPH in chloroplasts is the reverse of NADH oxidation in mitochondria. The net electron flow in chloroplasts is the reverse of that in the mitochondria, although different carriers are involved.

Other than their use in nucleic acid synthesis, which energy source is used uniquely for the synthesis of lipids? ATP UTP GTP CTP None of these is unique to fatty acid synthesis.

CTP

Which pathway is central to virtually all living organisms? - glycolysis - photosynthesis - electron transport chain - citric acid cycle - fatty acid metabolism

Citric acid cycle

A molecule synthesized in the mitochondrial matrix and transported to the cytosol for subsequent reaction is arginine argininosuccinate citrulline ornithine

Citrulline

What function do vitamins usually serve? A) coenzymes or precursors of coenzymes B) intermediates of the TCA cycle C) precursors of amino acids D) hormones or second messengers

Coenzymes or precursors of coenzymes

Why is there no net gain of methionine if homocysteine is converted to methionine with S-adenosylmethionine as the methyl donor?

Conversion of homocysteine to methionine using S-adenosylmethionine as the methyl donor gives no net gain; one methionine is needed to produce another methionine.

What is the advantage to plants to have the option of both cyclic and noncyclic pathways for photophosphorylation?

Cyclic photophosphorylation can take place when the plant needs ATP but does not have a great need for NADPH. Non-cyclic photophosphorylation can take place when the plant needs both.

Where in the cell does fatty acid synthesis occur? A) Outer mitochondrial membrane B) Mitochondrial intermembrane space C) Inner mitochondrial membrane D) Cytoplasm E) Mitochondrial matrix

Cytoplasm

The addition of two-carbon units to a growing fatty acid chain is directly driven by reduction of NADP+ The formation of malonyl-CoA from malonate and coenzyme A. oxidation of NADPH decarboxylation of malonyl-CoA hydrolysis of ATP

Decarboxylation of malonyl-CoA

People on high-fiber diets often have less cancer (especially of the colon) and lower blood-cholesterol levels than people on low-fiber diets, even though fiber is not digestible. Suggest reasons for the benefits of fiber.

Diets high in fiber are usually lower in fats, especially saturated fats; fiber adsorbs many potentially toxic substances, such as cholesterol and halocarbons, preventing their absorption into the body; fiber decreases transit time through the intestine, so any toxic materials in food remain in the body for less time and have a smaller chance of being absorbed or otherwise causing problems.

How many ATPs are required for one round of the urea cycle? Where do these ATPs get used?

Each round of the urea cycle costs 4 ATP, two to make carbamoyl-phosphate and effectively two (ATP → AMP) to make arginosuccinate.

Uncouplers of oxidative phosphorylation in mitochondria also uncouple photoelectron transport and ATP synthesis in chloroplasts. Give an explanation for this observation.

Electron transport and ATP production are coupled to each other by the same mechanism in mitochondria and chloroplasts. In both cases, the coupling depends on the generation of a proton gradient across the inner mitochondrial membrane or across the thylakoid membrane, as the case may be.

In fatty acid synthesis, malonyl-CoA, rather than acetyl-CoA, is used as a "condensing group." Suggest a reason for this.

Energy is needed to condense an acetyl group to the growing fatty acid. In theory, such could be done with acetyl-CoA, using ATP. In practice, the ATP is used to convert acetyl-CoA to malonyl-CoA; the condensation of the acetyl moiety of malonyl-CoA is driven in part by the accompanying decarboxylation and requires no additional energy. A possible reason for this is to avoid a metabolic confusion of pathways, perhaps particularly important in (uncompartmented) prokaryotes; one could envision an acetyl-CoA from degradation being used immediately for synthesis. Malonyl-CoA says "synthesis"; acetyl-CoA says "degradation."

Why might a person who is an alcoholic have a "fatty liver"?

Ethanol is converted to acetaldehyde and then to acetic acid. Humans can use that acetic acid only for energy, or they can convert it to fatty acids and other lipids.

You hear a fellow student say that the oxidation of unsaturated fatty acids requires exactly the same group of enzymes as the oxidation of saturated fatty acids. Is the statement true or false? Why?

False - You hear a fellow student say that the oxidation of unsaturated fatty acids requires exactly the same group of enzymes as the oxidation of saturated fatty acids. Is the statement true or false? Why?

Individuals with a mutation in the gene for apolipoprotein B-100 produce very low levels of this protein, which is a component of LDL. How would accumulation of fat in the liver be affected and would this result in hypercholesterolemia or hypocholesterolemia?

Fat would accumulate in the liver and the individual would exhibit hypocholesterolemia.

What do we mean when we say that there is no storage form for protein? How is this different from fats and carbohydrates?

Fats and carbohydrates can be stored when they are consumed in excess. Fats are stored as triacylglycerols and carbohydrates are stored as glycogen. However, proteins consumed in excess are not stored. The extra protein is broken down. The amino groups are released as urea and the carbon skeletons are stored as carbohydrate or fat.

It has been stated many times that fatty acids cannot yield a net gain in carbohydrates. Why can odd-chain fatty acids be thought to break this rule to a small extent?

Fats cannot produce a net yield of glucose because they must enter the citric acid cycle as the two-carbon unit acetyl-CoA. In the first few steps, two carbons are released as CO2. However, an odd-chain fatty acid can be considered partially glucogenic because the final three carbons become succinyl-CoA and enter the citric acid cycle after the decarboxylation steps. Thus, if an extra succinyl-CoA is added, it can then be drawn off later as malate and used for gluconeogenesis without removing the steady-state level of citric acid cycle intermediates.

Comment briefly on the usefulness to organisms of feedback control mechanisms in long biosynthetic pathways.

Feedback control mechanisms slow down long biosynthetic pathways at or near their beginnings, saving energy for the organism.

The final carrier of electrons from PSI is ____, which can then dispose of electrons to produce either ATP or NADPH. A) Plastocyanin B) Manganese C) Ferredoxin D) Cytochrome c E) None of the above

Ferredoxin

How many α-amino acids participate directly in the urea cycle? Of these, how many can be used for protein synthesis?

Five α-amino acids are involved directly in the urea cycle (ornithine, citrulline, aspartate, arginosuccinate, and arginine). Of those, only aspartate and arginine are also found in proteins.

What are the unique enzymes needed to β-oxidize a monounsaturated fatty acid?

For a monounsaturated fatty acid, an additional enzyme is needed, the enoyl-CoA isomerase.

What are the unique enzymes needed to β-oxidize a polyunsaturated fatty acid?

For a polyunsaturated fatty acid, two additional enzymes are needed, the enoyl-CoA isomerase and 2,4-dienoyl-CoA reductase.

Describe briefly how β-oxidation of an odd-chain fatty acid is different from that for an even-chain fatty acid.

For an odd-chain fatty acid, β-oxidation proceeds normally until the last round. When five carbons are left, that round of β-oxidation releases one acetyl-CoA and one propionyl-CoA. Propionyl-CoA cannot be further metabolized by β-oxidation; however, a separate set of enzymes converts propionyl-CoA into succinyl-CoA, which can then enter the citric acid cycle.

Calculate the net ATP yield from the complete processing of a saturated fatty acid containing 17 carbons. Consider the β-oxidation steps, processing of acetyl-CoA through the citric acid cycle, and electron transport.

From seven cycles of β-oxidation: 7 acetyl-CoA, 1 propionyl-CoA, 7 FADH2, 7 NADH. From the processing of 7 acetyl-CoA in the citric acid cycle: 7 FADH2, 21 NADH, and 7 GTP. From the processing of the propionyl-CoA: -1 ATP for conversion to succinyl-CoA, -1 GTP from the citric acid cycle, and 1 NADH and 1 FADH2 from the citric acid cycle. From reoxidation of all FADH2 and NADH: 22.5 ATP from 15 FADH2, and 72.5 ATP from 29 NADH. From 8 GTP: 8 ATP. Subtotal: 103 ATP. Subtract a 2-ATP equivalent used in activation step and a 1-ATP equivalent used in the conversion to succinyl-CoA for a grand total of 100 ATP.

Calculate the ATP yield for the complete oxidation of one molecule of palmitic acid (16 carbons). How does this figure differ from that obtained for stearic acid (18 carbons)? Consider the β-oxidation steps, processing of acetyl-CoA through the citric acid cycle, and electron transport.

From seven cycles of β-oxidation: 8 acetyl-CoA, 7 FADH2, and 7 NADH. From the processing of 8 acetyl-CoA in the citric acid cycle: 8 FADH2, 24 NADH, and 8 GTP. From re-oxidation of all FADH2 and NADH: 22.5 ATP from 15 FADH2, 77.5 ATP from 31 NADH. From 8 GTP: 8 ATP. Subtotal: 108 ATP. A 2-ATP equivalent was used in the activation step. Grand total: 106 ATP. The grand total for stearic acid was 120 ATP.

The urea cycle is linked to the citric acid cycle by A) arginine B) citrulline C) fumarate D) ornithine

Fumarate

What is the difference between a G protein and a receptor tyrosine kinase? Give an example of a hormone that uses each.

G proteins get their name because they bind GTP as part of their effect. An example is the G protein that is linked to the epinephrine receptor and leads to the production of cAMP as a second messenger. Receptor tyrosine kinases have a different mode of action. When they bind their hormone, they phosphorylate tyrosine residues on themselves and other target proteins, which then act as a second messenger. Insulin is an example of a hormone that binds to a receptor tyrosine kinase.

Activation of a G protein in response to hormone binding requires binding of _____ to the _____ subunit.

GTP to the α subunit

Which of these hormones causes elevation of the level of glucose in human blood? A) Insulin B) Glucagon C) Glucagon and Epinephrine D) Insulin and Glucagon E) All of these Hormones elevate the blood glucose levels.

Glucagon and Epinephrine

When amino acids are catabolized, what are the end products of the carbon skeletons for glucogenic amino acids? For ketogenic amino acids?

Glucogenic amino acids are degraded to pyruvate or one of the citric acid cycle intermediates found after the decarboxylation steps, such as succinate or malate. Ketogenic amino acids are degraded to acetyl-CoA or acetoacetyl-CoA.

If photosynthesizing plants are grown in the presence of 14CO2, is every carbon atom of the glucose that is produced labeled with the radioactive carbon? Why or why not?

Glucose synthesized by photosynthesis is not uniformly labeled because only one molecule of CO2 is incorporated into each molecule of ribulose-1,5-bisphosphate, which then goes on to give rise to sugars.

How does the level of glutamic acid affect the urea cycle?

Glutamate brings ammonia groups to the matrix of the mitochondria for the urea cycle. High levels of glutamate stimulate the urea cycle.

Adrenalin (epinephrine) is the "fight-or-flight hormone". One of its physiological roles is to mobilize fuel stores in preparation for activity. Given this, which of the following does NOT typically occur in response to the release of adrenalin? - Glycolysis in the muscle - Glycogen synthesis in the liver - Gluconeogenesis in the liver - Glycogen breakdown in the muscle

Glycogen synthesis in the liver

If an amino acid is ketogenic, then it: A) has a catabolic pathway that leads to acetyl-CoA or acetoacetyl-CoA B) has a one-step pathway to a fatty acid C) enters the citric acid cycle as pyruvate D) enters the citric acid cycle as fumarate E) none of these

Has a catabolic pathway that leads to acetyl-CoA or acetoacetyl-CoA

Are all hormones closely related in their chemical structure?

Hormones can have several different kinds of chemical structures, including steroids, polypeptides, and amino acid derivatives.

When PIP3 is hydrolyzed, why does IP3 diffuse into the cytosol while DAG remains in the membrane?

IP3 is a polar compound and can dissolve in the aqueous environment of the cytosol; DAG is nonpolar and interacts with the side chains of the membrane phospholipids.

In the mitochondrion is a short-chain carnitine acyltransferase that can take acetyl groups from acetyl-CoA and transfer them to carnitine. How might this be related to lipid biosynthesis?

If acetyl-carnitine forms in the matrix of the mitochondrion, it can be translocated to the cytosol via the carnitine translocase. Thus, this could represent another way of shuttling acetyl units out of the mitochondria for synthesis.

A deficiency of carnitine results in muscle cramps, which are exacerbated by fasting or exercise. Give a biochemical explanation for the muscle cramping, and explain why cramping increases during fasting and exercise.

If carnitine is deficient, fatty acid transport from the cytosol to the mitochondrial matrix (the site of β-oxidation) is impaired. Muscle cramping is exacerbated by fasting because the decreased concentration of circulating glucose and depleted glycogen stores. Exercise also increases muscle cramping because the demand for ATP by the muscle is greater.

Why might a doctor smell the breath of a person known to have diabetes who has just passed out?

If the reason for passing out is uncontrolled diabetes, the doctor expects to smell acetone on the breath, since the otherwise unused sugars are being converted to fats and ketone bodies.

In cyclic photophosphorylation in photosystem I, ATP is produced, even though water is not split. Explain how the process takes place.

In cyclic photophosphorylation, the excited chlorophyll of photosystem I passes electrons directly to the electron transport chain that normally links photosystem II to photosystem I. This electron transport chain is coupled to ATP production.

What are the two places where light energy is required in the light reaction of photosynthesis? Why must energy be supplied at precisely these points?

In photosystem I and in photosystem II, light energy is needed to raise the reaction-center chlorophylls to a higher energy level. Energy is needed to generate strong enough reducing agents to pass electrons to the next of the series of components in the pathway.

What is an important difference between the biosynthesis of purine nucleotides and that of pyrimidine nucleotides?

In purine nucleotide biosynthesis, the growing purine ring is covalently bonded to ribose; the ribose is added after the ring is synthesized in pyrimidine nucleotide biosynthesis.

When glucose is unavailable, the liver begins to break down fatty acids to supply the rest of the body with metabolic fuel. Explain why fatty acid-derived acetyl-CoA is not catabolized by the citric acid cycle but is instead diverted to ketogenesis when no glucose is available.

In the absence of glucose, fat-derived acetyl-CoA cannot enter the TCA cycle due to insufficient oxaloacetate levels. When carbohydrate amounts decreases, any available oxaloacetate goes to gluconeogenesis to produce the glucose that is necessary for red blood cell and brain function (as ketone bodies are less desirable for these bodily functions). If fat-derived acetyl-CoA cannot enter the TCA cycle, it is converted to ketone bodies to support other areas in the body that need energy.

Given the nature of the hormonal activation of lipases, what carbohydrate pathways would be activated or inhibited under the same conditions?

In the liver, glycogen breakdown and gluconeogenesis would occur. In the muscle, glycogen breakdown and glycolysis would occur.

How are the two redox reactions of β-oxidation different from their counterparts in fatty acid synthesis?

In β-oxidation, FAD is the coenzyme for the first oxidation reaction, while NAD+ is the coenzyme for the second. In fatty acid synthesis, NADPH is the coenzyme for both. The β-hydroxy-acyl group in β-oxidation has the L-configuration, while it has the D-configuration in fatty acid synthesis.

The second messenger _____ opens Ca2+ channels in the endoplasmic reticulum.

Inositol trisphosphate

How is it possible that both insulin and epinephrine stimulate muscle glycolysis?

Insulin and epinephrine normally have opposite effects, but they both stimulate muscle glycolysis. Epinephrine is the hormone that signals the need for quick energy, which means the muscle cells must be able to use glucose via glycolysis. Insulin stimulates pathways that use up glucose so that the blood glucose lowers, so it makes sense for it to stimulate glycolysis as well. Epinephrine stimulates muscle glycolysis by activating adenylate cyclase, which makes cAMP; cAMP then activates protein kinase A, which phosphorylates phosphofructokinase-2 and fructose-bisphosphatase-2. In the muscle, phosphorylation of phosphofructokinase-2 activates it, producing more fructose-2,6-bisphosphate, which activates phosphofructokinase-1 and glycolysis. In muscle, insulin stimulates glycolysis by activating phosphofructokinase and pyruvate dehydrogenase.

What is the primary function of insulin?

Insulin's primary function is to stimulate the transport of glucose out of the blood and into the cell.

What are the principal metal ions used in electron transfer in chloroplasts? Compare them to the ions found in mitochondria.

Iron and manganese in chloroplasts; iron and copper in mitochondria. Note that all these are transition metals, which can easily undergo redox reactions.

Under what conditions are ketone bodies produced?

Ketones are produced when there is an imbalance in lipid catabolism, compared with carbohydrate catabolism. If fatty acids are being β-oxidized to produce acetyl-CoA, but there is insufficient oxaloacetate because it is being drawn off for gluconeogenesis, the acetyl-CoA molecules combine to form ketone bodies.

What is leptin, and how does it work?

Leptin is a hormone that affects metabolism. It affects the brain to suppress appetite and it affects metabolism directly by stimulating fatty-acid oxidation and inhibiting fatty-acid synthesis.

Which amino acid generates the following product in a transamination reaction with α-ketoglutarate? A) Met B) Arg C) Leu D) Val E) Tyr

Leu The carbonyl carbon of the keto acid is where the amine group will be transferred - so this becomes the alpha carbon. The hydrocarbon portion of the molecule linked to the keto group is then the amino acid side chain. Identify the side chain and you identify the amino acid.

Why are linoleate and linolenate considered essential fatty acids? What step in production of polyunsaturated fatty acids are mammals unable to perform?

Linoleate and linolenate cannot be synthesized by the body and must therefore be obtained from dietary sources. Mammals cannot produce a double bond beyond carbon atom 9 of fatty acids.

Explain what insulin and low-carbohydrate diets have to do with one another.

Low-carbohydrate diets are designed to prevent the high blood sugar levels that arise when large quantities of carbohydrates are consumed. High blood sugar leads to a rapid rise in insulin. Insulin is known to stimulate fat synthesis and to inhibit fatty-acid oxidation. Thus, low-carbohydrate diets are thought to help fight weight gain.

A cat named Lucullus is so spoiled that he will eat nothing but freshly opened canned tuna. Another cat, Griselda, is given only dry cat food by her far less indulgent owner. Canned tuna is essentially all protein, whereas dry cat food can be considered 70% carbohydrate and 30% protein. Assuming that these animals have no other sources of food, what can you say about the differences and similarities in their catabolic activities? (The pun is intended.)

Lucullus breaks down the protein in the tuna to amino acids, which in turn undergo the urea cycle and the breakdown of the carbon skeleton, eventually leading to the citric acid cycle and electron transport. In addition to protein catabolism, Griselda breaks down the carbohydrates to sugars, which then undergo glycolysis and enter the citric acid cycle. (Gratuitous information: Lucullus was a notorious Roman gourmand. In medieval literature, Griselda was the name usually given to a forbearing, long-suffering woman.)

Why is it somewhat misleading to study biochemical pathways separately?

Many compounds, such as oxaloacetate, pyruvate, and acetyl-CoA, play a role in a number of reactions. More to the point, the end products of some pathways are the starting points of others. Each pathway is one aspect of an overall metabolic scheme.

What kinds of organisms can fix nitrogen? Which ones cannot?

Nitrogen-fixing bacteria (symbiotic organisms that form nodules on the roots of leguminous plants, such as beans and alfalfa) and some free-living microbes and cyanobacteria can fix nitrogen. Plants and animals cannot.

Do all the chlorophyll molecules in a photosynthetic reaction center play the same roles in the light reactions of photosynthesis?

No - most chlorophylls are light-harvesting molecules that transfer energy to the special pair that takes part in the light reactions.

If photosynthesizing plants are grown in the presence of 14CO2, is every carbon atom of the glucose that is produced labeled with the radioactive carbon? Why or why not?

No - ribulose-1,5-bisphosphate is carboxylated in the initial step of dark reactions, and the Calvin Cycle reactions that follow lead to glucose synthesis; only one of the 6 carbons in glucose would be labeled with a radioactive carbon instead of every carbon.

The first three reactions of the β-oxidation cycle of fatty acids produce A) one mole each of NADH and FADH2 B) two moles of NADH C) two moles of FADH2 D) two moles of ATP

One mole each of NADH and FADH2

Compare the energy yields from the oxidative metabolism of glucose and of stearic acid. To be fair, calculate it on the basis of ATP equivalents per carbon and also ATP equivalents per gram.

One obtains 6.7 ATP per carbon and 0.42 ATP per gram for stearic acid versus 5 ATP per carbon and 0.17 ATP per gram for glucose. More energy is available from stearic acid than from glucose.

Suggest a reason why plants contain light-absorbing pigments in addition to chlorophylls a and b.

Only a relatively small portion of the visible spectrum is absorbed by chlorophylls. The accessory pigments absorb light at additional wavelengths. As a result, most of the visible spectrum can be harnessed in light-dependent reactions.

Describe citrulline and ornithine based on their similarity to one of the 20 standard amino acids.

Ornithine is similar to lysine, but it has one fewer methylene group in the side chain. Citrulline is a keto version of arginine with a side chain C=NH2+ replaced by C=O.

Would you expect ‍H2O or CO2 to be the source of the oxygen produced in photosynthesis? Give the reason for your answer.

Oxygen produced in photosynthesis comes from water. The oxygen-evolving complex is part of the series of electron-transfer reactions from water to NADPH. Carbon dioxide is involved in the dark reactions, which are different reactions that take place in another part of the chloroplast.

How are nitrogen-utilizing pathways controlled by feedback inhibition?

Pathways that use nitrogen to make amino acids, purines, and pyrimidines are controlled by feedback inhibition. The final product, such as CTP, inhibits the first or an early step in its synthesis.

Stimulation of the insulin receptor by ligand binding and autophosphorylation eventually leads to the activation of glycogen synthase and the translocation of glucose transporters to the plasma membrane. One strategy for treating diabetes is to develop drugs that act as inhibitors of the phosphatase enzymes that remove phosphate groups from the phosphorylated tyrosines on the insulin receptor. Why might this be an effective treatment for diabetes?

Phosphatases would turn the insulin signaling pathway off and protein kinases B and C would not be activated; so, glycogen synthase is inactive and glycogen cannot be synthesized from glucose, and glucose transporters are not translocated to the membrane and glucose remains in the blood. Drugs that act as inhibitors would increase insulin receptor action, allowing the receptor to remain active with a lower concentration of ligand.

What is implied about the energy requirements of photosystems I and II by the fact that there is a difference in the minimum wavelength of light needed for them to operate (700 nm for photosystem I and 680 nm for photosystem II)?

Photosystem II requires more energy than photosystem I. The shorter wavelength of light means a higher frequency. Frequency, in turn, is directly proportional to energy.

The action of Ca2+ in a second messenger scheme A) involves the calcium-binding protein calmodulin as the actual second messenger B) relies entirely on intercellular reservoirs of Ca2+ in the endoplasmic reticulum C) does not involve the G protein D) produces sustained responses by controlling the flow of Ca2+ into the cell

Produces sustained responses by controlling the flow of Ca2+ into the cell

A key intermediate in the catabolism of fatty acids with uneven numbers of carbon atoms is A) malonyl-CoA B) propionyl-CoA C) oxaloacetate D) phosphoenolpyruvate

Propionyl-CoA

Which of the following is not an advantage of fatty acids as a form of energy storage? A) Fatty acids contain more highly reduced carbons than carbohydrates. B) Their hydrophobic nature allows close packing in adipose tissue. C) Storage of fats in muscle tissue makes it readily available for use during activity. D) Fats contain more energy per gram than sugars. E) They do not require water for storage.

Storage of fats in muscle tissue makes it readily available for use during activity.

How is tetrahydrofolate important to purine synthesis?

Tetrahydrofolate is a carrier of carbon groups. Two of the carbons in the purine ring are donated by tetrahydrofolate.

Chemotherapy patients receiving cytotoxic (cell-killing) agents such as FdUMP (the UMP analogue that contains fluorouracil) and methotrexate temporarily go bald. Why does this take place?

The DNA of fast-growing cells, such as those of the hair follicles, is damaged by chemotherapeutic agents.

What stops a hormone response when a G protein is involved?

The G protein is bound to GTP. Eventually, the GTP is hydrolyzed to GDP, which causes it to dissociate from adenylate cyclase. This stops the hormone response until the hormone dissociates from the receptor, the G protein trimers are rejoined, and the process starts over again.

What is the activating group used in the formation of phosphoacylglycerols?

The activating group found on the acylglycerol is cytidine diphosphate.

Why is arginine an essential amino acid, when it is made in the urea cycle?

The amounts of arginine necessary in the urea cycle are only catalytic. If arginine from the cycle is used for protein synthesis, the cycle becomes depleted.

Does the whole Calvin cycle represent carbon dioxide fixation? Why or why not?

The condensation of ribulose-1,5-bisphosphate with carbon dioxide to form two molecules of 3-phosphoglycerate is the actual carbon dioxide fixation. The rest of the Calvin cycle regenerates ribulose-1,5-bisphosphate.

Does the conversion of IMP to GMP use or produce ATP either directly or indirectly? Justify your answer.

The conversion of IMP to GMP produces one NADH and uses the equivalent of 2 ATP because an ATP is converted to AMP. Because NADH gives rise to 2.5 ATP if it goes into the electron transport chain, we can say that the conversion results in a net production of ATP.

The oxidation of an unsaturated fatty acid leads to the production of fewer ATPs than a saturated fatty acid with the same number of carbons because ____ A) the oxidation of the fatty acid stops when it reaches the double bond B) unsaturated fatty acids cannot be metabolized C) the double bond results in the bypassing of the first oxidation step of the pathway, thereby eliminating one FADH2 product D) none of these

The double bond results in the bypassing of the first oxidation step of the pathway, thereby eliminating one FADH2 product

Describe some similarities between the electron transport chains in chloroplasts and in mitochondria.

The electron transport chain in chloroplasts, like that in mitochondria, consists of proteins, such as plastocyanin, and protein complexes, such as the cytochrome b6-f complex. It also contains mobile electron carriers, such as pheophytin and plastoquinone (equivalent to coenzyme Q), which is also true of the mitochondrial electron transport chain.

What is the source of the glycerol in triacylglycerol synthesis?

The glycerol comes from degradation of other acylglycerols or from glycerol-3-phosphate derived from glycolysis.

Why is it better, when running a marathon, to drink a beverage with sugar for energy rather than one with amino acids?

The metabolism of amino acids encourages urine formation and actually a greater thirst and need for water.

A friend who is trying to lose weight complains about the odd taste in his mouth in the mornings. He says it seems like a filling has broken loose, and the metallic sensation is bothersome. What would you say?

The metallic taste may be due to acetone, which means that your friend may have a mild state of ketosis. Ask if your friend has consulted a doctor about the diet regimen, and perhaps recommend either backing off from such a low-calorie diet or drinking more water to flush the system more thoroughly.

What is the difference between the old food pyramid and the newer version?

The old pyramid assumed that all carbohydrates and fats were the same and that carbohydrates were good and all fats were bad. The newer pyramid recognized that not all carbohydrates are good and not all fats are bad. Complex carbohydrates were placed lower down on the newer pyramid, whereas processed ones were placed higher. Essential fats and oils were included as necessary food types. Also, dairy consumption recommendations were reduced.

How many high-energy phosphate bonds must be hydrolyzed in the pathway that produces GMP from guanine and PRPP by the PRPP salvage reaction, compared with the number of such bonds hydrolyzed in the pathway leading to IMP and then to GMP?

The purine salvage reaction that produces GMP requires the equivalent of 2 ATP. The pathway to IMP and then to GMP requires the equivalent of 8 ATP.

One way in which the anabolism of pyrimidine nucleotides differs from that of purine nucleotides is that

The pyrimidine ring is assembled before being bonded to the ribose phosphate, whereas the purine ring is bonded to the ribose phosphate as it is formed

Some GPCR's are associated with a protein called RGS (regulator of G protein signaling). RGS stimulates the GTPase activity of the G protein associated with the receptor. What effect does RGS have on the signaling process? A) This will have no effect on the signaling process. B) This will diminish the signal. C) This will enhance the signal.

This will diminish the signal.

In which part of the chloroplast does production of oxygen during photosynthesis primarily occur? thylakoid disks thylakoid space grana stroma

Thylakoid disks

Accessory pigments A) transfer electrons to reaction center chlorophylls. B) transfer energy to reaction center chlorophylls. C) transfer electrons from reaction center chlorophylls to pheophytins. D) transfer electrons from reaction center chlorophylls to ferredoxin. E) All of these statements are true.

Transfer energy to reaction center chlorophylls

Amino acid and protein hormones usually act by binding to a receptor on the cell surface. A) True B) False

True

Many hormones come in pairs, which have opposing metabolic activity.

True

The use of cyclic AMP to mobilize fatty acids from adipose tissue is analogous to cyclic AMP's role in mobilization of sugars from glycogen in the liver. A) True B) False

True

Briefly outline the reactions involved in ketone production.

Two acetyl-CoA molecules combine to form acetoacetyl-CoA. This can then release coenzyme A to yield acetoacetate, which can be converted either to β-hydroxybutyrate or to acetone.

What is the link between insulin binding to the receptor and the eventual second messenger?

When insulin binds to its receptor, the β-subunit of the receptor kinase autophosphorylates. When this happens, the receptor kinase is able to phosphorylate tyrosines on the insulin receptor kinase.

Albert Szent-Gyorgi, a pioneer in early photosynthesis research, stated, "What drives life is a little electric current, kept up by the sunshine." What did he mean by this?

With very few exceptions, life directly or indirectly depends on photosynthesis. The electric current is the flow of electrons from water to NADP+, a light-requiring process. The "current" continues in the light-independent reactions, with electrons flowing from NADPH to bisphosphoglycerate, which ultimately yields glucose.

If an amino acid's degradation pathway leads to α-ketoglutarate, is it glucogenic? Yes No Only if the organism has a glyoxylate pathway. Only if the organism does not have a glyoxylate pathway.

Yes

Give three examples of second messengers.

cAMP, Ca2+, insulin receptor substrate

Draw a transamination reaction between α-ketoglutarate and alanine.

α-ketoglutarate + alanine → L-glutamate + pyruvate