Grabner Exam 6 Review Questions

How does the presence of alpha bonds versus beta bonds influence the digestibility of glucose polymers by humans? Hint: There are two effects.

Humans can digest glucose polymers containing alpha-bonds but not beta. For this reason, glycogen and starch are digestible, whereas cellulose is not. In addition, cellulose is fibrous rather than branched, which makes the polymer insoluble in water and inaccessible to digestion by humans.

Explain how the pentose phosphate pathway can respond to a cell's need for ATP, NADPH, and ribose-5-phosphate.

If a cell needs NADPH, all the reactions of the pentose phosphate pathway take place. If a cell needs ribose-5-phosphate, the oxidative portion of the pathway can be bypassed; only the nonoxidative reshuffling reactions take place. The pentose phosphate pathway does not have a significant effect on the cell's supply of ATP.

How does the role of glucose-6-phosphate in gluconeogenesis differ from that in glycolysis?

In gluconeogenesis, glucose-6-phosphate is dephosphorylated to glucose (the last step of the pathway); in glycolysis, it isomerizes to fructose-6-phosphate (an early step in the pathway).

How does phosphorolysis differ from hydrolysis?

In phosphorolysis, a bond is cleaved by adding the elements of phosphoric acid across that bond, whereas in hydrolysis, the cleavage takes place by adding the elements of water across the bond.

What is the metabolic basis for the observation that many adults cannot ingest large quantities of milk without developing gastric difficulties?

In some cases, the enzyme that degrades lactose (milk sugar) to its components—glucose and galactose—is missing. In other cases, the enzyme isomerizes galactose to glucose for further metabolic breakdown.

How does control of the glucose-6-phosphatase reaction differ from that of the fructose-1,6-bisphosphatase reaction?

In the glucose-6-phosphatase reaction, the concentration of substrate is the main determinant of reaction velocity. In the fructose-1,6-bisphosphatase reaction, allosteric effects are the main determinant of reaction velocity.

What are the metabolic effects of not being able to produce the M subunit of phosphofructokinase?

Individuals who lack the gene that directs the synthesis of the M form of the enzyme can carry on glycolysis in their livers but experience muscle weakness because they lack the enzyme in muscle.

What role does insulin play in glycogen synthesis?

Insulin triggers the series of events that leads to glycogen synthesis.

You are a teaching assistant in a general chemistry lab. The next experiment is to be an oxidation-reduction titration involving iodine. You get a starch indicator from the stockroom. Why do you need it?

Iodine is the reagent that will be added to the reaction mixture in the titration. When the end point is reached, the next drop of iodine will produce a characteristic blue color in the presence of the indicator.

Why would enzymes be found as isozymes?

Isozymes allow for subtle control of the enzyme to respond to different cellular needs. For example, in the liver, lactate dehydrogenase is most often used to convert lactate to pyruvate, but the reaction is often reversed in the muscle. Having a different isozyme in the muscle and liver allows for those reactions to be optimized.

Define isozymes and give an example from the material discussed in this chapter.

Isozymes are oligomeric enzymes that have slightly different amino acid compositions in different organs. ex) Lactate dehydrogenase, phosphofructokinase.

In metabolism, glucose-6-phosphate (G6P) can be used for glycogen synthesis or for glycolysis, among other fates. What does it cost, in terms of ATP equivalents, to store G6P as glycogen, rather than to use it for energy in glycolysis? Hint: The branched structure of glycogen leads to 90% of glucose residues being released as glucose-1-phosphate and 10% as glucose.

It "costs" one ATP equivalent (UTP to UDP) to add a glucose residue to glycogen. In degradation, about of the glucose residues do not require ATP to produce glucose-1-phosphate. The other require ATP to phosphorylate glucose. On average, this is another ATP. Thus, the overall "cost" is ATP, compared with the three ATP that can be derived from glucose-6-phosphate by glycolysis.

UDP-glucose pyrophosphorylase works by this mechanism: It adds a phosphate group to glucose from UTP, leaving behind UDP. It adds a UMP molecule to glucose-1-phosphate by splitting out pyrophosphate. It adds a pyrophosphate group to glucose, using UTP. It adds a UDP molecule to glucose by splitting out phosphate. None of these

It adds a UMP molecule to glucose-1-phosphate by splitting out pyrophosphate.

A researcher claims to have discovered a variant form of glycogen. The variation is that it has very few branches (every 50 glucose residues or so) and that the branches are only three residues long. Is it likely that this discovery will be confirmed by later work?

It is unlikely that this finding will be confirmed by other researchers. The highly branched structure of glycogen is optimized for release of glucose on demand.

Suggest a reason or reasons why the Cori cycle takes place in the liver and in muscle.

Muscle tissue uses large quantities of glucose, producing lactate in the process. The liver is an important site of gluconeogenesis to recycle the lactate to glucose.

The glycolytic pathway must be supplied with its primary oxidizing agent ________ .

NAD+

Which of the enzymes discussed in this chapter are NADH-linked dehydrogenases?

NADH-linked dehydrogenases: Glyceraldehyde-3-phosphate dehydrogenase, lactate dehydrogenase, and alcohol dehydrogenase.

Avidin, a protein found in egg whites, binds to biotin so strongly that it inhibits enzymes that require biotin. What is the effect of avidin on glycogen formation? On gluconeogenesis? On the pentose phosphate pathway?

Of the three processes—glycogen formation, gluconeogenesis, and the pentose phosphate pathway—only one, gluconeogenesis, involves an enzyme that requires biotin. The enzyme in question is pyruvate carboxylase, which catalyzes the conversion of pyruvate to oxaloacetate, an early step in gluconeogenesis.

Show, by a series of equations, the energetics of phosphorylation of ADP by phosphoenolpyruvate.

Phosphoenolpyruvate--> pyruvate +Pi free energy= -61.9 KJ/mol AD{+Pi--> ATP free energy= 30.5 KJ/mol Phosphoenolpyruvate+ADP--> Pyruvate +ATP free energy= -31.4 KJ/mol

Glycolysis that starts with glycogen instead of glucose can be considered to have a higher energy yield because: Phosphorolysis reactions cleave bonds with phosphate instead of water. Phosphorylase is a better enzyme than hexokinase Phosphorylase produces a glucose phosphate without spending an ATP to do it All of these None of these is true because glycolysis starting from glycogen does not have a higher energy yield

Phosphorylase produces a glucose phosphate without spending an ATP to do it

What are some of the main differences between the cell walls of plants and those of bacteria?

Plant cell walls- cellulose bacteria cell walls- polysaccharides with peptide crosslink

What are the possible metabolic fates of pyruvate?

Pyruvate can be converted to lactate, ethanol (yeast), or acetyl-CoA. In humans, only acetyl-coA and lactate.

Which reaction or reactions discussed in this chapter (18) require ATP? Which reaction or reactions produce ATP? List the enzymes that catalyze the reactions that require and that produce ATP.

Reactions that require ATP: formation of UDP-glucose from glucose-1-phosphate and UTP (indirect requirement, because ATP is needed to regenerate UTP), regeneration of UTP, and carboxylation of pyruvate to oxaloacetate. Reactions that produce ATP: none. Enzymes that catalyze ATP-requiring reactions: UDP-glucose phosphorylase (indirect requirement), nucleoside phosphate kinase, and pyruvate carboxylase. Enzymes that catalyze ATP-producing reactions: none.

Which reaction or reactions that we have met in this chapter require ATP? Which reaction or reactions produce ATP? List the enzymes that catalyze the reactions that require and that produce ATP.

Reactions that require ATP: phosphorylation of glucose --> glucose-6-phosphate phosphorylation of fructose-6-phosphate --> fructose-1,6-bisphosphate. Reactions that produce ATP: transfer of phosphate group from 1,3-bisphosphoglycerate to ADP transfer of phosphate group from phosphoenolpyruvate to ADP. Enzymes that catalyze reactions requiring ATP: hexokinase, glucokinase, and phosphofructokinase. Enzymes that catalyze reactions producing ATP: phosphoglycerate kinase and pyruvate kinase.

Which reaction or reactions that we have met in this chapter require NADH? Which reaction or reactions require NAD+? List the enzymes that catalyze the reactions that require NADH and that require NAD+

Reactions that require NADH: reduction of pyruvate to lactate and reduction of acetaldehyde to ethanol. Reactions that require NAD+ : oxidation of glyceraldehyde-3-phosphate-->1,3-diphosphoglycerate. Enzymes that catalyze reactions requiring NADH: lactate dehydrogenase and alcohol dehydrogenase. Enzymes that catalyze reactions requiring NAD+: glyceraldehyde-3-phosphate dehydrogenase.

What reactions in this chapter require acetyl-CoA or biotin?

Reactions that require acetyl-CoA: none. Reactions that require biotin: carboxylation of pyruvate to oxaloacetate.

In the muscles, glycogen is broken down via the following reaction: (Glucose)n+Pi--> GLucose-1-phosphate+ (Glucose)n-1 What would be the ATP yield per molecule of glucose in the muscle if glycogen were the source of the glucose?

Starting with glucose-1-phosphate, the net yield is three ATP, because one of the priming reactions is no longer used. Thus, glycogen is a more efficient fuel for glycolysis than free glucose.

Earlier biochemists called substrate cycles "futile cycles." Why might they have chosen such a name? Why is it something of a misnomer?

Substrate cycles are futile in the sense that there is no net change except for the hydrolysis of ATP. However, substrate cycles allow for increased control over opposing reactions when they are catalyzed by different enzymes.

Suggest a reason why sugar nucleotides, such as UDPG, play a role in glycogen synthesis, rather than sugar phosphates, such as glucose-6-phosphate.

Sugar nucleotides are diphosphates. The net result is hydrolysis to two phosphate ions, releasing more energy and driving the addition of glucose residues to glycogen in the direction of polymerization.

Name two control mechanisms that play a role in glycogen biosynthesis. Give an example of each.

Synthase is INactivated by phosphorylation and activated by dephosphorylation. It is allosterically inhibited by ATP and activated by its substrate - glucose-6-phosphate (G6P). To understand the inhibition by ATP, keep in mind that having enough energy to store glucose isn't sufficient - the cell must also have a high concentration of G6P. As [ATP] increases, it binds to synthase causes a conformational change, so that a higher concentration of G6P is required. This mechanism ensures that there is not only sufficient energy for glycogen storage - but also sufficient glucose.

What is the Cori cycle?

The Cori cycle is a pathway in which there is cycling of glucose due to glycolysis in muscle and gluconeogenesis in liver. The blood transports lactate from muscle to liver and glucose from liver to muscle.

What does the material of this chapter have to do with beer? What does it have to do with tired and aching muscles?

The bubbles in beer are , produced by alcoholic fermentation. Tired and aching muscles are caused in part by a buildup of lactic acid, a product of anaerobic glycolysis.

How can the synthesis and breakdown of fructose-2,6-bisphosphate be controlled independently?

The concentration of fructose-2,6-bisphosphate in a cell depends on the balance between its synthesis (catalyzed by phosphofructokinase-2) and its breakdown (catalyzed by fructose-2,6-bisphosphatase). The separate enzymes that control the formation and breakdown of fructose-2,6-bisphosphate are themselves controlled by a phosphorylation/dephosphorylation mechanism.

Which reactions are the control points in glycolysis?

The control points in glycolysis are the reactions catalyzed by hexokinase, phosphofructokinase, and pyruvate kinase.

No animal can digest cellulose. Reconcile this statement with the fact that many animals are herbivores that depend heavily on cellulose as a food source.

The digestive tract of these animals contains bacteria that have the enzyme to hydrolyze cellulose.

Would eating candy bars, high in sucrose rather than complex carbohydrates, help build up glycogen stores?

The disaccharide sucrose can be hydrolyzed to glucose and fructose, which can both be readily converted to glucose-1-phosphate, the immediate precursor of glycogen. This is not the usual form of "glycogen-loading."

Show how the estimate of 33% efficiency of energy use in anaerobic glycolysis is derived.

The energy released by all the reactions of glycolysis is 184.5 KJ/mol glucose. The energy released by glycolysis drives the phosphorylation of two ADP to ATP for each molecule of glucose, trapping 61 KJ/mol glucose . The estimate of efficiency comes from the calculation (61/1845)100.

Why is it useful to have a primer in glycogen synthesis?

The enzyme that catalyzes addition of glucose residues to a growing glycogen chain cannot form a bond between isolated glucose residues; thus we have the need for a primer.

Why is it reasonable to expect that glucose-6-phosphate will be oxidized to a lactone (see Question 52) rather than to an open-chain compound?

The ester bond is more easily broken than any of the other bonds that form the sugar ring. Hydrolysis of that bond is the next step in the pathway.

Define substrate-level phosphorylation and give an example from the reactions discussed in this chapter.

The free energy of hydrolysis of a substrate is the energetic driving force in substrate-level phosphorylation. An example is the conversion of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate.

Is the glycogen synthase reaction exergonic or endergonic? What is the reason for your answer?

The glycogen synthase reaction is exergonic overall because it is coupled to phosphate ester hydrolysis.

How does the result in Question 41 differ from the gross yield of ATP?

The gross yield is four ATP molecules per glucose molecule, but the reactions of glycolysis require two ATP per glucose.

How does the hydrolysis of fructose-1,6-bisphosphate bring about the reversal of one of the physiologically irreversible steps of glycolysis?

The hydrolysis of fructose-1,6-bisphosphate is a strongly exergonic reaction. The reverse reaction in glycolysis, phosphorylation of fructose-6-phosphate, is irreversible because of the energy supplied by ATP hydrolysis.

What is unique about TPP that makes it useful in decarboxylation reactions?

The important part of TPP is the five-membered ring, in which a carbon is found between nitrogen and sulfur. This carbon forms a carbanion that is extremely reactive, making it able to perform a nucleophilic attack on carbonyl groups, leading to decarboxylation of several compounds in different pathways.

What should be the net ATP yield for glycolysis when fructose, mannose, and galactose are used as the starting compounds? Justify your answer.

The net yield of ATP from glycolysis is the same, two ATP, when either of the three substrates is used. The energetics of the conversion of hexoses to pyruvate are the same, regardless of hexose type.

Many NADH-linked dehydrogenases have similar active sites. Which part of glyceraldehyde-3-phosphate dehydrogenase would be the most conserved between other enzymes?

The part of the active site that binds to NADH would be the part that is most conserved, since many dehydrogenases use that coenzyme.

If lactic acid is the buildup product of strenuous muscle activity, why is sodium lactate often given to hospital patients intravenously?

The problem with lactic acid is that it is an acid. The produced from lactic acid formation causes the burning muscle sensation. Sodium lactate is the conjugate weak base of lactic acid. It is reconverted to glucose by gluconeogenesis in the liver. Giving sodium lactate intravenously is a good way to supply an indirect source of blood glucose.

What is the metabolic purpose of lactic acid production?

The purpose of the step that produces lactic acid is to re duce pyruvate so that NADH can be oxidized to , which is needed for the step catalyzed by glyceraldehyde-3-phosphate dehydrogenase.

Is the reaction of 2-phosphoglycerate to phosphoenolpyruvate a redox reaction? Give the reason for your answer.

The reaction of 2-phosphoglycerate to phosphoenolpyruvate is a dehydration (loss of water) rather than a redox reaction.

Which reactions in glycolysis are coupled reactions?

The reactions catalyzed by hexokinase, phosphofructokinase, glyceraldehyde-3-phosphate dehydrogenase, phosphoglycerokinase, and pyruvate kinase.

The concentration of lactate in blood rises sharply during a sprint and declines slowly for about an hour afterward. What causes the rapid rise in lactate concentration? What causes the decline in lactate concentration after the run?

The sprint is essentially anaerobic and produces lactate from glucose by glycolysis. Lactate is then recycled to glucose by gluconeogenesis.

Which steps in glycolysis are physiologically irreversible?

The steps catalyzed by hexokinase, phosphofructokinase, and pyruvate kinase.

What is the metabolic advantage of having both hexokinase and glucokinase to phosphorylate glucose?

The two enzymes can have different tissue locations and kinetic parameters. The glucokinase has a higher Km for glucose than hexokinase. Thus, under conditions of low glucose, the liver does not convert glucose to glucose-6-phosphate, using the substrate that is needed elsewhere. When the glucose concentration is much higher, however, glucokinase helps phosphorylate glucose so that it can be stored as glycogen. review ntr notes

Does the net gain of ATP in glycolysis differ when glycogen, rather than glucose, is the starting material? If so, what is the change?

There is a net gain of three, rather than two, ATP when glycogen, not glucose, is the starting material of glycolysis.

What is the net gain of ATP molecules derived from the reactions of glycolysis?

There is a net gain of two ATP molecules per glucose molecule consumed in glycolysis.

Why is it essential that the mechanisms that activate glycogen synthesis also deactivate glycogen phosphorylase?

These two pathways occur in the same cellular compartment, and, if both are on at the same time, a futile ATP hydrolysis cycle results. Using the same mechanism to turn them on/off or off/on is highly efficient.

Briefly discuss the role of thiamine pyrophosphate in enzymatic reactions, using material from this chapter to illustrate your points.

Thiamine pyrophosphate is a coenzyme in the transfer of two-carbon units. It is required for catalysis by pyruvate decarboxylase in alcoholic fermentation.

Beriberi is a disease caused by a deficiency of vitamin (thiamine) in the diet. Thiamine is the precursor of thiamine pyrophosphate. In view of what you have learned in this chapter, why is it not surprising that alcoholics tend to develop this disease?

Thiamine pyrophosphate is a coenzyme required in the reaction catalyzed by pyruvate carboxylase. Because this reaction is a part of the metabolism of ethanol, less will be available to serve as a coenzyme in the reactions of other enzymes that require it.

Which steps of glycolysis are irreversible? What bearing does this observation have on the reactions in which gluconeogenesis differs from glycolysis?

Three reactions of glycolysis are irreversible under physiological conditions. They are the production of pyruvate and ATP from phosphoenolpyruvate, the production of fructose-1,6-bisphosphate from fructose-6-phosphate, and the production of glucose-6-phosphate from glucose. These reactions are bypassed in gluconeogenesis; the reactions of gluconeogenesis differ from those of glycolysis at these points and are catalyzed by different enzymes.

How does the cyclization of sugars introduce a new chiral center?

Two different orientations with respect to the sugar ring are possible for the hydroxyl group at the anomeric carbon. The two possibilities give rise to the new chiral center.

Most metabolic pathways are relatively long and appear to be very complex. For example, there are individual chemical reactions in glycolysis, converting glucose to pyruvate. Suggest a reason for the complexity.

With few exceptions, a biochemical reaction typically results in only one chemical modification of the substrate. Accordingly, several to many steps are needed to reach the ultimate goal.

What is a furanose?

a cyclic sugar- 5 membered ring similar to furan

What is a glycosidic bond?

acetal linkage that joins two sugars

Show that the reaction glucose--> 2 glyceraldehyde3phosphate is slightly endergonic ; that is, it is not too far from equilibrium. Use the data in Table 17.1. free energy=2.2 KJ/mol

add free energy standard for the reactions from glucose to glyceraldehyde 3 phosphate 2.5KJ/mol

Using Table 17.1, predict whether the following reaction is thermodynamically possible: phosphoenolpyruvate +Pi+ 2 ADP--> Pyruvate +2 ATP

add together phosphoenolpyruvate +ADP--> Pyruvate +ATP free energy = -31.4 KJ/mol ADP+Pi--> ATP free energy= 30.5 total= -.9 KJ/mol reaction is thermodynamically possible under standard conditions

What reaction types are catalyzed by aldolases?

aldol cleavage of a sugar to yield two smaller sugars or sugar derivatives

Name which, if any, of the following groups are not aldose-ketose pairs: D-ribose and D-ribulose, D-glucose and D-fructose, D-glyceraldehyde and dihydroxyacetone.

all groups are aldose-ketose pairs

The molecules α-D-glucose and β-D-glucose are examples of epimers enantiomers anomers Both enantiomers and anomers

anomers

What is the main structural difference between cellulose and starch?

both polymers of glucose cellulose: monomers are joined by beta glycosidic linkage starch- joined by alpha glycosidic linkage

What reaction types are catalyzed by isomerases?

changing the form of a molecule without changing its empirical formula (replacing one isomer with another)

What is a glycoprotein?

covalent bonding of sugars to protein

What is a pyranose?

cyclic sugar- 6 membered rings similar to pyran

Which enzyme cleaves the α(1 → 6) bonds in glycogen? glycogen phosphorylase phosphoglucomutase glycogen synthase debranching enzyme None of these because there are no α(1 → 6) bonds in glycogen.

debranching enzyme

What is the difference between an enantiomer and a diasteromer?

enantiomer- nonsuperimposable, mirror-image stereoisomers diastereomer- nonsuperimposable, nonmirrorimage stereoisomers

Following are Fischer projections for a group of five-carbon sugars, all of which are aldopentoses. Identify the pairs that are enantiomers and the pairs that are epimers. (The sugars shown here are not all of the possible five-carbon sugars.) 1) CHO 2) CHO 3)CHO H-C-OH H-C-OH H-C-OH H-C-OH HO-C-H H-C-OH H-C-OH HO-C-H HO-C-H CH2OH CH2OH CH2OH 4)CHO 5)CHO 6) CHO HO-C-H H-C-OH HO-C-H H-C-OH HO-C-H HO-C-H H-C-OH H-C-OH HO-C-H CH2OH CH2OH CH2OH

enantiomers: 1,6 2,4 epimers:2,5 3,1 3,2 1,4 1,5 2,6

What is the commited step in glycolysis?

formation of fructose -1,6- bisphosphate by phosphofructokinase (key allosteric control enzyme) - undergo reactions of glycolysis - components of the pathway up to this point can have other metabolic fates (G6P and F6P can feed into other pathways)

Draw a Haworth projection for the disaccharide gentibiose, given the following information: (a)It is a dimer of glucose. (b)The glycosidic linkage is beta 1-6 (c) The anomeric carbon not involved in the glycosidic linkage is in the configuration.

gentibiose 2 D glucose review notes c4 OH down on the right

The regulatory enzymes in gluconeogenesis are hexokinase, phosphofructokinase, and pyruvate kinase glucose-6-kinase, aldolase, and enolase pyruvate carboxylase, aldolase, and phosphofructokinase glucose-6-phosphatase, fructose-1,6-bisphosphatase, pyruvate carboxylase, and phosphoenolpyruvate carboxykinase

glucose-6-phosphatase, fructose-1,6-bisphosphatase, pyruvate carboxylase, and phosphoenolpyruvate carboxykinase

The compound uridine diphosphate glucose (UDPG) plays a role in glycogen breakdown glycogen synthesis glycolysis gluconeogenesis none of these

glycogen synthesis

In the Cori cycle lactic acid is transported from the liver to muscle by the blood. lactic acid is transported from the liver to the kidneys by the blood. glycolysis takes place in muscle and gluconeogenesis in the liver. glycolysis takes place in the liver and gluconeogenesis in muscle. none of these Quiz

glycolysis takes place in muscle and gluconeogenesis in the liver.

For the α anomer of a D-sugar, the anomeric hydroxyl in a Haworth projection has an upward projection (on the same side as the terminal CH2OH group). has a downward projection (on the opposite side from the terminal CH2OH group). may be either up or down, it depends on the individual sugar. is non-existent; anomers are a consideration only in Fischer projections.

has a downward projection (on the opposite side from the terminal CH2OH group).

In what way is the observed mode of action of hexokinase consistent with the induced-fit theory of enzyme action?

hexokinase molecule changes shape drastically on binding to substrate, consistent with the induced fit theory of an enzyme adapting itself to its substrate

What reaction types are catalyzed by kinases?

high-energy phosphate phosphorylates a substrate

The phosphorylation of glucose to glucose 6-phosphate is so strongly exergonic that it does not require a catalyst. is an exergonic reaction not coupled to any other reaction. is an endergonic reaction that takes place because it is coupled to the exergonic hydrolysis of ATP. is an exergonic reaction that is coupled to the endergonic hydrolysis of ATP.

is an endergonic reaction that takes place because it is coupled to the exergonic hydrolysis of ATP.

When glycogen synthase is phosphorylated its activity decreases its activity increases its activity is unaffected none of these because that enzyme doesn't get phosphorylated

its activity decreases

What is an oligosaccharide?

linking monosaccharides by glycosidic bonds

The fate of pyruvate produced during glycolysis depends primarily on the availability of NAD+ to keep the pathway going. ADP for conversion to ATP. coenzyme A for further metabolism of pyruvate. phosphoric acid for the synthesis of ATP. molecular oxygen.

molecular oxygen.

How many chiral centers are there in the open-chain form of glucose? In the cyclic form?

open chain: 4 (C2-C5) cyclic: 5 (including hemiacetal formation)

The step that commits the cell to metabolize glucose is catalyzed by hexokinase phosphoglucomutase aldolase phosphofructokinase

phosphofructokinase

What kind of reaction is used to release glucose units from glycogen? hydrolysis dehydration oxidation dehydrogenation phosphorolysis

phosphorolysis

What is a polysaccharide?

polymer of simple sugars -compounds that contain a single carbonyl group and several hydroxyl groups

13) Convert the following Haworth projections to a Fischer projection. Name the monosaccharides you have drawn.

review notes

16) Starting with the open-chain form of D-ribose, write equations for the cyclization reactions that form the pyranose and the furanose forms.

review notes - nucleophilic attack of OH (C4/C5) on the carbonyl carbon

What is the chemical difference between a sugar phosphate and a sugar involved in a glycosidic bond?

sugar phosphate- ester bond formed between one of the sugar OH and phosph. acic glycosidic bond- acetal, hydrolyzed to make the 2 original sugar OH

What is a ketose?

sugar with a ketone group

What is an aldose?

sugar with an aldehyde group

The primary function of the pentose phosphate pathway is to synthesize NAD+ and pentose phosphates. to synthesize NADPH and pentose phosphates. to produce NADH. to convert pentose phosphates to metabolic intermediates for oxidative phosphorylation.

to synthesize NADPH and pentose phosphates.

Hydrolysis of pyrophosphate is an important energy driving force in the synthesis of glycogen. True False

true

The ΔG values for glycolytic reactions at physiological conditions may be exergonic, even though the ΔG°′′ at "standard" conditions, may be endergonic. True False

true

Briefly describe "going for the burn" in a workout in terms of the material in this chapter.

"Going for the burn" in a workout refers to the sensation that accompanies lactic acid buildup. This in turn arises from anaerobic metabolism of glucose in muscle.

What is the effect on gluconeogenesis and glycogen synthesis of (a) increasing the level of ATP, (b) decreasing the concentration of fructose-1,6-bisphosphate, and (c) increasing the concentration of fructose-6-phosphate?

(a) Increasing the level of ATP favors both gluconeogenesis and glycogen synthesis. (b) Decreasing the level of fructose-1,6-bisphosphate would tend to stimulate glycolysis, rather than gluconeogenesis or glycogen synthesis. (c) Levels of fructose-6-phosphate do not have a marked regulatory effect on these pathways of carbohydrate metabolism.

How many possible epimers of D glucose exist?

4 (C2-C5) can be inverted

Why are furanoses and pyranoses the most common cyclic forms of sugars?

5C and 6C ring structure are most stable and readily form

What is a reducing sugar?

A reducing sugar is one that has a free aldehyde group. The aldehyde is easily oxidized, thus reducing the oxidizing agent.

How does the action of allosteric effectors differ in the reactions catalyzed by phosphofructokinase and fructose-1,6-bisphosphatase?

AMP and fructose-2,6-bisphosphate are allosteric activators of phosphofructokinase. They are allosteric inhibitors of fructose-1,6-bisphosphatase.

How does ATP act as an allosteric effector in the mode of action of phosphofructokinase?

ATP inhibits phosphofructokinase, consistent with the fact that ATP is produced by later reactions of glycolysis.

Discuss the logic of the nature of the allosteric inhibitors and activators of glycolysis. Why would these molecules be used?

ATP is an inhibitor of several steps of glycolysis as well as other catabolic pathways. The purpose of catabolic pathways is to produce energy, and high levels of ATP mean the cell already has sufficient energy. Glucose-6-phosphate inhibits hexokinase and is an example of product inhibition. If the glucose-6-phosphate level is high, it may indicate that sufficient glucose is available from glycogen breakdown or that the subsequent enzymatic steps of glycolysis are going slowly. Either way, there is no reason to produce more glucose-6-phosphate. Phosphofructokinase is inhibited by a special effector molecule, fructose-2,6-bisphosphate, whose levels are controlled by hormones. It is also inhibited by citrate, which indicates that there is sufficient energy from the citric acid cycle, probably from fat and amino acid degradation. Pyruvate kinase is also inhibited by acetyl-CoA, the presence of which indicates that fatty acids are being used to generate energy for the citric acid cycle. The main function of glycolysis is to feed carbon units to the citric acid cycle. When these carbon skeletons can come from other sources, glycolysis is inhibited to spare glucose for other purposes.

Explain the origin of the name of the enzyme aldolase.

Aldolase catalyzes the reverse aldol condensation of fructose-1,6- bisphosphate to glyceraldehyde-3-phosphate and dihydroxyacetone phosphate

Name two forms of control of enzymatic action. Which of the two is more important in control of glycogen breakdown?

Allosteric effects and covalent modification are two important forms of control of enzymatic action. Covalent modification plays a more important role than allosteric effects in glycogen breakdown

What is the difference between an isomerase and a mutase?

An isomerase is a general term for an enzyme that changes the form of a substrate without changing its empirical formula. A mutase is an enzyme of the isomerase class that moves a functional group, such as a phosphate, to a new location in a substrate molecule.

Most hunters know that meat from animals that have been run to death tastes sour. Suggest a reason for this observation.

Animals that have been run to death have accumulated large amounts of lactic acid in their muscle tissue, accounting for the sour taste of the meat.

Consider the structures of arabinose and ribose. Explain why nucleotide derivatives of arabinose, such as ara-C and ara-A, are effective metabolic poisons. D-Ribose D-Arabinose CHO CHO H-COH HO-CH HCOH HCOH HCOH HCOH CH2OH CH2OH

Arabinose is an epimer of ribose. Nucleosides in which arabinose is substituted for ribose act as inhibitors in reactions of ribonucleosides

Glycogen is highly branched. What advantage, if any, does this provide an animal?

Because of the branching, the glycogen molecule gives rise to a number of available glucose molecules at a time when it is being hydrolyzed to provide energy. A linear molecule could produce only one available glucose at a time.

What is the role of biotin in gluconeogenesis?

Biotin is the molecule to which carbon dioxide is attached to the process of being transferred to pyruvate. The reaction produces oxaloacetate, which then undergoes further reactions of gluconeogenesis.

Show the carbon atom that changes oxidation state during the reaction catalyzed by glyceraldehyde-3-phosphate dehydrogenase. What is the functional group that changes during the reaction?

Carbon-1 of glyceraldehyde is the aldehyde group. It changes oxidation state to a carboxylic acid, which is phosphorylated simultaneously.

What reaction types are catalyzed by dehydrogenases?

Changing the oxidation state of a substrate by removing hydrogens while simultaneously changing the oxidation state of a coenzyme (NADH, FADH2)

How does chitin differ from cellulose in structure and function?

Chitin is a polymer of N-acetyl beta D glucosamine, whereas cellulose is a polymer of D glucose. Both polymers play a structural role, but chitin occurs in the exoskeletons of invertebrates and cellulose primarily in plants.

Why is the polysaccharide chitin a suitable material for the exoskeleton of invertebrates such as lobsters? What other sort of material can play a similar role?

Chitin is a suitable material for the exoskeleton of invertebrates because of its mechanical strength. Individual polymer strands are cross-linked by hydrogen bonding, accounting for the strength. Cellulose is another polysaccharide cross-linked in the same way, and it can play a similar role.

What is the metabolic advantage in the conversion of glucose to lactate, in which there is no net oxidation or reduction?

Conversion of glucose to lactate rather than pyruvate recycles NADH.

Two sugars are epimers of each other. Is it possible to convert one to the other without breaking covalent bonds?

Converting a sugar to an epimer requires inversion of configuration at a chiral center. - break and reform covalent bonds

Name which, if any, of the following are epimers of D glucose: D mannose, D galactose, D ribose

D mannose and D galactose (6C) - inversion of configuration around C2 and C4 D ribose 5C

Explain how the minor structural difference between alpha and beta glucose is related to the differences in structure and function in the polymers formed from these two monomers.

Differences in structure: cellulose consists of linear fibers, but starch has a coil form. Differences in function: cellulose has a structural role, but starch is used for energy storage.

What is the source of the energy needed to incorporate glucose residues into glycogen? How is it used?

Each glucose residue added to a growing phosphate chain comes from uridine diphosphate glucose. The cleavage of the phosphate ester bond to the nucleoside diphosphate moiety supplies the needed energy.

Briefly outline the role of UDPG in glycogen biosynthesis.

Each glucose residue is added to the growing glycogen molecule by transfer from UDPG.

You are planning to go on a strenuous hike and are advised to eat plenty of high-carbohydrate foods, such as bread and pasta, for several days beforehand. Suggest a reason for the advice.

Eating high-carbohydrate foods for several days before strenuous activity is intended to build up glycogen stores in the body. Glycogen will be available to supply required energy.

Why would you expect to see that reactions of substrate cycles involve different enzymes for different directions?

Enzymes, like all catalysts, speed up the forward and reverse reaction to the same extent. Having different catalysts is the only way to ensure independent control over the rates of the forward and reverse process.

Why is it reasonable to expect that control can be exerted near the end of a pathway as well as near the beginning?

Exerting control at the end of a pathway is a good example of feedback control, so it is reasonable to expect it.

At what point in glycolysis are all the reactions considered doubled?

From the point at which aldolase splits fructose-1,6-bisphosphate into dihydroxyacetone phosphate and glyceraldehyde-3-phosphate; all reactions of the pathway are doubled (only the path from one glyceraldehyde-3-phosphate is usually shown).

How does fructose-2,6-bisphosphate play a role as an allosteric effector?

Fructose-2,6-bisphosphate is an allosteric activator of phosphofructokinase (a glycolytic enzyme) and an allosteric inhibitor of fructose-1,6-bisphosphatase (an enzyme in the pathway of gluconeogenesis).

Explain how fructose-2,6-bisphosphate can play a role in more than one metabolic pathway.

Fructose-2,6-bisphosphate is an allosteric activator of phosphofructokinase (a glycolytic enzyme) and an allosteric inhibitor of fructose-1,6-bisphosphatase (an enzyme in the pathway of gluconeogenesis). It thus plays a role in two pathways that are not exactly the reverse of each other.

The order of compounds in the conversion of glucose to pyruvic acid is as follows:(PEP = phosphoenolpyruvate) Fructose-bisphosphate, fructose-6-phosphate, 1,3-phosphoglyceric acid, 3-phosphoglyceric acid, PEP. Fructose-6-phosphate, fructose-bisphosphate, PEP, 1,3-phosphoglyceric acid, 3-phosphoglyceric acid. Fructose-6-phosphate, fructose-bisphosphate, 1,3-phosphoglyceric acid, 3-phosphoglyceric acid, PEP. Fructose-6-phosphate, fructose-bisphosphate, 3-phosphoglyceric acid, 1,3-phosphoglyceric acid, PEP. Fructose-bisphosphate, fructose-6-phosphate, 3-phosphoglyceric acid, 1,3-phosphoglyceric acid, PEP.

Fructose-6-phosphate, fructose-bisphosphate, 1,3-phosphoglyceric acid, 3-phosphoglyceric acid, PEP.

What roles do glucagon and epinephrine play in glycogen breakdown?

Glucagon and epinephrine start the chain of events leading to glycogen breakdown.

What are four possible metabolic fates of glucose-6-phosphate?

Glucose-6-phosphate can be converted to glucose (gluconeogenesis), glycogen, pentose phosphates (pentose phosphate pathway), or pyruvate (glycolysis).

High levels of glucose-6-phosphate inhibit glycolysis. If the concentration of glucose-6-phosphate decreases, activity is restored. Why?

Glucose-6-phosphate inhibits hexokinase, the enzyme responsible for its own formation. Because G-6-P is used up by additional reactions of glycolysis, the inhibition is relieved.

Why is it advantageous that breakdown of glycogen gives rise to glucose-6-phosphate rather than to glucose?

Glucose-6-phosphate is already phosphorylated. This saves one ATP equivalent in the early stages of glycolysis.

How does glycogen differ from starch in structure and function?

Glycogen and starch differ mainly in the degree of chain branching. Both polymers serve as vehicles for energy storage, glycogen in animals and starch in plants.

What is the main structural difference between glycogen and starch?

Glycogen exists as a highly branched polymer. Starch can have both a linear and a branched form, which is not as highly branched as that of glycogen.

How is it advantageous for animals to convert ingested starch to glucose and then to incorporate the glucose into glycogen?

Glycogen is more extensively branched than starch. It is a more useful storage form of glucose for animals because the glucose can be mobilized more easily when there is a need for energy.

What are glycoproteins? What are some of their biochemical roles?

Glycoproteins are ones in which carbohydrates are covalently bonded to proteins. They play a role in eukaryotic cell membranes, frequently as recognition sites for external molecules. Antibodies (immunoglobulins) are glycoproteins.

Suggest a reason why a different reducing agent (NADPH) is used in anabolic reactions rather than NADH, which plays a role in catabolic ones.

Having different reducing agents for anabolic and catabolic pathways keeps the pathways separate metabolically. Thus, they are subject to independent control and do not waste energy.

Why is it advantageous for two control mechanisms—allosteric control and covalent modification—to be involved in the metabolism of glycogen?

Having two control mechanisms allows for fine-tuning of control and for the possibility of amplification. Both mechanisms are capable of rapid response to conditions, milliseconds in the case of allosteric control and seconds to minutes in the case of covalent modification.

Blood samples for research or medical tests sometimes have heparin added. Why is this done?

Heparin is an anticoagulant. Its presence prevents blood clotting.

How do glucokinase and hexokinase differ in function?

Hexokinase can add a phosphate group to any of several six-carbon sugars, whereas glucokinase is specific for glucose. Glucokinase has a lower affinity for glucose than does hexokinase. Consequently, glucokinase tends to deal with an excess of glucose, particularly in the liver. Hexokinase is the usual enzyme for phosphorylating six-carbon sugars.

Which molecules act as inhibitors of glycolysis? Which molecules act as activators?

Hexokinase is inhibited by glucose-6-phosphate. Phosphofructokinase is inhibited by ATP and citrate. Pyruvate kinase is inhibited by ATP, acetyl-CoA, and alanine. Phosphofructokinase is stimulated by AMP and fructose-2,6-bisphosphate. Pyruvate kinase is stimulated by AMP and fructose-1,6-bisphosphate.