Chapter 18
If oxidation of acetyl-CoA yields 10 ATPs per mole through the citric acid cycle, how many ATPs will be derived from the complete metabolic oxidation of 1 mole of alanine in a mammal?
12.5 ATP
Place the following sequence of events for a transamination reaction that requires pyridoxal phosphate in the correct order: 1) deprotonation by lysine at the α-carbon of the amino acid tethered to pyridoxal phosphate 2) exchange of the amino acid that makes the Schiff base, releasing free lysine 3) hydrolysis via a cabinolamine intermediate, yielding an α-ketoacid and pyridoxamine phosphate 4) reprotonation at the α-carbon, resulting in tautomerization of the imine carbon atom 5) a second α-ketoacid reacting with pyridoxamine to provide a new amino acid 2-1-3-4-5 1-2-4-5-3 2-1-4-3-5 2-4-1-5-3
2-1-4-3-5
Calculate the proportions of phosphatidylcholine synthesized by the phosphatidylserine pathway Calculate the proportions of phosphatidylcholine synthesized by the pathway starting from free choline.
25 % 75 %
How much energy would be derived from the metabolic oxidation of 1 mole of isoleucine to CO2, H2O, and NH3? How much energy would be derived from the metabolic oxidation of 1 mole of tyrosine to CO2, H2O, and NH3?
33.5 35.5
What one additional substrate or cofactor is required by enzyme B? methenyltetrahydrofolate cyclohydrolase 5-Methyldihydrofolate 10-formyltetrahydrofolate 5-Methyltetrahydrofolate
5-Methyltetrahydrofolate
Glutamate dehydrogenase has a KM for ammonia (NH3) of ~1 mM. However, at physiological pH the dominant ionic species is ammonium ion, NH4+ (pKa=9.2). Calculate the velocity (as a fraction of Vmax) that would be achieved by glutamate dehydrogenase if the total intracellular ammonia concentration (NH3+NH4+) is 100 μM (approximate physiological concentration). Assume a mitochondrial matrix pH of 8.0
5.90×10−3
Most bacterial mutants that require isoleucine for growth also require valine. Why? Because the same enzymes are involved in comparable steps of both isoleucine and valine biosynthesis. Because isoleucine and valine are interchangeable in the process of bacterial growth. Because the valine biosynthesis increases some spets of isoleucine biosynthesis.
Because the same enzymes are involved in comparable steps of both isoleucine and valine biosynthesis
Some forms of the condition described in Part B can be successfully treated by injection of rather massive doses of vitamin B12. What kind of genetic alteration in the enzyme would be consistent with this result? Some forms of Parkinsonism. Phenylketonuria (PKU). Decreased affinity of enzyme C for B12 coenzyme (KM mutant) Deficiency of cystathionine β-synthase.
Decreased affinity of enzyme C for B12 coenzyme (KM mutant)
Choose necessary reactants for getting this intermediate. Check all that apply. H+ NH3 CO2 H2O
H+ and H2O
Put the enzymes in the urea cycle in the correct order, from first to last. argininosuccinase arginase arginosuccinate syntetase ornithine transcarbomolyase
Order... ornithine transcarbomolyase arginosuccinate syntetase argininosuccinase arginase
Nitrifying bacteria convert _____ to _____. nitrogen gas ... ammonium nitrogen gas ... nitrates ammonium ... nitrites nitrates ... nitrogen gas ammonium ... nitrogen gas
ammonium ... nitrites
N2 gas can be converted to NH3 by: all plants. legumes. all bacteria. most organisms. mammals.
legumes
What coenzyme is involved? S-Adenosylmethionine methylcobalamin tetrahydrofolate (THF) pyridoxal phosphate
methylcobalamin
Denitrifying bacteria convert _____ to _____. nitrogen gas ... nitrites nitrates ... nitrogen gas nitrogen gas ... ammonium ammonium ... nitrogen gas nitrogen gas ... nitrates
nitrates ... nitrogen gas
Formaldehyde reacts nonenzymatically with tetrahydrofolate to generate 5,10-methylenetetrahydrofolate. [14C]Formaldehyde can be used to prepare serine labeled in the β-carbon. What else would be needed? Check all that apply. serine hydroxymethyltransferase tetrahydrofolate serine S-Adenosyl-L-methionine glycine
serine hydroxymethyltransferase tetrahydrofolate glycine
All amino acids can be synthesized from intermediates of the: glycolytic pathway alone. the glycolytic pathway, the pentose phosphate pathway and citric acid cycle. glycolytic pathway and the pentose phosphate pathway. the glycolytic pathway and citric acid cycle. citric acid cycle alone.
the glycolytic pathway, the pentose phosphate pathway and citric acid cycle.
Part complete The proteasome is a large multisubunit ATP-dependent protease that degrades proteins that have been modified by the attachment of ________.
ubiquitin
Part A Part complete Which of the following amino acids is both glucogenic and ketogenic? Arginine Cysteine Lysine Leucine Tryptophan
Tryptophan
Transamination reactions have equilibrium constants close to unity. What does this mean? 1) In the absence of any other forces, the ratio of products to starting materials is nearly one. 2) Intracellular conditions can dictate the direction the equilibrium favors. 3) The forward reaction is not favored in cells. 4) Aminotransferases can alter the position of the equilibrium.
1,2
Antimetabolites are clinically useful for which of the following reasons? 1) They interfere with the utilization of the natural substrate (the metabolite) because they are mimics of it. 2) They can interfere with the biosynthesis of thymidine and then DNA synthesis. 3) They can block proliferation of cells, especially cancer cells. 4) They can allosterically alter the function of dihydrofolate reductase.
1,2,3
Part A Part complete The effects of folate deficiencies are manifested in which of the following ways? 1) elevated levels of homocysteine that are correlated with an increased incidence of heart attack 2) the presence of reduced amounts of erythrocytes that are immature and enlarged, a condition known as megaloblastic anemia 3) a decrease in the synthesis of vitamin B12 4) neural tube defects in early stages of pregnancy
1,2,4
Which of the following statements about the ubiquitination of proteins are true?
1,2,4
Which of the following molecules make up naturally occurring folates? 1) p-aminobenzoic acid 2) a tail of aspartate residues ranging from 3 to 8 residues 3) a bicyclic, heterocyclic pteridine ring 4) a tail of glutamate residues ranging from 3 to 8 residues
1,3,4
Using the principles described in the text regarding pyridoxal phosphate mechanisms, propose a schema for the reaction catalyzed by serine hydroxymethyltransferase.
1. Displacement → 2. Deprotonation → 3. Reprotonation → 4. Hydrolysis → 5. Second reaction
Assimilation is indicated by the letter(s) _____. Nitrogen-fixing bacteria is(are) indicated by the letter(s) _____. Nitrification is indicated by the letter(s) _____.
A DE B
Essential amino acids must be obtained from the diet, whereas nonessential amino acids can be synthesized in the body. Which of the following statements about essential and nonessential amino acids are true? Serine cannot be synthesized in the human body. About half of the 20 amino acids must be acquired from food. Meat and eggs have incomplete amino acids and must be eaten with complementary sources of protein. Many of the nonessential amino acids are derived from other amino acids. Nonessential amino acids are synthesized by the human body. Humans obtain phenylalanine from their diet. The synthesis of essential amino acids is generally completed in fewer steps than for nonessential amino acids.
About half of the 20 amino acids must be acquired from food. Many of the nonessential amino acids are derived from other amino acids. Nonessential amino acids are synthesized by the human body. Humans obtain phenylalanine from their diet.
Tetrahydrofolate is a coenzyme involved in the mobilization and utilizatiTetrahydrofolate is a coenzyme involved in the mobilization and utilization of single carbon functional units in the: biosynthesis of thymine. biosynthesis of formylmethionyl-tRNA. metabolism of serine, glycine, methionine, and histidine. biosynthesis of purine nucleotides. all of the above. on of single carbon functional units in the:
All of the above.
Would the corresponding energy yield in a fish be higher or lower? Why? All of these pathways would generate a little more energy in a fish because it is not necessary to consume ATP in converting ammonia to urea for excretion. All of these pathways would generate a little less energy in a fish because besides to consuming ATP in converting ammonia to urea for excretion, also it is necessary to consider the energy of protonation at each stage of transformation. It is mpossible to determine, because the amount of generated energy depends on the environment of the fish.
All of these pathways would generate a little more energy in a fish because it is not necessary to consume ATP in converting ammonia to urea for excretion.
Which folate structure (from the list below)
BCDEAFC methionine
Why is phenylketonuria resulting from dihydropteridine reductase deficiency a more serious disorder than PKU resulting from phenylalanine hydroxylase deficiency? Because a pteridine reductase deficiency would impair all tetrahydrobiopterin-dependent reactions. Because phenylketonuria is a terminal disease. Because phenylalanine hydroxylase deficiency can be compensated with dihydropteridine reductase or other compound.
Because a pteridine reductase deficiency would impair all tetrahydrobiopterin-dependent reactions.
Deprotonation Hydrolysis Second Reaction Displacement Reprotonation
Displacement Deprotonation Reprotonation Hydrolysis Second Reaction
Nitrate can be reduced to ammonia by virtually all organisms.
Fale
Nitrogen fixation requires the hydrolysis of 8 ATPs to produce 2 NH3 molecules from 1 N2.
False
The H2S produced by metabolism of cysteine is a waste product with no physiological or biochemical function. True False
False
The Krebs-Henseleit urea cycle takes place in liver cells entirely within the mitochondrion. True False
False
Tyrosine is the precursor of serotonin which has a number of roles in the nervous system. True False
False
Given that the nitrogen of glutamate can be redistributed by transamination, glutamate should be a good supplement for nutritionally poor proteins. False. Glutamate can be used to synthesize essential amino acids only if the carbon skeletons are available as keto acids. True. Glutamate will be a good supplement, because transamination reactions run with quaternary ammonium compound formation.
False. Glutamate can be used to synthesize essential amino acids only if the carbon skeletons are available as keto acids.
What histidine metabolite would you expect to accumulate in a folate- or B12- deficient patient, and why? 10-formyltetrahydrofolate synthetase, because the next reaction in its catabolism requires formate. Formiminoglutamate, because the next reaction in its catabolism requires glutamate. Formiminoglutamate, because the next reaction in its catabolism requires tetrahydrofolate.
Formiminoglutamate, because the next reaction in its catabolism requires tetrahydrofolate.
Which of the following enzymes is NOT involved in the conversion of NH3 to an organic nitrogen-containing compound? Carbamoyl phosphate dehydrogenase Glutamate-oxaloacetate transaminase Glutamine synthetase Glutamate dehydrogenase Asparagine synthetase
Glutamate-oxaloacetate transaminase
The structure shown above is an intermediate in the synthesis of which biogenic amine? Spell out the full name of the compound.
Histamine
Which of the following amino acids cannot be used to provide an intermediate of the citric acid cycle? Phenylalanine Proline Tyrosine Histidine Lysine
Lysine
Genetic deficiency in animals of enzyme C would result in excessive urinary excretion of what compound? Methylmalonate Cystathionine β-synthase S-Adenosyl-L-methionine 5,10-methylenetetrahydrofolate reductase
Methylmalonate
Since all organs degrade amino acids, which leads to the production of ammonia, which of the following statements regarding the fate of ammonia is NOT correct? Most tissues convert ammonia directly to urea. In the liver, alanine is converted to pyruvate at the same time α-ketoglutarate is converted to Gln; the Gln is deaminated to Glu, and the resulting NH4 is converted to urea. In muscle, glutamate is converted to α-ketoglutarate acid while also producing alanine, which is used as the ammonia carrier to the liver. Some tissues use glutamine as the carrier form of ammonia.
Most tissues convert ammonia directly to urea.
The reaction above has NH3 as a reactant, instead of NH4+, which is far more abundant at physiological pH. Why is NH3 preferred? NH3 has an unshared electron pair that can initiate nucleophilic attack on the electron-poor carbonyl carbon atom. NH+4 occupies much more space due to the extra hydrogen atom which prevents interaction between nitrogen and carbon atoms. NH3 is stronger nucleophile than NH+4, so nucleophilic attack will be originated faster.
NH3 has an unshared electron pair that can initiate nucleophilic attack on the electron-poor carbonyl carbon atom.
Which one of these is a nitrite? NO2 - NO3 - PO4 - NH2 NH4 -
NO2 -
Which one of these is a nitrate? NO2 - NO3 - NH4 - SH NH2
NO3 -
_____ removes nitrogen from the atmosphere. Denitrification Nitrification Mineralization Nitrogen fixation Assimilation
Nitrogen fixation
Explain the basis for the following statement: As a coenzyme, pyridoxal phosphate is covalently bound to enzymes with which it functions, yet during catalysis the coenzyme is not covalently bound. The ability of PLP to form an unstable Schiff base is the key to its versatility in enzyme catalyzed reactions.The planarity of its structure results in a large conjugated π molecular orbital system, which is essential for catalysis. PLP forms a covalent Schiff base between the aldehyde carbon of the coenzyme and an ϵ-amino group of a lysine residue in the active site of the enzyme. This bond must be broken for the coenzyme to form a Schiff base with an amino acid substrate. All of the known reactions of PLP enzymes can be described mechanistically in the same way: formation of a planar Schiff base or aldimine intermediate, followed by bond cleavage and formation of a resonance-stabilized carbocation with an amino acid substrate.
PLP forms a covalent Schiff base between the aldehyde carbon of the coenzyme and an ϵ-amino group of a lysine residue in the active site of the enzyme. This bond must be broken for the coenzyme to form a Schiff base with an amino acid substrate.
Nitrogen is a limited resource, despite the presence of strong demand. Which of the following is NOT a metabolic consequence of the inability to store nitrogen? Proteolysis is a non-selective, exergonic process. Limited proteolysis has wide-ranging effects, including a role in signaling apoptosis, otherwise known as programmed cell death. Nitrogen must be continually replenished through diet. Amino acids from proteins that are turned over are reused in the synthesis of other proteins.
Proteolysis is a non-selective, exergonic process.
Identify the reactants and the products of the preparation step to the urea cycle. 2 ADP h2N(CO)OPO32- carboma... 2 ATP Pi NH4+ CO2 H2O
Reactants: 2ATP, H2O, NH4+, CO2 Products 2ADP Pi H2N(CO)....
Pyridoxal phosphate is a versatile coenzyme as it is capable of forming a stable ________ base between an amino acid substrate and the coenzyme.
Schiff
The thermodynamic equilibrium for the reaction greatly favors α-ketoglutarate reduction, yet in mitochondria the enzyme acts primarily to oxidize glutamate to α-ketoglutarate. Explain. The direction of a reaction depends on the concentrations of reactants. NH3 constantly increasing, the forward reaction is more preferable. The direction of a reaction depends on the equilibrium constant. In mitochondria the value of the equilibrium constant greatly differs from the standard conditions, so the reaction drives toward α-ketoglutarate. The direction of a reaction depends both on the equilibrium constant and the concentrations of reactants and products. The intramitochondrial [NAD+]/[NADH] ratio is high, and this drives the reaction toward α-ketoglutarate.
The direction of a reaction depends both on the equilibrium constant and the concentrations of reactants and products. The intramitochondrial [NAD+]/[NADH] ratio is high, and this drives the reaction toward α-ketoglutarate.
Part A Part complete Which of the following statements about glutamate dehydrogenase is NOT true? The reaction it catalyzes is readily reversible. Low energy charge activates the pathway. The enzyme is under allosteric control in that ATP stimulates its activity, whereas ADP inhibits its function. The reaction it catalyzes generates feedstocks for the citric acid cycle and electrons that can enter the mitochondrial respiratory chain.
The enzyme is under allosteric control in that ATP stimulates its activity, whereas ADP inhibits its function.
What further information would you need for your calculated values to reflect the true rates of these processes? Check all that apply. Radioactive decay constant for a particular intermediate. The half-life of the final intermediate in each pathway. The rate of degradation of the product. The specific radioactivity of the final intermediate in each pathway.
The rate of degradation of the product. The specific radioactivity of the final intermediate in each pathway.
Which enzyme or reaction would be defective in a mutant requiring only isoleucine (not valine) for growth? Threonine dehydratase Serine-threonine dehydratase Pyridoxal phosphate L-serine
Threonine dehydratase
Determine whether each reaction represents a transamination or an oxidative deamination. Glutamate aminotransferase catalyzes a reaction. Alanine dehydrogenase, which requires a coenzyme, catalyzes a reaction. An amino group in glutamate is removed as an ammonium ion. Glutamate aminotransferase catalyzes a reaction.
Transaminination Aspartic acid is transferred from an amino acid to a keto acid. Glutamate aminotransferase catalyzes a reaction. Oxidatitive deamination Alanine dehydrogenase, which requires a coenzyme, catalyzes a reaction. An amino group in glutamate is removed as an ammonium ion.
Arginine and methionine can be synthesized by mammals but are generally classed as essential amino acids. True False
True
In general, the metabolic oxidation of protein in mammals is less efficient, in terms of energy conserved, than the metabolic oxidation of carbohydrate or fat. True.The complete catabolism of amino acids yields CO2, H2O, and ammonia. However, the ammonia must be converted to urea for detoxification and excretion, and this requires ATP, which decreases the net ATP yield. False. The complete metabolic oxidation of carbohydrate or fat requires at least 25 ATP for the Kreb's cycle, which decreases the net ATP yield.
True
The oxidation of branched chain amino acids shares a similar chemical strategy with β-oxidation of fatty acids. True False
True
With appropriate nutrition, animals maintain nitrogen intake and excretion at equivalent rates. True False
True
In the degradation of amino acids in muscle NH4+ is carried to the liver for conversion into urea by: either alanine or glutamine. any amino acid. glutamine. asparagine. alanine.
alanine.
Genetic deficiency in animals of enzyme B will result in excessive urinary excretion of what amino acid? Spell out the full name of the amino acid.
homocysteine
