Chapter 18

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..

Pyridoxal phosphate is a versatile coenzyme as it is capable of forming a stable ________ base between an amino acid substrate and the coenzyme.

Schiff

Transamination reactions have equilibrium constants close to unity. What does this mean? 1) In the absence of any other forces 2) Intracellular conditions 3) The forward reaction is 4) Aminotransferases can alter

Statements 1 and 2 are accurate.

Antimetabolites are clinically useful for which of the following reasons? 1) They interfere 2) They can interfere 3) They can block

Statements 1, 2, and 3 are correct.

The effects of folate deficiencies are manifested in which of the following ways? 1) elevated levels of homocysteine 2) the presence of reduced amounts of erythrocytes 3) a decrease in the synthesis of vitamin B12 4) neural tube defects

Statements 1, 2, and 4 are correct.

T or F The oxidation of branched chain amino acids shares a similar chemical strategy with β-oxidation of fatty acids.

True

T or F With appropriate nutrition, animals maintain nitrogen intake and excretion at equivalent rates.

True

Which folate structure (from the list below) Is the substrate for the enzyme that is inhibited by methotrexate and trimethoprim?

B, C, D, E, methionine, A, F, C

Place the following sequence of events for a transamination reaction that requires pyridoxal phosphate in the correct order:

(2-1-4)-3-5

Starting with carbamoyl phosphate, place the sequence of events for the urea cycle in the correct order:

(4-3)-1-2

What histidine metabolite would you expect to accumulate in a folate- or B12- deficient patient, and why?

(Formiminoglutamate), because the next reaction in its catabolism requires (tetrahydrofolate)

Why is phenylketonuria resulting from dihydropteridine reductase deficiency a more serious disorder than PKU resulting from phenylalanine hydroxylase deficiency?

Because a pteridine reductase deficiency...

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?

-Humans obtain phenylalanine from their diet. -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 B) DRAW

Which of the following molecules make up naturally occurring folates? 1) p-aminobenzoic acid 2) a tail of aspartate 3) a bicyclic, heterocyclic pteridine ring 4) a tail of glutamate residues

1, 3, and 4

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?

A) 12.5 ATPS B) All these pathways would generate a little (more) C) 33.5 ATPS D) 35.5 ATPS

Methyl-labeled [14C]methionine...

A) 25 % B) 75% C) The rate& The specific radioactivity

Most bacterial mutants that require isoleucine for growth also require valine.

A) Because the same enzymes.. B)Threonine dehydratase

The structure shown above is an intermediate in the synthesis of which biogenic amine?

A) Histamine B) H+, H2O

Consider the following questions about glutamate dehydrogenase.

A) NH3 has unshared electron pair B) 5.9 x10^-3 C) NAD+/NADH

Ammonia is a toxic byproduct of protein metabolism. To get rid of it, the body converts it to water-soluble urea through a process called the urea cycle.

A) Reactants: NH4, H20, 2ATP, CO2 Products: H2N(CO)OPO3^2-, Pi, 2ADP B) Ornithine transcarbamolyase-> argininosuccinate synthetase-> argininosuccinate -> arginase

Indicate whether each of the following statements is true or false, and briefly explain your answer.

A) True B) False C) False D) False

Part complete What one additional substrate or cofactor is required by enzyme B?

A)5-Methyl(tetra)hydrofolate B) Methylmalonate C)Decreased affinity of enzyme C for B12 coenzyme D) homocysteine

T or F Nitrate can be reduced to ammonia by virtually all organisms.

False

T or F Nitrogen fixation requires the hydrolysis of 8 ATPs to produce 2 NH3 molecules from 1 N2.

False

T or F The H2S produced by metabolism of cysteine is a waste product with no physiological or biochemical function.

False

T or F The Krebs-Henseleit urea cycle takes place in liver cells entirely within the mitochondrion.

False

T or F Tyrosine is the precursor of serotonin which has a number of roles in the nervous system.

False

Which of the following enzymes is NOT involved in the conversion of NH3 to an organic nitrogen-containing compound?

Glutamate-oxaloacetate transaminase

Which of the following amino acids cannot be used to provide an intermediate of the citric acid cycle?

Lysine

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.

Which of the following statements about the ubiquitination of proteins are true? 1) Ubiquitin is a relatively smal. 2) It is covalently attached 3) A carboxyl group of ubiquitin forms amide bonds 4) Target selection

Only statements 1, 2, and 4 are correct.

Explain the basis for the following statement: As a coenzyme, pyridoxal phosphate is covalently bound to enzymes with which it functions,

PLP...

Which of the following statements about glutamate dehydrogenase is NOT true?

The enzyme is under allosteric control

T or F Arginine and methionine can be synthesized by mammals but are generally classed as essential amino acids.

True

Which of the following amino acids is both glucogenic and ketogenic?

Tryptophan

Tetrahydrofolate is a coenzyme involved in the mobilization and utilization of single carbon functional units in the:

all of the above.

In the degradation of amino acids in muscle NH4+ is carried to the liver for conversion into urea by:

alanine.

Formaldehyde reacts nonenzymatically with tetrahydrofolate to generate 5,10-methylenetetrahydrofolate. [14C]Formaldehyde can be used to prepare serine labeled in the β-carbon. (check all that apply)

glycine, serine hydroxymethyltransferase, tetrahydrofolate

N2 gas can be converted to NH3 by:

legumes.

All amino acids can be synthesized from intermediates of the:

the glycolytic pathway, the pentose phosphate pathway and citric acid cycle.

The proteasome is a large multisubunit ATP-dependent protease that degrades proteins that have been modified by the attachment of ________.

ubiquitin