Question 1

Which of these is not a protease that acts in the small intestine? A) Chymotrypsin B) Elastase C) Enteropeptidase D) Secretin E) Trypsin

Accepted Answer

Secretin

Question 2

In the digestion of protein that occurs in the small intestine, which enzyme is critical in the activation of zymogens?

Accepted Answer

Enteropeptidase

Question 3

Which of the following is a zymogen that can be converted to an endopeptidase that hydrolyzes peptide bonds adjacent to Lys and Arg residues?

Accepted Answer

Trypsinogen

Question 4

In amino acid catabolism, the first reaction for many amino acids is a(n):

Accepted Answer

transamination requiring pyridoxal phosphate (PLP).

Question 5

The coenzyme required for all transaminations is derived from:

Accepted Answer

pyridoxine (vitamin B6).

Question 6

The coenzyme involved in a transaminase reaction is:

Accepted Answer

pyridoxal phosphate (PLP)

Question 7

Transamination from alanine to ?-ketoglutarate requires the coenzyme:

Accepted Answer

pyridoxal phosphate (PLP)

Question 8

Pyridoxal phosphate is a cofactor in this class of reactions:

Accepted Answer

transamination

Question 9

Which of the following is not true of the reaction catalyzed by glutamate dehydrogenase? A) It is similar to transamination in that it involves the coenzyme pyridoxal phosphate (PLP). B) NH4+ is produced. C) The enzyme can use either NAD+ or NADP+ as a cofactor. D) The enzyme is glutamate-specific, but the reaction is involved in oxidizing other amino acids. E) ?-Ketoglutarate is produced from an amino acid.

Accepted Answer

A) It is similar to transamination in that it involves the coenzyme pyridoxal phosphate (PLP).

Question 10

Glutamate is metabolically converted to ?-ketoglutarate and NH4+ by a process described as:

Accepted Answer

oxidative deamination.

Question 11

The conversion of glutamate to an ?-ketoacid and NH4+:

Accepted Answer

is catalyzed by glutamate dehydrogenase.

Question 12

Which of the following conversions require more than one step?1. Alanine ? pyruvate2. Aspartate ? oxaloacetate3. Glutamate ? ??ketoglutarate4. Phenylalanine ? hydroxyphenylpyruvate 5. Proline ? glutamate

Accepted Answer

4 & 5

Question 13

Urea synthesis in mammals takes place primarily in tissues of the:

Accepted Answer

Liver

Question 14

Which substance is not involved in the production of urea from NH4+ via the urea cycle? A) Aspartate B) ATPC) Carbamoyl phosphate D) Malate E) Ornithine

Accepted Answer

Malate

Question 15

Which of these directly donates a nitrogen atom for the formation of urea during the urea cycle?

Accepted Answer

Aspartate

Question 16

Conversion of ornithine to citrulline is a step in the synthesis of:

Accepted Answer

Urea

Question 17

In the urea cycle, ornithine transcarbamoylase catalyzes:

Accepted Answer

formation of citrulline from ornithine and another reactant.

Question 18

Which of the following statements is false in reference to the mammalian synthesis of urea?

Accepted Answer

The process of urea production is an energy-yielding series of reactions.

Question 19

Which of the following amino acids are essential for humans?

Accepted Answer

Threonine

Question 20

If a person's urine contains unusually high concentrations of urea, which one of the following diets has he or she probably been eating recently?

Accepted Answer

Very low carbohydrate, very high protein

Question 21

Which of these amino acids can be directly converted into a citric acid cycle intermediate by transamination?

Accepted Answer

glutamic acid

Question 22

Which of these amino acids are both ketogenic and glucogenic?1. Isoleucine2. Valine3. Histidine4. Arginine5. Tyrosine A) 1 and 5 B) 1, 3, and 5 C) 2 and 4 D) 2, 3, and 4 E) 2, 4, and 5

Accepted Answer

Isoleucine & Tyrosine

Question 23

Tetrahydrofolate (THF) and its derivatives shuttle between different substrates.

Accepted Answer

one carbon units

Question 24

The amino acids serine, alanine, and cysteine can be catabolized to yield:

Accepted Answer

pyr

Question 25

serine or cysteine may enter the citric acid cycle as acetyl-CoA after conversion to:

Accepted Answer

pyr

Question 26

The human genetic disease phenylketonuria (PKU) can result from:

Accepted Answer

inability to convert phenylalanine to tyrosine.

Question 27

In the human genetic disease maple syrup urine disease, the metabolic defect involves:

Accepted Answer

oxidative decarboxylation.

Question 28

Describe ____ fx in in protein digestion.  - gastrin - pepsinogen - cholecystokinin - enteropeptidase

Accepted Answer

Gastrin - The hormone gastrin stimulates secretion of HCl and pepsinogen into the stomachPepsinogen - as pepsin, begins protein degradation in the stomachCCK - stimulates the secretion of the pancreatic zymogens chymotrypsinogen, trypsinogen, and procarboxypeptidase into the small intestineEnteropeptidase - proteolytic enzyme that activates trypsinogen in the small intestine. Trypsin in turn activates chymotripsinogen, procarboxypeptidases, proelastase, and trypsinogen itself.

Question 29

In the treatment of diabetes, insulin is given intravenously. Why can't this hormone, a small protein, be taken orally?

Accepted Answer

First, the biological activity of insulin would be destroyed by the low pH of the gastric juice and the proteases that act in the small intestine. Furthermore, even if insulin escaped degradation in the intestine, it would not enter the bloodstream from the intestine; the transport systems in the cells that line the intestinal lumen transport free amino acids, not intact proteins.

Question 30

Define zymogen and describe the role of one zymogen in protein digestion.

Accepted Answer

inactive form of E, a* by proteolytic cleavage. a* by PC after release into SI - pepsinogen - chymotrypsinogen - trypsinogen  - procarboxypeptidases (A & B)

Question 31

Transamination rxs are catalyzed by a family of enzymes, all of which require __________ as a coenzyme. In the first step of a transamination, the coenzyme in the aldehyde form condenses with the _________ group of an amino acid to form a(n) _________.

Accepted Answer

pyridoxal phosphate (PLP)?-aminoSchiff base (or imine or aldimine)

Question 32

Give the name and draw the structure of the ?-keto acid resulting when the following amino acids undergo transamination with ?-ketoglutarate: (a) aspartate; (b) alanine.

Accepted Answer

Ans: (a) oxaloacetate; (b) pyruvate; see Fig. 18-4, p. 677.

Question 33

Describe, by showing the chemical intermediates, the role of pyridoxal phosphate (PLP) in the transamination of an amino acid.

Accepted Answer

Ans: See the yellow screened portion of Fig. 18-6, p. 679.

Question 34

Describe the roles of glutamine synthetase and glutaminase in the metabolism of amino groups in mammals.

Accepted Answer

in tissues metabolizing carbon skeletons of AAs, the amino groups are transferred by transamination to glutamate, then released as ammonia. Glutamine Synthetase - uses ATP to combine Ammonia (toxic) + glutamate ? glutamineGlutamine moves from extrahepatic tissues ? liver & kidneysGluatminase - releases the amino group from glutamine ? glutamate + ammonia ammonia in the liver is converted to urea, then excreted.

Question 35

Show the reaction in which ammonia is formed from glutamate; include any required cofactors.

Accepted Answer

this is the reaction catalyzed by glutamate dehydrogenase; see Fig. 18-7, p. 680.

Question 36

Describe the reactions and the role of the glucose-alanine cycle.

Accepted Answer

Toxic ammonia dt AA catabolism in musc is tp'd to liver as alanine (nontoxic) In muscle: Transamination of pyr ? alanine.Glutamate Dehydrogenase forms glutamate as the amino donor. In liver: Transamination of alanine ? pyr (its amino group eventually converted to urea, the pyruvate undergoes glucneogenesis and the glucose goes to muscle)

Question 37

Why does a mammal go to all of the trouble of making urea from ammonia rather than simply excreting ammonia as many bacteria do?

Accepted Answer

bacteria rel ammonia into surrounding medium ? diluted to nontoxic levels. ammonia prod by AA catabolism in mammals cannot be sufficiently diluted in tissues/blood to avoid accumulating at toxic levels. Urea is much less toxic than ammonia.

Question 38

Describe the three general mechanisms for disposing of excess nitrogen obtained in the diet. Which organisms use each mechanism?

Accepted Answer

(1) Ammonotelic: release into the surrounding medium as NH4+ (bacteria and many marine organisms)(2) Uricotelic: production of uric acid (birds and reptiles)(3) Ureotelic: production and excretion of urea (land-dwelling animals).

Question 39

Amino acid catabolism involves the breakdown of 20 amino acids all of which contain nitrogen but have different carbon skeletons. What overall strategy is used to deal with this problem? Illustrate the strategy with two examples.

Accepted Answer

Nitrogen is removed by transamination to glutamate. This converts the amino acid to an ?-keto acid that either is an intermediate in carbohydrate catabolism or is converted to one. (See Fig. 18-2, p. 675.) Examples are shown in Fig. 18-10, p. 683.

Question 40

During starvation, more urea production occurs. Explain this observation (in 50 words or less).

Accepted Answer

During starvation, cellular proteins are degraded and their carbon skeletons are oxidized for energy. The first step in amino acid catabolism is removal of the amino groups, which are ultimately excreted as urea.

Question 41

Describe (a) the fundamental nutritional problem faced by individuals with genetic defects in enzymes involved in urea formation and (b) two approaches to treatment of these diseases.

Accepted Answer

Ans: (a) A defect in urea synthesis can result in the formation of toxic blood levels of ammonia from the breakdown of ingested proteins. Thus, it is desirable to limit the intake of amino acids. However, some amino acids are essential for humans (i.e., not biosynthesized) and hence must be ingested in adequate amounts. (b) One approach is to administer compounds that deplete the supply of glycine and glutamine. The replenishment of these amino acids removes ammonia from the blood. Another approach is to administer compounds that allow the liver to bypass the enzyme that is defective in the individual.

Question 42

Name four amino acids that can be converted directly (in one step) into pyruvate or a citric acid cycle intermediate, and name the intermediate formed from each.

Accepted Answer

aspartate ? oxaloacetateglutamate ? ?-ketoglutaratealanine ? pyruvateserine ? pyruvate

Question 43

Name one amino acid whose oxidation proceeds via the intermediate shown: (a) pyruvate; (b) oxaloacetate; (c) ? -ketoglutarate; (d) succinyl-CoA; (e) fumarate.

Accepted Answer

(a) A, W, G, S, C (b) D, N (c) E, Q, R, H, P (d) I, T, M, V (e) F, Y.

Question 44

Degradation of amino acids yields compounds that are common intermediates in the major metabolic pathways. Explain the distinction between glucogenic and ketogenic amino acids in terms of their metabolic fates.

Accepted Answer

glucogenic AAs - catabolized to intermediates that can serve as substrates for gluconeogenesis: (pyruvate & any 4-5C CAC intermediates) Ketogenic AAs - catabolized to yield acetyl-CoA or acetoacetyl-CoA, (ketone body precursors)

Question 45

There are bacteria for which alanine can serve as the chief energy source; they oxidize the carbon skeleton of this amino acid, thereby generating ATP. Describe the first step in alanine degradation; show any cofactors that participate.

Accepted Answer

The first step is removal of the amino group by transamination, in which pyridoxal phosphate (PLP) serves as an essential cofactor and ?-ketoglutarate is the usual amino group acceptor:  Alanine + ?-ketoglutarate --(PLP)? pyruvate + glutamate

BIOC #18 QUIZ

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