Chapter 30 Amino Acid Degradation and the Urea Cycle
Matching Questions
Use the following to answer questions 1–10:
Choose the correct answer from the list below. Not all of the answers will be used.
a) hyperammonemia
b) aminotransferase
c) methionine
d) S-adenosylmethionine
e) glutamate dehydrogenase
f) oxaloacetate
g) biopterin
h) glutamate
i) hippurate
j) urea
k) dehydratase
l) dioxygenases
m) ammonia disorder
1.
The molecule _______________ undergoes oxidative deamination to an ammonium ion.
2.
Serine and threonine are deaminated by the enzyme _______________.
Ans:
k
Section: 30.1
3.
The ______________ enzymes transfer an -amino group from amino acids to -ketoglutarate.
Ans:
b
Section: 30.1
4.
The _______________ enzyme catalyzes an oxidative deamination and can utilize either NAD+
or NADP+.
Ans:
e
Section: 30.1
5.
_______________ is caused by defects in the urea cycle, which lead to an elevated level of
ammonia in blood.
Ans:
a
Section: 30.2
6.
The molecule _______________ is formed from excess NH4+ by ureotelic organisms.
Ans:
j
Section:30.2
Ans:
h
Section: 30.1
Chapter 30 Amino Acid Degradation and the Urea Cycle
2
7.
The product _______________ results when aspartate is transaminated with -ketoglutarate.
8.
The methyl group donor _______________ is the product of the first step of methionine
degradation.
Ans:
d
Section: 30.3
9.
The class of enzymes that cleaves most aromatic rings in biological systems is _____________.
Ans:
l
Section: 30.3
10.
The cofactor required by phenylalanine hydroxylase is _______________.
Ans:
g
Section: 30.3
Fill–in-the-Blank Questions
11.
_____ is the prosthetic group used in deaminations by dehydratases.
Ans: pyridoxal phosphate (PLP) Section: 30.1
12.
Oxidative deamination of glutamate results in free ammonium ions and _____.
Ans: -ketoglutarate Section: 30.1
13.
In adults, the urea cycle produces sufficient quantities of _____ so that it is not considered an
essential amino acid.
Ans: arginine Section: 30.2
14.
The hydrolysis of arginine by arginase produces ornithine and _____.
Ans: urea Section: 30.2
15.
Nitrogen is transported from muscle to liver in the form of _____ or _____.
Ans: alanine or glutamine Section: 30.1
16.
The urea cycle is linked to gluconeogenesis via formation of _____, a precursor of glucose
synthesis.
Ans: fumarate Section: 30.2
17.
Serine dehydratase catalyzes the conversion of serine into NH4+ and _____.
Ans: pyruvate Section: 30.1
18.
In the degradation of amino acids, the amino nitrogens can eventually become the amino group
of _____.
Ans: glutamate Section: 30.2
19.
In the first step of the urea cycle, CO2 and NH4+ are converted into _____.
Ans: carbamoyl phosphate Section: 30.2
Ans:
f
Section: 30.1
Chapter 30 Amino Acid Degradation and the Urea Cycle
3
20.
The genetic deficiency of the enzyme _____ results in a condition referred to as
phenylketonuria.
Multiple-Choice Questions
21.
Surplus amino acids are
A)
stored in proteasomes.
B)
stored in protein scaffolds.
C)
used as metabolic fuel.
D)
All of the above.
E)
None of the above.
22.
Which of the following is an allosteric activator of mammalian carbamoyl phosphate
synthetase?
A)
α-ketoglutarate
B)
N-acetylaspartate
C)
N-acetylglutamate
D)
glutamine
E)
None of the above.
Ans: C Section: 30.2
23.
Which of the following amino acids is glucogenic?
A)
asparagine
B)
methionine
C)
valine
D)
lysine
E)
A, B, and C.
Ans: E Section: 30.3
24.
In the urea cycle, the second nitrogen of urea enters the cycle in the form of which of the
following metabolites?
A)
alanine
B)
glutamine
C)
ornithine
D)
aspartate
E)
arginine
Ans: D Section: 30.2
25.
The urea cycle is
A)
a system for protein degradation and turnover.
B)
responsible for nitrogen fixing.
C)
responsible for the filtering of nitrogen by the kidney.
D)
a system involved in transporting nitrogen from muscle to the liver.
E)
None of the above.
Ans: E Section: 30.2
Ans: phenylalanine hydroxylase Section: 30.3
Chapter 30 Amino Acid Degradation and the Urea Cycle
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26.
Which amino acids can be directly deaminated to produce NH4+?
A)
serine and threonine
B)
serine, asparagine, and threonine
C)
proline and threonine
D)
serine and valine
E)
None of the above.
27.
In the urea cycle, free NH4+ is coupled with carboxyphosphate to form
A)
uretic phosphate.
B)
pyruvate.
C)
carbamic acid.
D)
All of the above.
E)
None of the above
Ans: C Section: 30.2
28.
Which amino acid is a metabolite in the urea cycle, but is not used as a building block of
proteins?
A)
ornithine
B)
citrulline
C)
glutamate
D)
A and B.
E)
A, B, and C.
Ans: D Section: 30.2
29.
In the urea cycle, the carbon skeleton of aspartate is preserved as
A)
succinate.
B)
fumarate.
C)
urea.
D)
All of the above.
E)
None of the above.
Ans: B Section: 30.2
30.
Which of the following amino acids is/are ketogenic?
A)
asparagine
B)
methionine
C)
valine
D)
lysine
E)
A, B, and C.
Ans: D Section: 30.3
31.
Ammoniotelic organisms excrete excess nitrogen as
A)
N2H8.
B)
NH4+.
C)
urea.
D)
All of the above.
E)
None of the above.
Ans: B Section: 30.2
Ans: A Section: 30.1
Chapter 30 Amino Acid Degradation and the Urea Cycle
5
32.
Uricotelic organisms release nitrogen as
A)
NH4+.
B)
arginine.
C)
uric acid.
D)
All of the above.
E)
None of the above.
33.
Ketogenic amino acids are degraded to which of the following metabolites?
A)
pyruvate
B)
acetyl CoA
C)
acetoacetate
D)
All of the above.
E)
B and C.
Ans: E Section: 30.3
34.
Which amino acids supply carbons for eventual entry into metabolism as succinyl CoA?
A)
methionine, valine, leucine
B)
methionine, isoleucine, valine
C)
isoleucine, valine, leucine
D)
All of the above.
E)
None of the above.
Ans: B Section: 30.3
35.
Which amino acid is converted to tyrosine in the degradative pathway?
A)
phenylalanine
B)
tryptophan
C)
methionine
D)
A and B.
E)
A, B, and C.
Ans: A Section: 30.3
36.
Three enzymes of the urea cycle require ATP, but when counting high-transfer-potential
phosphoryl groups spent, the number is four. Explain this apparent discrepancy.
A)
Carbamoyl phosphate synthetase is regulated allosterically by ATP, so the
phosphorylation / dephosphorylation reactions utilize twice the ATP.
B)
The urea cycle starts in the mitochondrial matrix and ends in the cytoplasm. ATP is
required for the translocation of citrulline across the mitochondrial matrix.
C)
Ornithine transcarbamoylase is regulated allosterically by ATP, so the phosphorylation /
dephosphorylation reactions utilize twice the ATP.
D)
Argininosuccinate synthetase is driven by the cleavage of ATP into AMP and
pyrophosphate and by the subsequent hydrolysis of pyrophosphate.
E)
Glutamine synthetase is driven by the cleavage of ATP into AMP and pyrophosphate and
by the subsequent hydrolysis of pyrophosphate.
Ans: D Section: 30.3
Ans: C Section: 30.2
Chapter 30 Amino Acid Degradation and the Urea Cycle
6
37.
Individuals with phenylketonuria can ameliorate the symptoms by consuming a diet low in
phenylalanine. Would the same strategy (a diet low in one amino acid) work for individuals with
citrullinemia?
A)
Yes, citrullinema is due to a deficiency of α-Aminoadipic semialdehyde dehydrogenase,
which is usually benign.
B)
Yes, citrullinema is due to an inability to degrade lysine. A diet low in lysine would work.
C)
No, citrullinema is due to a deficiency of argininosuccinase, an enzyme that all amino
acids ultimately utilize for nitrogen disposal.
D)
No, citrullinema is due to an inability to transport citrulline out of the mitochondrial
matrix, eventually shutting down the citric acid cycle.
E)
No, citrullinema is due to the inability of ornithine to be carried into the mitochondrial
matrix with the concomitant loss of the ability to dispose of nitrogen.
38.
As shown in Chapter 28, excess alcohol consumption not only makes you overweight, it can
also lead to ammonia poisoning. How is alcohol metabolism linked to increased levels of blood
ammonia?
A)
Excess alcohol consumption can cause liver damage, the primary site of urea formation.
Loss of liver function can lead to increases of ammonia in the blood.
B)
Excess alcohol consumption can cause a buildup of acetaldehyde, a reactive species that
short circuits the urea cycle at the formation of cambamoyl phosphate.
C)
Excess alcohol consumption makes the liver more acidic, driving the equilibrium from
ammonium ion to free ammonia.
D)
Alcohol acts as an allosteric inhibitor of carbamoyl phosphate reducing the rate of
ammonium ion removal.
E)
Excess alcohol facilitates the formation of an abortive complex for glutamate
dehydrogenase.
Ans: A Section: 30.1
39.
Metabolic pathways are said to be variations on a theme. Which of the following would
accurately illustrate this statement?
A)
All amino groups in amino acids enter the urea cycle through glutamate.
B)
Defects in the urea cycle lead to increased transamination reactions in muscle.
C)
Cleavages of aromatic rings in amino acids are catalyzed by oxygenases.
D)
The carbon skeletons of amino acids are metabolized by the citric acid cycle and the
pentose phosphate pathway.
E)
Like monosaccharides, amino acids are oxidized by NADH.
Ans: C Section: 30.3
Ans: C Section: 30.3
Chapter 30 Amino Acid Degradation and the Urea Cycle
7
40.
You are studying a species of ground squirrel that hibernates and you want to determine if
nitrogen is salvaged in the same way it is salvaged in bears. You feed the ground squirrels a
stable isotope of nitrogen-labeled food. Where would you look for the labeled nitrogen during
hibernation?
A)
in ammonia released into the hibernating den
B)
in NH4+ in the blood of the ground squirrel
C)
in proteins in the bacteria in the intestine of the ground squirrel
D)
in urea in liver mitochondria of the ground squirrel
E)
in NH4+ in in liver mitochondria of the ground squirrel
Short-Answer Questions
41.
In addition to increased fatty acid accumulation in the liver, excess consumption of ethanol can
also induce two further stages of liver damage. What are they?
Ans:
The second stage is alcoholic hepatitis and the third stage is cirrhosis. In the third stage,
ammonia accumulates because of the liver’s inability to utilize the urea cycle.
Section: 30.2
42.
Why are more uricotelic organisms found in the desert than in the tropics?
Ans:
Uricotelic organisms remove toxic ammonium ions directly as uric acid, which requires
very little water.
Section: 30.2
43.
Draw the urea cycle and indicate which component is in which compartment of the cell.
Ans:
See Figure 30.2.
Section: 30.2
44.
Why do you think that weight lifters take large amounts of branched-chain amino acids?
secondary source of ATP production without high levels of carbohydrates.
Section: 30.3
45.
What is the structure of compound X in the following metabolic reaction?
X
+–O2CCH2CH2CCO2–
O
–O2CCH2CH2CHCO2–
NH3+
+
CH3CHCH2CHCO2–
CH3NH3+
Section: 30.3
Ans: C Section: 30.2
Chapter 30 Amino Acid Degradation and the Urea Cycle
8
46.
Describe the glucose-alanine cycle and its significance in amino acid metabolism.
47.
How is the urea cycle linked to the citric acid cycle?
30.3 in the text shows this integration.
Section: 30.1
48.
Write the net equation of the urea cycle and indicate whether energy is produced or consumed
by the cycle.
Ans:
conversion of NH4+ to urea requires the equivalent of four ATP molecules.
Section: 30.2
49.
What type of damage occurs if there is a defect in the urea cycle?
damage follow, most likely caused by the high levels of glutamate and glutamine in the
brain.
Section: 30.2
50.
What is the basic strategy by which proteins are degraded?
carbon skeleton is then converted into major metabolic intermediates that can be
converted into glucose or oxidized by the citric acid cycle.
Section: 30.1
51.
Which amino acids can be converted to pyruvate by a single enzyme-catalyzed reaction?
Ans:
Alanine can be converted to pyruvate by transamination. Serine can be converted to
pyruvate by a deamination, in a reaction catalyzed by serine dehydratase.
Section: 30.3
52.
What general feature is involved in the degradation of aromatic amino acids?
Ans:
The cleavage of aromatic rings is catalyzed by dioxygenases.
pyruvate is converted to glucose by gluconeogenesis and the glucose is transported back
to the muscle via the bloodstream.
Section: 30.2
Chapter 30 Amino Acid Degradation and the Urea Cycle
9
53.
Biopterin is a complicated coenzyme required for the conversion of phenylalanine to tyrosine.
Why is biopterin not considered a vitamin?
54.
What is the cause of PKU? Give the structure of the abnormal metabolite that accumulates from
which this condition gets its name.
Section: 30.3