Chapter 25—Nitrogen Acquisition and Amino Acid Metabolism
MULTIPLE CHOICE
1. What is nitrogen fixation?
a.
The reduction of NO3− to NH4+.
b.
The formation of NH4+ from N2 gas.
c.
The oxidation of NH4+ to N2.
d.
The oxidation of NH4+ to NO3−.
e.
The formation of NO2− from NO3−.
2. Animals are totally dependent on plants and microorganisms for nitrogen fixation and nitrate
assimilation because animals:
a.
do not have the reducing power (NADPH) to fix nitrogen.
b.
lack the enzyme aminotransferase.
c.
lack the enzymes to reduce N2 and NO3− to NH4+.
d.
lack carbamoyl-phosphate synthetase I.
e.
lack glutamate dehydrogenase.
3. Nitrifying bacteria obtain their chemical energy from:
a.
the reduction of NO3− to NH4+.
b.
the formation of NH4+ from N2 gas.
c.
the oxidation of NH4+ to N2.
d.
the oxidation of NH4+ to NO3−.
e.
the formation of NO2− from NO3−.
4. Denitrifying bacteria are capable of all EXCEPT:
a.
reducing NO3− to N2.
b.
using NO3− as an electron acceptor in their energy-producing pathways.
c.
oxidizing NH4+to NO3−.
d.
reducing the combined-nitrogen levels.
e.
being utilized in water treatment plants to reduce nitrogen entering lakes and streams.
5. Nitrate assimilation occurs in two steps: reduction of nitrate to ____ and further reduction to ____.
a.
ammonia; nitrogen
b.
nitrogen; ammonia
c.
nitrite; nitrogen
d.
nitrite; ammonia
e.
none are correct
6. The correct sequence for electron-transport in nitrate reductase is:
A.
NADH
B.
−SH
C.
FAD
D.
Cyt b557
E.
MoCo
F.
NO3−
a.
A, B, C, D, E, F
b.
B, C, A, D, F, E
c.
D, A, B, C, E, F
d.
A, C, B, D, F, E
e.
C, A, B, D, E, F
7. Nitrite reductase requires ____ electrons to reduce NO2− to NH4+ and the electrons are provided in
higher plants through photosynthesis as reduced ____.
a.
2; plastoquinone
b.
3; plastoquinone
c.
4; heme
d.
5; heme
e.
6; ferredoxin
8. Nitrogen fixation involves the enzyme ____ catalyzing the reaction:
N2 + 10 ____ + ____ e− → 2 NH4+ + H2
a.
nitrate reductase; H2O; 6
b.
nitrogenase; H+; 8
c.
nitrogenase; H+; 2
d.
nitrogen reductase; H2O; 6
e.
none are true
9. All N2-fixing systems are nearly identical and have all of the requirements EXCEPT:
a.
ATP.
b.
O2-free conditions.
c.
the enzyme nitrogenase.
d.
strong reductant such as ferredoxin.
e.
all are true.
10. The involvement of ATP in the nitrogenase complex is because ATP is needed to:
a.
provide electrons.
b.
phosphorylate the enzyme.
c.
break the N2 triple bond.
d.
transfer electrons from ferredoxin.
e.
all are true.
11. Control over nitrogenase activity is accomplished by which of the following?
a.
ATP serves as an allosteric activator
b.
AMP serves as an allosteric inhibitor
c.
NH4+ prevents expression of the genes that encode nitrogenase
d.
phosphorylation prevents activity
e.
none of the above
12. N-acetylglutamate functions in ammonium incorporation into metabolic intermediates as:
a.
a coenzyme for glutamine synthetase (GS).
b.
a competitive inhibitor for glutamine synthetase (GS).
c.
an allosteric activator for carbamoyl-phosphate synthetase I (CPS-I).
d.
the energy source needed for the reductive amination of -ketoglutarate (-KG) to yield
glutamate.
e.
the substrate for the amination of the -glutamylphosphate.
13. What reaction does glutamate dehydrogenase (GDH) catalyze?
a.
The reductive amination of –ketoglutarate to yield glutamate.
b.
Phosphorylation of carbamate to yield carbamoyl-phosphate.
c.
The amidation of the -carboxyl group of glutamate to form glutamine.
d.
The deadenylation of glutamine synthetase.
e.
The adenylation of glutamine synthetase.
14. The reaction, glutamate + NAD(P)+ + H2O → NH4+ + -ketoglutarate + NAD(P)H + H+, is catalyzed
by:
a.
Nitrogenase.
b.
carbamoyl-phosphate synthetase (CPS-I).
c.
glutamate dehydrogenase (GDH).
d.
glutamine synthetase (GS).
e.
none are true.
15. Glutamine synthetase is activated by which of the following molecules?
a.
glycine
b.
AMP
c.
CTP
d.
carbamoyl phosphate
e.
none of the above
16. The reaction, 2 NH4+ + -ketoglutarate + NADPH + ATP → glutamine + NADP+ + ADP + Pi + H2O,
is the combined result of what two enzymes?
a.
first, glutamate synthase, then glutamate dehydrogenase
b.
first, glutamate dehydrogenase ,then glutamine synthetase
c.
first, glutamine synthetase, then glutamate synthase
d.
first, glutamine synthetase, then glutamate dehydrogenase
e.
none of the above
Garrett/Grisham 5e Test Bank 4
17. Glutamate synthase catalyzes the reaction ____ + ____ + ____ → 2 glutamate + NADP+
a.
-ketoglutarate; NADPH; glutamine
b.
glutamate; -ketoglutarate; NADH
c.
glutamyl-phosphate; glutamine; NADPH
d.
N-acetylglutamate; -ketoglutarate; NADH
e.
none are true
18. Bacterial glutamine synthetase monomers are inactive because they must be:
a.
phosphorylated for activity.
b.
activated by binding glutamine.
c.
stacked for interface active site development.
d.
combined for allosteric regulation.
e.
none are true.
19. All of the following act in feedback inhibition of glutamine synthetase in prokaryotes EXCEPT:
a.
AMP.
b.
glucosamine-6-phosphate.
c.
histidine.
d.
proline.
e.
CTP.
20. All of the following are characteristic features of ATP:GS:adenylyl transferase (AT) EXCEPT:
a.
It catalyzes the adenylation of glutamine synthetase.
b.
It exists in two forms, PIIA and PIID. PIIA causes the deadenylation of glutamine synthetase
and its activity increases with an increase in glutamine.
c.
AT:PIIA is inhibited by -ketoglutarate.
d.
AT:PIIA needs ATP to complete the reaction.
e.
It is an enzyme that causes the adenylation/deadenylation of a specific tyrosine residue.
21. What would be a result of a high [Gln]/[-KG] ratio?
a.
Increase in deadenylation of glutamine synthetase.
b.
An increase in activity of glutamine synthetase.
c.
An increase in the degree of adenylation of glutamine synthetase.
d.
Inhibition of AT:PIIA and stimulation of AT:PIID.
e.
A need for ammonium fixation by glutamine synthetase.
22. In plants and microorganisms, amino acid biosynthesis is a matter of synthesizing the appropriate ____
followed by transamination with ____.
a.
acetyl-CoA derivatives; glutamine
b.
-ketoacid; glutamate
c.
phospho-carbon skeleton; alanine
d.
nitrogenous base; glutamate
e.
none are true
23. In the process of amino acid biosynthesis, how are glutamic acid, glutamine, proline, and arginine all
related?
a.
They are all derived from –KG.
b.
They are all derivatives of acetyl CoA.
c.
They are all derivatives of pyruvate.
d.
They are all derived from aspartate.
e.
They are all derivatives of 3-phosphoglycerate.
24. All of the following are true of transamination EXCEPT:
a.
It is characterized by the transfer of an -amino group from an amino acid to the -keto
position of an -keto acid.
b.
The amino donor becomes an -keto acid.
c.
The coenzyme needed is thiamin pyrophosphate (TPP).
d.
Humans are capable of synthesizing the -keto acid analog of nonessential amino acids
and using transamination to form the amino acids, but are not able to construct carbon
skeletons of the essential amino acids.
e.
The -keto acid acceptor becomes an -amino acid.
25. Which of the following is both an essential amino acid (for humans) and also an exclusively ketogenic
amino acid?
a.
proline
b.
valine
c.
glutamine
d.
leucine
e.
phenylalanine
26. Which of the following is NOT a commonly used transamination pair?
a.
Phe / phenylpyruvate
b.
Asp / oxaloacetate
c.
Glu / -ketoglutarate
d.
Ala / pyruvate
e.
all are correct
27. ____ is an intermediate in biosynthesis of ornithine and a regulator of carbamoyl-phosphate
synthetase-I
a.
-Glutamyl-phosphate
b.
-Ketoglutarate
c.
N-Acetylglutamate
d.
Glutamine
e.
Glutamine-5-phosphate
28. All of the carbons and nitrogens of ornithine come biosynthetically from:
a.
proline.
b.
valine.
c.
alanine.
d.
glutamate.
e.
glycine.
29. Ornithine serves three metabolically important roles, but is not found in proteins. What is one of the
important roles of ornithine?
a.
amine donor for many transamination reactions
b.
an intermediate in the urea cycle
c.
involved in the synthesis of serine, which serves as a precursor for glycine and cysteine
d.
its reaction with methenyl tetrahydrofolate produces lysine
e.
none of the above
30. All of the following are true of the urea cycle EXCEPT:
a.
It helps in the excretion of excess nitrogen.
b.
It is mainly confined to the liver.
c.
It is linked to the citric acid cycle through fumarate.
d.
Stimulation of carbamoyl-phosphate synthetase I (CPS-I) decreases the activity of the urea
cycle.
e.
It is completed by the regeneration of ornithine from arginine.
31. Which of the following directly serves as one of the nitrogen atoms of a urea molecule?
a.
the amide N of Asn
b.
the N of Asp
c.
the R-group N of Lys
d.
the N of glucosamine
e.
none of the above
32. Aspartate is formed from transamination of:
a.
asparagine.
b.
aspartame.
c.
oxaloacetate.
d.
citrate.
e.
-ketoglutarate.
33. Homoserine and homocysteine are related to serine and cysteine by having:
a.
an extra amino group.
b.
an additional methylene group.
c.
an additional carboxyl group.
d.
an aldehyde group.
e.
a phosphoryl group.
34. Amino acids biosynthesized from aspartate include all EXCEPT:
a.
asparagine.
b.
threonine.
c.
methionine.
d.
lysine.
e.
glutamate.
35. Elevated levels of homocysteine in the blood is related to all EXCEPT:
a.
homocysteinuria at very high levels.
b.
folic acid deficiency.
c.
folic acid-dependent conversion of homocysteine to methionine.
d.
higher risk of heart attack and stroke.
e.
all of the above are correct.
36. Transamination of pyruvate with glutamate as amino donor gives:
a.
alanine.
b.
serine.
c.
cysteine.
d.
aspartate.
e.
valine.
37. All of the following are true statements regarding the 3-Phosphoglycerate (3-PG) family EXCEPT:
a.
3-PG is diverted from glycolysis by phosphoglycerate mutase.
b.
Glycine can be derived from serine.
c.
Serine, glycine and cysteine are all nonessential amino acids.
d.
Production of glycine also produces N5, N10-methylene-THF which is important in the
biosynthesis of purines.
e.
Serine production is regulated by a direct feedback mechanism.
38. Chorismate biosynthesis occurs via the shikimate pathway and is an important precursor in the
synthesis of all of the following EXCEPT:
a.
vitamin K.
b.
folic acid.
c.
threonine.
d.
tyrosine.
e.
tryptophan.
39. Herbicides (e.g., “Roundup”) that inhibit biosynthesis of “essential” amino acids are supposedly safe
for animal exposure because animals do not have enzymes for:
a.
synthesis of the members of the 3-phosphoglycerate family
b.
the glyoxylate cycle.
c.
the glutamine synthesis pathway.
d.
Phe, Val, Leu, and Ile biosynthetic pathways.
e.
none are correct.
40. All are convergent metabolic intermediates of -amino acid carbon skeletons EXCEPT:
a.
Citrate.
b.
-ketoglutarate.
c.
Oxaloacetate.
d.
succinyl-CoA.
e.
Fumarate.
41. The term ketogenic amino acids refers to amino acids:
a.
that are precursors for glucose synthesis.
b.
degraded to yield acetyl CoA or acetoacetate.
c.
that can not be converted to fatty acids or ketone bodies.
d.
degraded to yield succinyl-CoA, pyruvate, -ketoglutarate, fumarate and oxaloacetate.
e.
none of the above.
42. The carbon skeletons of all of the amino acids below converge to pyruvate EXCEPT:
a.
alanine.
b.
serine.
c.
cysteine.
d.
glycine.
e.
glutamate.
43. Degradation of valine, isoleucine and methionine leads to succinyl-CoA via the sequence of:
A.
carboxylation using biotin and ATP
B.
mutase
C.
methylmalonyl-CoA
D.
epimerase
E.
Propionyl-CoA
a.
E, B, A, C, D, C
b.
E, A, C, D, C, B
c.
A, C, D, C, E, B
d.
C, A, D, B, C, E
e.
D, A, C, B, C, E
44. All are characteristics of Maple Syrup Urine Disease EXCEPT:
a.
hereditary enzyme defect.
b.
elevated levels of val, leu, and ile and their corresponding -keto acids in blood and urine.
c.
restrictive intake of proteins with val, leu and ile is required.
d.
fatal without detection and treatment.
e.
elevated blood levels of acetoacetate and -hydroxybutyrate.
45. All of the following are characteristics of phenylketonuria EXCEPT:
a.
excretion of phenylpyruvate.
b.
air oxidation causes urine to turn dark on standing.
c.
treated by putting patient on a diet low in phenylalanine.
d.
untreated patients suffer severe mental retardation.
e.
deficiency or defect in phenylalanine hydroxylase.
46. All of the following are characteristics of alkaptonuria EXCEPT:
a.
excretion of homogentisate
b.
air oxidation causes urine to turn dark on standing
c.
results in mental retardation due to accumulation of Phe metabolites
d.
homogentisate accumulation in joints may cause arthritis
e.
all are true
47. Glutamine synthetase plays an important role in the incorporation of ammonia (or ammonium ions)
into glutamine. Which of the following mechanisms of covalent modification controls the activity of
glutamine synthetase?
a.
Phosphorylation/dephosphorylation
b.
Adenylation/deadenylation
c.
Acetylation/deacetylation
d.
Methylation/demethylation
e.
None of the above
48. Microorganisms that fix nitrogen have a symbiotic relationship with certain plants. What molecule is
produced by plants to prevent oxygen poisoning of the nitrogenase enzyme?
a.
-ketoglutarate
b.
leghemoglobin
c.
glutamic acid
d.
glucose
e.
none of the above
49. Which of the following sequence of events best explains the mechanism of carbamoyl phosphate
synthetase?
a.
CO2 and ATP form a carbonic-phosphoric anhydride (CPA), then NH3 performs a
nucleophilic attack on the CPA to form carbamate
b.
Activated CO2 is transferred to biotin, then reacts with NH3 to produce carbamate
c.
NH3 reacts with ATP, which then reacts with CO2 to produce carbamate
d.
NH3 reacts with CO2 to produce carbamate followed by reaction with ATP
e.
None of the above
50. Which of the following amino acids plays the most central role in nitrogen and amino acid
metabolism?
a.
Asn
b.
Glu
c.
Gly
d.
Cys
e.
none of the above
51. Glutamine synthetase is an important enzyme in the regulation of ammonia concentrations. Which of
the following would be an activator of Gln synthetase?
a.
-ketoglutarate
b.
glutamine
c.
histidine
d.
alanine
e.
none of the above