Biochem chapter 25

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Nitrogen Acquisition and Amino Acid Metabolism

Question Answer
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-.
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.
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-.
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.
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
The correct sequence for electron-transport in nitrate reductase is:
B. –SH
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
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
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
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.
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.
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


Question Answer
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 α-ketoglutarate (α-KG) to yield glutamate.
e. the substrate for the amination of the γ-glutamylphosphate.
What reaction does glutamate dehydrogenase (GDH) catalyze?
a. The reductive amination of α-ketoglutarate to yield glutamate.
b. Phosphorylation of carbamate to yield carbamoyl-phosphate.
c. The amidation of the γ-carboxyl group of glutamate to form glutamine.
d. The deadenylation of glutamine synthetase.
e. The adenylation of glutamine synthetase.
The reaction, glutamate + NAD(P)+ + H2O → NH4+ + α-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.
Glutamine synthetase (GS) belongs to what class of enzymes?
a. isomerases
b. oxidoreductase
c. ligase
d. lyase
e. transferase
The reaction, 2 NH4+ + α-ketoglutarate + NADPH + ATP → glutamine + NADP+ + ADP + Pi + H2O, is the combined result of what two enzymes?
a. nitrogenase and glutamate dehydrogenase
b. glutamate dehydrogenase and glutamine synthetase
c. glutamine synthetase and nitrogenase
d. glutamate dehydrogenase and nitrogenase
e. all are correct
Glutamate synthase (GOGAT) catalyzes the reaction __________ + _________ + ___________ → 2 glutamate + NADP+
a. α-ketoglutarate; NADPH; glutamine
b. glutamate; α-ketoglutarate; NADH
c. glutamyl-phosphate; glutamine; NADPH
d. N-acetylglutamate; α-ketoglutarate; NADH
e. none are true
Bacterial glutamine synthetase (GS) 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.
All of the following act in feedback inhibition of glutamine synthetase (GS) in prokaryotes EXCEPT:
a. AMP.
b. glucosamine-6-phosphate.
c. histidine.
d. proline.
e. CTP.
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 α-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.
What would be a result of a high [Gln]/[α-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.
In plants and microorganisms, amino acid biosynthesis is a matter of synthesizing the appropriate __________ followed by transamination with __________.
a. acetyl-CoA derivatives; glutamine
b. α-ketoacid; glutamate
c. phospho-carbon skeleton; alanine
d. nitrogenous base; glutamate
e. none are true
In the process of amino acid biosynthesis, how are glutamic acid, glutamine, proline, and arginine all related?
a. They are all derived from α-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.
All of the following are true of transamination EXCEPT:
a. It is characterized by the transfer of an α-amino group from an amino acid to the α-keto position of an α-keto acid.
b. The amino donor becomes an α-keto acid.
c. The coenzyme needed is thiamin pyrophosphate (TPP).
d. Humans are capable of synthesizing the α-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 α-keto acid acceptor becomes an α-amino acid.
Which of the following is an essential amino acid?
a. proline
b. valine
c. glutamine
d. aspartate
e. glycine
Which of the following is NOT a commonly used transamination pair?
a. Phe / phenylpyruvate
b. Asp / oxaloacetate
c. Glu / α-ketoglutarate
d. Ala / pyruvate
e. all are correct
____________ is an intermediate in biosynthesis of ornithine and a regulator of carbamoyl-phosphate synthetase-I
a. γ-Glutamyl-phosphate
b. α-Ketoglutarate
c. N-Acetylglutamate
d. Glutamine
e. Glutamine-5-phosphate
All of the carbons and nitrogens of ornithine come biosynthetically from:
a. proline.
b. valine.
c. alanine .
d. glutamate.
e. glycine.
Ornithine serves three metabolically important roles, but is not found in proteins. What is one of the important roles of ornithine?
a. precursor in the synthesis of aspartate
b. an intermediate in the urea cycle
c. formation of ornithine from serine
d. it condenses with aspartate to form argininosuccinate
e. it is derived from glutamate-5-phosphate
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.
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
Aspartate is formed from transamination of:
a. asparagine.
b. aspartame.
c. oxaloacetate.
d. citrate.
e. α-ketoglutarate.
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.
Amino acids biosynthesized from aspartate include all EXCEPT:
a. asparagine.
b. threonine.
c. methionine.
d. lysine.
e. glutamate.
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.
Transamination of pyruvate with glutamate as amino donor gives:
a. alanine .
b. serine.
c. cysteine.
d. aspartate.
e. valine.
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.
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.
Herbicides (e.g., "Roundup") that inhibit biosynthesis of "essential" amino acids should be relatively safe for animal exposure because animals do not have enzymes of:
a. photosynthesis.
b. glyoxylate cycle.
c. glutamine synthesis pathway.
d. Phe, Val, Leu, Ile and His biosynthetic pathways.
e. none are correct.
All are convergent metabolic intermediates of α-amino acid carbon skeletons EXCEPT:
a. Citrate.
b. α-ketoglutarate.
c. Oxaloacetate.
d. succinyl-CoA.
e. Fumarate.
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, α-ketoglutarate, fumarate and oxaloacetate.
e. none of the above.
The carbon skeletons of all of the amino acids below converge to pyruvate EXCEPT:
a. alanine.
b. serine.
c. cysteine.
d. glycine.
e. glutamate.
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

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