Deck 25: Nitrogen Acquisition and Amino Acid Metabolism

Question 1

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

Answer 1

B

Question 2

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.

Answer 2

C

Question 3

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.

Answer 3

C

Question 4

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.

Question 5

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−.

Question 6

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

Question 7

The reaction, 2 NH₄⁺ + α-ketoglutarate + NADPH + ATP → glutamine + NADP⁺ + ADP + P_i + 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

Question 8

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

Question 9

Glutamate synthase 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

Question 10

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.

Question 11

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.

Question 12

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−.

Question 13

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

Question 14

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.

Question 15

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 16

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.

Question 17

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, P_IIA and P_IID. P_IIA causes the deadenylation of glutamine synthetase and its activity increases with an increase in glutamine.
C) AT:P_IIA is inhibited by α-ketoglutarate.
D) AT:P_IIA needs ATP to complete the reaction.
E) It is an enzyme that causes the adenylation/deadenylation of a specific tyrosine residue.

Question 18

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.

Question 19

Glutamine synthetase is activated by which of the following molecules?

A) glycine
B) AMP
C) CTP
D) carbamoyl phosphate
E) none of the above

Question 20

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.

Question 21

All of the carbons and nitrogens of ornithine come biosynthetically from:

A) proline.
B) valine.
C) alanine.
D) glutamate.
E) glycine.

Question 22

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.

Question 23

Aspartate is formed from transamination of:

A) asparagine.
B) aspartame.
C) oxaloacetate.
D) citrate.
E) α-ketoglutarate.

Question 24

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.

Question 25

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.

Question 26

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:P_IIA and stimulation of AT:P_IID.
E) A need for ammonium fixation by glutamine synthetase.

Question 27

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

Question 28

Amino acids biosynthesized from aspartate include all EXCEPT:

A) asparagine.
B) threonine.
C) methionine.
D) lysine.
E) glutamate.

Question 29

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

Question 30

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

Question 31

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

Question 32

____ 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

Question 33

Transamination of pyruvate with glutamate as amino donor gives:

A) alanine.
B) serine.
C) cysteine.
D) aspartate.
E) valine.

Question 34

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.

Question 35

All are convergent metabolic intermediates of α-amino acid carbon skeletons EXCEPT:

A) Citrate.
B) α-ketoglutarate.
C) Oxaloacetate.
D) succinyl-CoA.
E) Fumarate.

Question 36

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.

Question 37

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.

Question 38

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

Question 39

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.

Question 40

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.

Question 41

Discuss the components of the metalloproteins that form the nitrogenase complex.​

Question 42

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

Question 43

Explain the regulation of glutamine synthetase through gene regulation in E. coli.​

Question 44

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.

Question 45

Discuss the products of tryptophan degradation.​

Question 46

All are characteristics of Maple Syrup Urine Disease EXCEPT:

A) hereditary enzyme defect.
B) elevated levels of val, leu, and ile and their corresponding α-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 β-hydroxybutyrate.

Question 47

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.

Question 48

Glutamine synthetase is an important enzyme in the regulation of ammonia concentrations. Which of the following would be an activator of Gln synthetase?

A) α-ketoglutarate
B) glutamine
C) histidine
D) alanine
E) none of the above

Question 49

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

Question 50

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

Question 51

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

Question 52

Explain the biosynthesis of glycine.​

Question 53

The carbon skeletons of all of the amino acids below converge to pyruvate EXCEPT:

A) alanine.
B) serine.
C) cysteine.
D) glycine.
E) glutamate.

Question 54

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

Question 55

Discuss the significance of glutamine synthetase.​