Biochemistry Chapter 25 PQ

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-.

b. The formation of NH4+ from N2 gas.

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.

c. lack the enzymes to reduce N2 and NO3- to NH4+.

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-.

d. the oxidation of NH4+ to 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.

c. oxidizing NH4+to NO3-.

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

d. nitrite; ammonia

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

e. 6; ferredoxin

Nitrogen fixation involves the enzyme ____ catalyzing the reaction:

N2 +10____+____e- Æ2NH4+ +H2

a. nitrate reductase; H2O; 6

b. nitrogenase; H+; 8

c. nitrogenase; H+; 2

d. nitrogen reductase; H2O; 6

e. none are true

b. nitrogenase; H+; 8

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.

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.

c. break the N2 triple bond.

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

c. NH4+ prevents expression of the genes that encode nitrogenase

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 a-ketoglutarate (a-KG) to yield glutamate.

e. the substrate for the amination of the γ-glutamylphosphate.

d. the energy source needed for the reductive amination of a-ketoglutarate (a-KG) to yield glutamate.

What reaction does glutamate dehydrogenase (GDH) catalyze?

a. The reductive amination of a-ketoglutarate to yield glutamate.

b. Phosphorylation of carbamate to yield carbamoyl-phosphate.

c. The amidation of the g-carboxyl group of glutamate to form glutamine.

d. The deadenylation of glutamine synthetase.

e. The adenylation of glutamine synthetase.

a. The reductive amination of a-ketoglutarate to yield glutamate.

The reaction, glutamate + NAD(P)+ + H2O Æ NH4+ + a-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.

c. glutamate dehydrogenase (GDH).

Glutamine synthetase is activated by which of the following molecules?

a. glycine

b. AMP

c. CTP

d. carbamoyl phosphate

e. none of the above

e. none of the above

The reaction, 2 NH4+ + a-ketoglutarate + NADPH + ATP Æ 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

c. first, glutamine synthetase, then glutamate synthase

Glutamate synthase catalyzes the reaction ____ + ____ + ____ Æ 2 glutamate + NADP+

a. a-ketoglutarate; NADPH; glutamine

b. glutamate; a-ketoglutarate; NADH

c. glutamyl-phosphate; glutamine; NADPH

d. N-acetylglutamate; a-ketoglutarate; NADH

e. none are true

a. a-ketoglutarate; NADPH; glutamine

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.

c. stacked for interface active site development.

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.

d. proline.

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 a-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.

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.

What would be a result of a high [Gln]/[a-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.

d. Inhibition of AT:PIIA and stimulation of AT:PIID.

In plants and microorganisms, amino acid biosynthesis is a matter of synthesizing the appropriate ___ followed by transamination with ___.

Infollowed by transamination with ____.

a. acetyl-CoA derivatives; glutamine

b. a-ketoacid; glutamate

c. phospho-carbon skeleton; alanine

d. nitrogenous base; glutamate

e. none are true

b. a-ketoacid; glutamate

In the process of amino acid biosynthesis, how are glutamine, proline, and arginine all related?

a. They are all derived from a-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.

a. They are all derived from a-KG.

All of the following are true of transamination EXCEPT:

a. It is characterized by the transfer of an a-amino group from an amino acid to the a-keto

position of an a-keto acid.

b. The amino donor becomes an a-keto acid.

c. The coenzyme needed is thiamin pyrophosphate (TPP).

d. Humans are capable of synthesizing the a-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 a-keto acid acceptor becomes an a-amino acid.

c. The coenzyme needed is thiamin pyrophosphate (TPP).

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

d. leucine

Which of the following is NOT a commonly used transamination pair?

a. Phe / phenylpyruvate

b. Asp / oxaloacetate

c. Glu / a-ketoglutarate

d. Ala / pyruvate

e. all are correct

a. Phe / phenylpyruvate

____ is an intermediate in biosynthesis of ornithine and a regulator of carbamoyl-phosphate synthetase-I

a. g-Glutamyl-phosphate

b. a-Ketoglutarate

c. N-Acetylglutamate

d. Glutamine

e. Glutamine-5-phosphate

c. N-Acetylglutamate

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

a. proline.

b. valine.

c. alanine.

d. glutamate.

e. glycine.

d. glutamate.

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

b. an intermediate in the urea cycle

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.

d. Stimulation of carbamoyl-phosphate synthetase I (CPS-I) decreases the activity of the urea cycle.

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

b. the N of Asp

Aspartate is formed from transamination of:

a. asparagine.

b. aspartame.

c. oxaloacetate.

d. citrate.

e. a-ketoglutarate.

c. oxaloacetate.

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.

b. an additional methylene group.

Amino acids biosynthesized from aspartate include all EXCEPT:

a. asparagine.

b. threonine.

c. methionine.

d. lysine.

e. glutamate.

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.

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.

a. alanine.

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.

a. 3-PG is diverted from glycolysis by phosphoglycerate mutase.

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.

c. threonine.

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.

d. Phe, Val, Leu, and Ile biosynthetic pathways.

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

a. Citrate.

b. a-ketoglutarate.

c. Oxaloacetate.

d. succinyl-CoA.

e. Fumarate.

a. Citrate.

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, a-ketoglutarate, fumarate and oxaloacetate.

e. none of the above.

b. degraded to yield acetyl CoA or acetoacetate.

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

a. alanine.

b. serine.

c. cysteine.

d. glycine.

e. glutamate.

e. glutamate.

Degradation of valine, isoleucine and methionine leads to succinyl-CoA via the sequence of:

A. carboxylation using biotin and ATP

B. Motors

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

b. E,A,C,D,C,B

All are characteristics of Maple Syrup Urine Disease EXCEPT:

a. hereditary enzyme defect.

b. elevated levels of val, leu, and ile and their corresponding a-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 b-hydroxybutyrate.

e. elevated blood levels of acetoacetate and b-hydroxybutyrate.

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.

b. air oxidation causes urine to turn dark on standing.

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

c. results in mental retardation due to accumulation of Phe metabolites

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

b. Adenylation/deadenylation

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. a-ketoglutarate

b. leghemoglobin

c. glutamic acid

d. glucose

e. none of the above

b. leghemoglobin

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

a. CO2 and ATP form a carbonic-phosphoric anhydride (CPA), then NH3 performs a nucleophilic attack on the CPA to form carbamate

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

b. Glu

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

a. a-ketoglutarate

b. glutamine

c. histidine

d. alanine

e. none of the above

a. a-ketoglutarate