Deck 23: The Metabolism of Nitrogen

Question 1

The nitrogen-fixing enzyme system,nitrogenase,is found exclusively in

A) plants
B) bacteria
C) some viruses
D) eukaryotic organisms

Answer 1

B

Question 2

In nitrogen fixation,the chemical change is

A) NH₄⁺ to NO₂^-
B) NO₃^- to NH₄⁺
C) N₂ to NH₄⁺ or NH₃
D) NO₂^- to N₂

Answer 2

C

Question 3

Which of the following is not an interconversion of nitrogen performed by living organisms?

A) Fixation of atmospheric nitrogen into ammonia.
B) Fixation of atmospheric nitrogen into nitrate.
C) Nitrification of soil nitrate into ammonia
D) Denitrification of nitrogen oxides to gaseous nitrogen.
E) All of these are common bioconversions in the nitrogen cycle.

Answer 3

E

Question 4

Nitrogen fixation in legumes may use as much as half of the ATP made in the plant.

Question 5

While the reduction of nitrogen to ammonia in plants follows the same net chemical reaction,the source of electrons for the process varies from plant to plant.

Question 6

Denitrification is carried out by

A) animals.
B) green plants.
C) soil bacteria.
D) all of the above.

Question 7

Glycine,alanine,serine,histidine,tryptophan,CTP,AMP,and carbamoyl-phosphate all have what in common?

A) They are all inhibitors of glutamine synthase
B) They are all amino acids
C) They are all cofactors in transamination reactions
D) They are activators of the urea cycle

Question 8

Pyridoxal phosphate forms a Schiff base with amino groups in transaminase reactions.

Question 9

The conversion of glutamate to glutamine is catalyzed by glutamine synthetase and requires ATP.

Question 10

Glutamine synthetase is subject to feedback inhibition by which of the following chemicals?

A) histidine
B) carbamoyl phosphate
C) cytidine triphosphate (CTP)
D) All of these

Question 11

The synthesis of amino acids occurs in about 5 or 6 related families of amino acids.

Question 12

A coenzyme frequently encountered in transamination reactions is

A) tetrahydrofolate.
B) pyridoxal phosphate.
C) thiamine pyrophosphate.
D) biotin.

Question 13

A characteristic feature of feedback inhibition is that

A) it is wasteful of energy and reducing power in anabolic pathways
B) it leads to buildup of the end products of anabolic pathways
C) it seldom occurs in the metabolism of compounds of nitrogen
D) it commonly appears in biosynthesis of amino acids

Question 14

Nitrogenase uses which of the following cofactors?

A) molybdenum-iron-sulfur complex
B) biotin
C) pyridoxal phosphate
D) thiamine pyrophosphate

Question 15

Which of the following is not a component of the nitrogenase complex?

A) a molybdenum complex
B) a nonheme iron-sulfur protein (Fe protein)
C) ferredoxin
D) pyridoxal phosphate

Question 16

How many electrons are required for the reduction of one nitrogen molecule (N₂) to two molecules of ammonia (NH₃) and molecule of hydrogen (H₂)?

A) 2 electrons.
B) 4 electrons.
C) 6 electrons.
D) 8 electrons.
E) 12 electrons.

Question 17

Nitrification is the conversion of

A) nitrogen gas to nitrate.
B) nitrogen gas to ammonia.
C) nitrate to ammonia.
D) nitrate to nitrogen gas.
E) ammonia to nitrogen gas.

Question 18

Which of the following is not true regarding ammonia and fertilizers?

A) Ammonia gas is seldom an effective plant fertilizer, since it is usually consumed by the bacteria in the soil.
B) Many fertilizers are made from ammonia synthesized by the Haber process
C) Researchers are looking for ways to incorporate the genes for nitrogenase into crop plants so that not as much fertilizer is needed
D) Both ammonium and nitrate ions are frequently used in fertilizers

Question 19

The conversion of a-ketoglutarate to glutamate is

A) a transamination.
B) a reductive amination.
C) an amidation.
D) an oxidation.

Question 20

How many ATP are required for the production of two ammonia molecules (NH₃) from one nitrogen molecule (N₂)?

A) 2 ATP.
B) 4 ATP.
C) 8 ATP.
D) 16 ATP.
E) 32 ATP.

Question 21

Essential amino acids cannot be synthesized in sufficient quantities within the human body.

Question 22

Which of the following statements concerning arginine is true?

A) Arginine is only marginally an essential amino acid, since it is made in the urea cycle.
B) Aspartic acid is the main amino acid precursor for the carbon chain in arginine.
C) Lysine could substitute for arginine as a dietary source for arginine.
D) Arginine is only essential in adults.

Question 23

The carbon skeleton used to make serine is:

A) 3-phosphoglycerate
B) glutamate
C) á-ketoglutarate
D) oxaloacetate
E) pyruvate

Question 24

By a simple transamination reaction,intermediates in glycolysis or the Kreb's Cycle can be converted in one step to all of these amino acids,except:

A) Alanine.
B) Aspartic Acid
C) Glutamic Acid.
D) Serine.
E) All of these amino acids are only one step away from the major metabolic pathways.

Question 25

Which of the following are all essential amino acids in humans?

A) leucine, lysine, valine
B) methionine, threonine, serine
C) arginine, histidine, cysteine
D) glutamate, glutamine, arginine
E) valine, isoleucine, glycine

Question 26

In the urea cycle,the molecule that is synthesized in the cytosol and transported to the mitochondrial matrix for subsequent reaction is

A) citrulline
B) ornithine
C) argininosuccinate
D) aspartate

Question 27

In animals,the synthesis of cysteine involves all of the following,except:

A) Addition of a carbon group to glycine.
B) Transfer of the sulfur group from methionine.
C) An intermediate of homocysteine.
D) Participation of ATP.
E) All of these reactions are involved in synthesizing cysteine in animals.

Question 28

Isoleucine,leucine and valine are all formed from the same amino acid root,aspartic acid.

Question 29

The sulfur group in cysteine arises from the same source in plants,animals and bacteria.

Question 30

The urea cycle is linked to the citric acid cycle by

A) arginine
B) citrulline
C) fumarate
D) ornithine

Question 31

Which of the following is not a characteristic of pyridoxal in its active form?

A) It is covalently bonded to its enzyme.
B) It requires being attached to a phosphate group.
C) It bonds to an amine group through by means of the phosphate group.
D) It has a particularly reactive carbonyl group.

Question 32

There is a curious diversity of nature for the two sulfur containing amino acids.Methionine is made from cysteine in bacteria,but cysteine is made from methionine in mammals.

Question 33

Which of the following is not an intermediate in the urea cycle?

A) arginine
B) citrulline
C) ornithine
D) lysine

Question 34

Pyridoxal phosphate is involved in which of the following reactions?

A) transaminations
B) decarboxylations
C) racemizations
D) movement of hydroxymethyl groups
E) all of these

Question 35

Kwashiorkor is a malnutrition disease in which high quality proteins are lacking,even if enough calories are available for survival.

Question 36

A metabolic intermediate that is not a precursor for an amino acid family is

A) a-ketoglutarate
B) pyruvate
C) glyceraldehyde-3-phosphate
D) oxaloacetate

Question 37

The serine family of amino acids includes synthesis of all the following amino acids,except:

A) Serine.
B) Glycine.
C) Cysteine.
D) Methionine.
E) The serine family includes synthesis of all four of these amino acids.

Question 38

Which of the following amino acids does not use glutamic acid as a precursor?

A) arginine
B) glutamine
C) lysine
D) proline

Question 39

Some amino acids can only be made in small quantities in humans and must be supplemented in the diet in children or during repair of injured tissue.

Question 40

Which group of small molecules best fit the boxes associated with the reaction shown?

A) I
B) II
C) III
D) IV

Question 41

High protein diets for humans result in the following.

A) Increased nitrogen metabolism.
B) A negative nitrogen balance for the body.
C) Increased demand for water in the diet.
D) Both increased nitrogen metabolism and need for water.
E) All of these are consequences of high protein diets.

Question 42

Purine salvage reactions

A) use bases as the substrate
B) use nucleosides as the substrate
C) produce nucleosides as products
D) require the hydrolysis of ATP

Question 43

If an amino acid's degradation pathway leads to a-ketoglutarate,is it glucogenic?

A) Yes.
B) No.
C) Only if the organism has a glyoxylate pathway.
D) Only if the organism does not have a glyoxylate pathway.

Question 44

A molecule synthesized in the mitochondrial matrix and transported to the cytosol for subsequent reaction is

A) arginine
B) argininosuccinate
C) citrulline
D) ornithine

Question 45

High protein foods are not advisable for the long distance runner during a race because they increase the need for water and for urination.

Question 46

Which of the following compounds does not serve as an intermediate for the entry of amino acid carbon chains into the major metabolic pathways?

A) Alpha-ketoglutarate.
B) Pyruvate.
C) Glyceraldehyde.
D) Acetyl CoA.
E) All of these are entry points for amino acid catabolism.

Question 47

If an amino acid is ketogenic,then it:

A) has a catabolic pathway that leads to acetyl-CoA or acetoacetyl-CoA
B) has a one step pathway to a fatty acid
C) enters the citric acid cycle as pyruvate
D) enters the citric acid cycle as fumarate

Question 48

The carbon skeletons of many amino acids can be used to replenish the intermediates of the citric acid cycle.

Question 49

Which best describes the synthesis of purine nucleotides?

A) The basic ring structure, inosine, is synthesized first and then linked to ribose. This is then modified to produce either GMP or AMP.
B) The basic ring structure, orotate, is synthesized stepwise on ribose. This is then modified to produce either GMP or AMP.
C) The basic ring structure, inosine, is synthesized stepwise on ribose. This is then modified to produce either GMP or AMP.
D) The basic ring structure, orotate, is synthesized first and then linked to ribose. This is then modified to produce either UMP or CMP.

Question 50

Humans produce these as the major nitrogen waste products.

A) Ammonia.
B) Urea.
C) Uric Acid.
D) Both Urea and Uric Acid.
E) All three of these are used to dispose of nitrogen wastes by humans.

Question 51

Which of the following is not associated with purine synthesis?

A) Synthesis of the base while ribose is attached.
B) Stimulation of ATP synthesis by GTP and of GTP synthesis by ATP
C) An Inosine intermediate.
D) Inhibition by pyrimidines.
E) All of these are features of purine synthesis.

Question 52

How is cancer chemotherapy related to purine biosyntheis?

A) rapidly dividing cells require more purine biosynthesis and some chemotherapy drugs inhibit a key enzyme in purine biosynthesis
B) aminopterin is a good chemotherapy drug because it inhibits aspartate transcarbamoylase
C) cancer cells lack folic acid, and chemotherapy drugs like sulfanamide act by overproducing folic acid, which is then poisonous to the cancer cells
D) none of these

Question 53

Which of the following amino acids is not glucogenic?

A) Glycine.
B) Alanine.
C) Aspartic Acid.
D) Leucine.
E) All of these amino acids are glucogenic.

Question 54

The form in which nitrogen waste is removed from the body is dependent mostly on the organism's availability of water.

Question 55

How many ATP equivalents are required to produce one molecule of urea from ammonia and aspartate?

A) 1
B) 2
C) 3
D) 4
E) 6

Question 56

What intermediate of the TCA cycle is produced from aspartic acid during the urea cycle,synthesis of IMP,and conversion from IMP to adenosine?

A) fumarate
B) malate
C) a-ketoglutarate
D) succinate

Question 57

The ultimate end products of xanthine catabolsim are:

A) two pyrimidines
B) urea and glyoxylate
C) uric acid
D) allantoin

Question 58

The following two amino acids are key to the transfers of amino groups during breakdown and synthesis of amino acids.

A) GLU and ASP
B) GLU and ARG
C) GLU and GLN
D) ALA and GLN
E) ASP and GLN

Question 59

In the synthesis of purine nucleotides,

A) the ring system is bonded to ribose phosphate only after the complete purine ring is assembled
B) the six-membered ring of the purine is assembled first, then the five-membered ring
C) there is no requirement for ATP
D) the ring system is synthesized onto the ribose phosphate moiety

Question 60

The two nitrogens in urea arise directly from:

A) Ammonia and aspartic acid.
B) Carbamoyl phosphate and aspartic acid.
C) Carbamoyl phosphate and glutamic acid.
D) Ammonia and glutamic acid.
E) Carbamoyl phosphate and glutamine.

Question 61

Many antibiotics,especially for viruses and cancer therapy involve inhibition of folic acid reactions,especially because folic acid is essential for this reaction:

A) Synthesis of many amino acids.
B) Conversion of uracil to thymine for DNA synthesis.
C) Synthesis of deoxyribose for DNA synthesis.
D) None of these reactions explains the reason why inhibitors of folic acid act as antibiotics.
E) All of these reactions are reasons why inhibitors of folic acid act as antibiotics.

Question 62

Ribonucleotide reductase uses which of the following cofactors?

A) biotin
B) thiamine pyrophosphate
C) tetrahydrofolate
D) thioredoxin

Question 63

The energy costs for synthesizing the bases for RNA and DNA is so high that salvage pathways have been developed to recycle existing bases and nucleotides.

Question 64

Which of the following is not associated with pyrimidine synthesis?

A) Synthesis of the base while ribose is attached.
B) Stimulation of synthesis by purines.
C) An orotic acid intermediate.
D) Inhibition by pyrimidines.
E) All of these are features of pyrimidine synthesis.

Question 65

Fluorouracil is an effective inhibitor of

A) ribonucleotide reductase
B) thymidylate synthase
C) xanthine oxidase
D) carbamoyl phosphate synthetase

Question 66

Folic acid is an important factor necessary for the production of three of the four deoxyribonucleotides needed for DNA synthesis.For which dNTP is this factor not necessary?

A) dATP
B) dCTP
C) dGTP
D) dTTP

Question 67

The initial,common intermediate in purine catabolism for both AMP and GMP is

A) xanthine
B) hypoxanthine
C) inosine
D) uric acid

Question 68

In the synthesis of pyrimidine nucleotides,the ring system formed just before the attachment of PRPP is

A) 5-aminoimidazole
B) inosine
C) orotate
D) xanthine

Question 69

Which reaction converts orotate monophosphate into uracil monophosphate?

A) a transamination
B) a methylation
C) a decarboxylation
D) a carboxylation

Question 70

Orotic acid is synthesized from

A) glycine and carbamoyl phosphate.
B) arginine and carbamoyl phosphate.
C) aspartic acid and carbamoyl phosphate.
D) aspartic acid and glutamine.

Question 71

Lesch-Nyhan syndrome involves a genetic error in the pyrimidine salvage pathways.

Question 72

The conversion of uracil to thymine requires

A) a one-carbon transfer with S-adenosylmethionine as the donor
B) a one-carbon transfer with tetrahydrofolate as the donor
C) a one-carbon transfer with biotin as the donor
D) a one-carbon transfer with oxaloacetate as the donor

Question 73

Cancer treatments which disrupt the function of folic acid have significant side effects,since this therapy also affects the growth of such rapidly dividing tissues as all of the following,except:

A) Skin cells.
B) Red and white blood cells.
C) Nerve tissue.
D) Sperm.
E) All of these cell types are affected by this therapy.

Question 74

Enzymes which carry out one-carbon transfers can use any of these compounds,except:

A) Biotin
B) Pyridoxal
C) Folic Acid
D) S-adenosylmethionine
E) All of these can carry out one-carbon transfers.

Question 75

One way in which the anabolism of pyrimidine nucleotides differs from that of purine nucleotides is that

A) the pyrimidine ring is assembled before being bonded to the ribose phosphate, whereas the purine ring is bonded to the ribose phosphate as it is formed
B) there is no feedback inhibition in the anabolism of pyrimidine nucleotides, while feedback inhibition is important in the synthesis of purine nucleotides
C) glycine appears in the pathway for formation of pyrimidine nucleotides but not in that for the formation of purine nucleotides
D) argininosuccinate is a key intermediate in pyrimidine biosynthesis

Question 76

Carbamoyl phosphate is made in both the mitochondria and the cytosol,because it is used in the synthesis of several different molecules which are made in either of the compartments.

Question 77

Deoxyribonucleotides are formed

A) by a pathway entirely different from that of ribonucleotides
B) exclusively by hydrolysis of the DNA of the organism
C) as monophosphates by reduction of ribonucleoside monophosphates
D) as diphosphates by reduction of ribonucleoside disphosphates

Question 78

Which of the following is not a carrier of one-carbon groups?

A) S-adenosylmethionine
B) biotin
C) tetrahydrofolate
D) thiamine pyrophosphate