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Medical Biochemistry > Nitrogen Metabolism > Flashcards

Nitrogen Metabolism Flashcards

Nitrogen Metabolism-Amino Acids, Purines, Pyrimidines and Products Derived from Amino Acids. Pyrimidine and Purine Synthesis and Metabolism

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USMLE® Step 1 flashcards
1
Q

The first committed and regulated step of purine biosynthesis

A

synthesis of phosphoribosylamine

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2
Q

Transports the bases and nucleosides to other tissue from liver

A

RBC

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3
Q

starting materials for purine breakdown

A

AMP and GMPAMP–> Adenosine –> inosine –> hypoxanthine –> xanthineAMP–> IMP–> inosine –>hypoxanthine –> xanthineGMP –> Guanosine –> Guanine –> xanthine

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4
Q

5 different precursors for purine synthesis

A

AspartateGlutamine (amide N) X2N10-Formyl-FH4 X2GlycineCo2

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5
Q

the amino group of adenine is from

A

Asp

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6
Q

dihydropyrimidine dehydrogenase

A

T+NADPH –> dihydroT + NADP

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7
Q

The two end product of the last steps of pyrimidine breakdown, Which can be used in TCA cycle

A

succinyl Co-A and Acetyl Co-AFormer from B-aminoisobutyrate and latter from B-alanine

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8
Q

The byproduct of IMP dehydrogenase

A

NADH + H+

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9
Q

the first reaction of pyrimidine breakdown starting from U

A

C —-> NH4+ + U (deamination)

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10
Q

the precursor for acetyl-coa and succinyl coa in pyrimidine breakdown

A

(U)B-alanine –>Acetyl Co-A

(T) B-aminoisobutyrate –> succinyl Co-A

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11
Q

The enzyme that catalyzes the following reaction:Adenylosuccinate –> fumarate + AMP

A

Adenylosuccinase

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12
Q

XMP

A

xanthosine monophosphate

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13
Q

adenylosuccinate synthetase catalyzes this reaction

A

Asp + IMP –> adenylosuccinate

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14
Q

Asp + IMP –> adenylosuccinate
is catalyzed by …….
Hint: the first step of AMP synthesis and requires GTP

A

adenylosuccinate synthetase

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15
Q

Most of the de novo synthesis of purine takes place in

A

liver

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16
Q

Are the sources of metabolic energy and precursors for RNA synthesis

A

ATP and GTP

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17
Q

Starting materials for pyrimidine breakdown

A

C, T

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18
Q

The products of purine breakdown

A

uric acid with low plasma solubility

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19
Q

Name all the four enzymes for the formation of AMP and GMP from IMP

A

adenylosuccinate synthetase
adenylosuccinase
IMP dehydrogenase
GMP synthase

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20
Q

The purpose and the requirements of reciprocal regulation

A

requires GTP and ATP
The purpose is to maintain balance between the synthesis of AMP and GMP:
1- AMP neg. feed back on adenylosuccinate synthetase AND
2- GMP neg. feedback on IMP dehydrogenase

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21
Q

The main source of ribose-5-phosphate

A

pentose phosphate pathway

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22
Q

The last step common for purine breakdown regardless of starting material

A

xanthine –> uric acid (Catalyzed by xanthine oxidase)

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23
Q

the first purine nucleotide

A

IMP- inosine monophosphate

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24
Q

Formation of dihydrothymine and dihydrouracil

A

rate determining steprequires NADPHEnzyme : dihydropyrimidine dehydrogenase

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25
Q

The enzyme, reactant, products and starting reactions of purine salvage pathway

A

startting- Guanine / hypoxanthineGuanine + PRPP –> GMPHypoxanthine +PRPP–> IMPThe enzyme that catalyzes both reactions is HGPRT

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26
Q

an inhibitor of xanthine oxidase

A

allopurinol

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27
Q

Due to the high energy expense, this pathway converts free bases and nucleosides to nucleotides

A

Salvage pathway

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28
Q

the enzyme that catalyzes the first committed step of purine biosynthesis

A

GPAglutamine phosphoribosyl amidotransferase

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29
Q

HGPRT defeciency

A

HGPRT converts Hypoxanthine and guanine to IMP and GMP respectively.It also requires PRPP. Therefore, deficiency leads toinc uric acid as a result of dec in HGPRTand dec in PRPP and ultimately inc uric acid productionoften seen in lesch-Nyhan syndrome

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30
Q

The energy requirement for one purine synthesis

A

~ 6ATP

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31
Q

The first step of GMP production from IMP

A

E: IMP dehydrogenasehypoxanthine base is oxidized by IMPdehydrogenase to xanthine- Specifically XMP

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32
Q

Three steps for regulation of purine synthesis

A

1- committed step - GPA inhib by end products2- second level- PRPP synthetase (also used in salvage pathway)3- reciprocal regulation- maintains balance between the synthesis of AMP and GMP

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33
Q

donates nitrogen to XMP to form GMP

A

glutaminesecond step of GMP production from IMP

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34
Q

Two step production of AMPindicate the step that requires energy

A

1- Asp + IMP –> adenylosuccinate* It requires energy from GTP2- adenylosuccinate –> fumarate + AMP

35
Q

Purine synthesis begins with

A

Attachment of amino group from Gln to ribose moiety.

36
Q

HGPRT

A

hypoxanthine-guanine Phosphoribosyl transferase

37
Q

Fumarate is released from this molecule to form AMP- Adenosine MonoPhosphate.

A

Adenylsuccinate

38
Q

The amino group of guanine is from

A

Gln

39
Q

The by products of Acetyl-CoA and Succinyl CoA synthesis in pyrimidine breakdown

A

Ammonia (used in urea cycle) and Co2

40
Q

The enzyme that catalyzes Guanine –> Xanthine +NH4+

A

Guanase.GMP–> Guanosine –> guanine –> xzanthine —> uric acid

41
Q

phsophorylate AMP and GMP

A

nucleoside monophosphate kinase

42
Q

Adenosine –?-? inosine –> hypoxanthinepurine breakdown- starting from AMP

A

ADAAdenosine deaminaseNH4+ forms

43
Q

The two enzymes for the production of GMP from IMP

A

1- IMP dehydrogenase 2-GMP synthetase

44
Q

The rate limiting step of pyrimidine breakdown

A

formation of dihydroT and dihydroU

45
Q

pathway for uric acid conversion to urine (general-schematic)

A

uric acid –> blood –> sodium urate –> kidney –> urine

46
Q

The enzyme(s) for the following reactions:
hypoxanthine —> xanthine —> uric acid
(Both release H2O2)

A

Xanthine oxidase

47
Q

the step of GMP production that requires energy and the associated enxyme

A

GMP synthaseRequires energy from ATP

48
Q

phosphorylates and forms ATP and GTP

A

nucleoside diphosphate kinase

49
Q

The base of the first purine nucleotide

A

the base of IMP is hypoxanthine

50
Q

the result of allopurinol

A

allopurinol is xanthine oxidase inhibitor.Xanthine oxidase catalyzes the following reactions:hypoxanthine –> xanthine –> uric acid(a series of oxidation reaction and H2O2 formation)Thus allopurinol decreases the uric acid level

51
Q

Synthesis of inosine monophosphate

A

Requires 10 steps4 enzymes use ATPTwo N10-formyl-FH4 are used to donate carbons.Requires adequate dietary folate.

52
Q

AMP–?-> IMP

A

AMP deaminaseNH4+ forms

53
Q

Converts ribose-5-phosphate to PRPP

A

PRPP synthetase

54
Q

megaloblastic anemia

A

cell division impaired due to inadequate folate

55
Q

The product of adenine degradation

A

hypoxanthine

56
Q

IMP

A

inosine monophosphate

57
Q

its synthesis is catalyzed by GPA

A

5-phosphoribosylamine

58
Q

the precursor for both AMP and GMP

A

IMP

59
Q

NAME?

A

Vitamine B12 or cobalamin

60
Q

If intracellular [PRPP] and [Gln] are usually below GPA’s Km, then an increase in their concentration leads to …

A

an increase in de novo purine synthesis

61
Q

Source of carbon for purine synthesis

A

two N10-formyl-FH4’s

62
Q

The product of guanine degradation

A

xanthine

63
Q

GPA

A

glutamine phosphoribosyl amidotransferase

64
Q

Impaired cell division as a result of inadequate folate

A

megaloblastic anemia

65
Q

PRPP reaction with glutamine to form

A

phosphoribosylamine

66
Q

ribose moiety

A

purine base is synthesized on it

67
Q

C8 and C2 are added by —— to the growing purine precursor.

A

Formyl-FH4

68
Q

committed step in purine synthesis

A

PRPP + Gln —> 5-phospshoribosylaminecatalyzed by GPA-glutamine phosphoribosyl amidotransferase

69
Q

xanthine oxidase requires —– to oxidize hypoxanthine.

A

molybdenum

70
Q

The base of the first purine nucleotide

A

Hypoxanthine

71
Q

PRPP synthetase

A

converts ribose-5-phosphate to PRPP, a ribose moiety on which purine is synthesized

72
Q

The degradation of adenine produces hypoxanthine, which is oxidized to xanthine by ———.

A

xanthine oxidase

73
Q

The reason that the first step of purine synthesis is regulated on the level of substrate

A

bc intracellular [PRPP] and [Gln] are usually below GPA’s Km.

74
Q

It can result in high turnover tissues damage, producing megaloblastic anemia, diarrhea and glossitis.

A

Vit. B12 deficiency in which folate gets sequestered as 5-methyltetrahydrofolate and can’t be used in purine synthesis.

75
Q

PRPP

A

5’-phosphoribosyl 1’-pyrophosphate

76
Q

AMP

A

Adenosine monophosphate

77
Q

Synthesis of phosphoribosylamine is catalyzed by

A

GPA- glutamine phosphoribosyl amidotransferase

78
Q

The condition in which folate gets sequestered as 5-methyltetrahydrofolate and cannot be used in purine synthesis.

A

B12 deficiency

79
Q

the reduction of the ribose moiety to deoxyribose occurs at ——— level, and is catalyzed by ——– which requires thioredoxin.

A

at diphosphate level and catalyzed by ribonucleotide reductase (RR)

80
Q

the base of inosine monophosphate (IMP)

A

hypoxanthine

81
Q

B12 deficiency and its impact on tetrahydrofolate

A

tetrahydrofolate can not be generated from 5- methyltetrahydrofolate

82
Q

the sequence of degradation of purine nucleotides

A

phosphate and ribose are removed first, then the nitrogenous base is oxidized.

83
Q

The first step in purine biosynthesis produces N9 of the purine ring and is inhibited by ——- and ——-.

A

AMP and GMP

84
Q

The first purine nucleotide

A

inosine monophosphate

Medical Biochemistry (12 decks)

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