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Metabolism > Simmon's Purine and Pyrimidine Nucleotide Metabolism > Flashcards

Simmon's Purine and Pyrimidine Nucleotide Metabolism Flashcards

1
Q

What are the two purines?

A

adenine-nitrogen and guanine-oxygen

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2
Q

What are the three pyrimidines?

A

thymine-mthyl
uracil-oxygen
cytosine-nitrogen

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3
Q

Bases vs. nucleosides vs.nucleotides

A

base-base
nucleoside-base, sugar
nucleotide-base, sugar, phosphate

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4
Q

deoxyribose vs ribose

A

deoxyribose missing oxygen on carbon 2

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5
Q

Purines are nutritionally nonessential. What are the sources of each atom of the purine ring?

A

AA (aspartate, glutamine, glycine), tetrahydrofolate derivative, CO2

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6
Q

Is the purine ring built up atom by atom on top of the ribose?

A

Yes

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7
Q

What is the first step to making a purine?

A

activation of ribose-5-phosphate–>PRPP (5 phospho-ribose-1-pyrophosphoric acid)

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8
Q

What is the source of ribose-5-phosphate

A

pentose pathway

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9
Q

What is the commitment step and major regulated step to making a purine?

A

PRPP–>5-phospho-ribosylamine

Enzyme: glutamine PRPP amidotransferase

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10
Q

Which nucleotide stands at the branch point leading to synthesis of AMP(adenylic acid) and GMP(guanylic acid)

A

IMP (inosinic acid)

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11
Q

What are 4 ways purine synthesis is allosterically regulated?

A
  1. PRPP- activation of glutamine PRPP amidotransferase
  2. AMP and GMP-inhibition of glutamine PRPP amdiotransferase
  3. IMP-inhibition of glutamine PRPP amidotransferase
  4. AMP and GMP- inhibition from: IMP-XMP-GMP
    IMP-Adenylosuccinate-AMP
    -negative feedback
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12
Q

Why would an inhibitor of purine biosynthesis be useful in slowing tumor growth?

A

Tumors need purine biosynthesis for purine nucleotides for DNA and RNA synthesis (normal cells recycle)

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13
Q

What pathway does the anit-tumor agent 6-mercaptopurine inhibit?

A
  1. 6-mercaptopurine is converted to nucleotide (by salvage pathway)
  2. inhibits the enzymes in the purine biosynthetic pathway that catalyze step 2(main regulatory and commitment step), 12a, 12b, 13a (from IMP to AMP and GMP)
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14
Q

What provides the atoms for the pyrimidine rings?

A

Glutamine, Aspartate, CO2

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15
Q

What is the regulated step in pyrimidine synthesis?

A

Carbamoyl phosphate synthetase 2-pyrimidine ring structure is formed

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16
Q

Is the pyrimidine ring structure formed first then attached to a ribose-5-phosphate?

A

yes

17
Q

What is the precursor for synthesis of other pyrimidine nucleotides?

A

UMP

18
Q

Which metabolic pathway is blocked by rheumatoid arthritis drug leflunomide?

A

inhibits dihyroorotate dehyrogenase

19
Q

How is dTMP formed?
Enzyme
Cosubstrate

A

dUMP–>dTMP

enzyme: Thymidylate synthase
cosubstrate: N5, N10-methylene-tetrahydrofolate (goes to dihyrofolate)

20
Q

How does 5-fluorouracil act as an antitumor agent? 2 steps

A
  1. 5-fluorouracil–>f-dUMP (suicide inhibitor)

2. f-dUMP acts as a pseudosubstrate and covalently binds to and inhibits thymidylate synthase

21
Q

What happens to the dihydrofolate formed from tetrahydrofolate during the dUMP–>dTMP reaction

A

-the dihydrofolate is toxic and must be converted back to tetrahydrofolate by dihyrofolate reductase to be used in purine nucleotide synthesis again

22
Q

What does methotrexate do to dihydrofolate reductase?

A

it is a competitive inhibitor of dihydrofolate reductase and increases dihydrofolate and cuases intracellular tetrahydrofolate deficiency

23
Q

What is the enzyme responsible for the synthesis of deoxyribonucleotides and the substrates for this reaction?
What does this reaction do?
What is the cofactor?

A

Substrates: ADP, GDP, UDP, CDP

Enzyme: Ribonucleotide reductase

Cofactor: thioredoxin

Action: 2’-hydroxl group of ribose is replaced wiht a hydrogen atom

24
Q

How is deoxyribonucleotide biosynthesis regulated during the cell cycle?

A
  1. Deoxyribonucleotides required for DNA synthesis–>increases greatly during S phase
  2. ribonucleotide reductase and thymidylate synthase increase during late G1/early S phase (these two enzymes also correlate with tumor growth rate)
25
Q

What classes of enzymes are involved in degradation of DNA and RNA?

A

nucleases (DNA–>nucleoside)
nucleotidases (nucleoside–>nucleotide–losing P)
nucleoside phosphorylases (nucleoside–>bases–losing deoxyribose 1-phosphate)

26
Q

What bases are formed by degradation of DNA and RNA?

A

uracil
thymine
guanine
hypoxanthine (from adenine)

27
Q

90% of free purine bases are reutilized in a salvage pathway, what enzymatic reaction is involved in salvaging hypoxanthine and guanine?

A

Hypoxanthine-guanine phosphoribosyltransferase (HGPRTase)

Hypoxanthine+PRPP–>IMP +Pi
Guanine+PRPP–>GMP+ Pi

28
Q

Lesch-Nyhan syndrome

A

genetic deficiency in HGPRTase

  • neurologic problems: spasticity, metal retardation, compulsive self-mutilation
  • affects only males
  • also have gout
29
Q

10% of purine bases are not salvaged and are degraded to what?

A

uric acid

30
Q

What is the enzyme that catalyzes the converstion of hypoxanthine to xanthin and xanthine to uric acid?

A

xanthine oxidase

31
Q

What is the form of uric acid in the blood?

A

urate (higher than 5.8 ph)

32
Q

What is the form of uric acid in the urine?

A

uric acid (lower than 5.8pH)

Uric acid exists in both enol form and keto form (enol form can be deprotonated to form urate)

33
Q

Which form of uric acid is more soluble?

A

urate

34
Q

What is the consequence of overproduction or underexcretion of uric acid?

A

gout

Hyperuricemia-(elevated urate in the blood)-deposition of sodium urate crystals in joints–>resulting tophi can elicit painful acute inflammatory responses

Hyperuricosuria(elevated uric acid in urine) can lead to acid stones in the kidney)

35
Q

How does allopurinol relieve the symptoms of gout?

A

allopurinol is an inhibitor of xanthine oxidase

  • blocks formation of xanthine from hypoxanthine and uric acid from xanthine
  • reduces urate and uric acid in blood and urine–>shrinks tophi and renal uric acid stones

Metabolism (13 decks)

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