Nucleotide Metabolism, Purine Degradation & Gout

Question 1

Nearly all nucleotide synthesis begins with which single activated molecule?

Accepted Answer

Phosphoribosyl pyrophosphate (PRPP)

Question 2

Which enzyme makes PRPP , and from what substrate?

Accepted Answer

PRPP synthetase , from ribose 5-phosphate (adding pyrophosphate from ATP )

Question 3

In purine de novo synthesis, is the ring built ON the ribose or assembled first and then attached?

Accepted Answer

Built directly ON ribose (atom by atom on PRPP )

Question 4

Which is the committed, key regulatory enzyme of purine de novo synthesis?

Accepted Answer

PRPP amidotransferase

Question 5

PRPP amidotransferase converts PRPP into which product, using which nitrogen donor?

Accepted Answer

5-phosphoribosylamine , using the amide nitrogen of glutamine

Question 6

PRPP amidotransferase is inhibited by which three nucleotides?

Accepted Answer

AMP, GMP, and IMP

Question 7

PRPP amidotransferase is allosterically activated by what?

Accepted Answer

PRPP (its own substrate)

Question 8

Pyrimidine de novo synthesis: is the ring assembled first or built on the ribose?

Accepted Answer

Ring assembled first (in cytoplasm ) as a free molecule, then attached to ribose

Question 9

Glutamine donates how many nitrogen atoms to the purine ring?

Accepted Answer

Two nitrogens

Question 10

Which amino acid donates two carbons and one nitrogen to the purine ring?

Accepted Answer

Glycine

Question 11

Which amino acid donates the single nitrogen contributed by an amino acid (other than Gln/Gly) to the purine ring?

Accepted Answer

Aspartate

Question 12

Which folate-derived carrier contributes two carbons to the purine ring (at two steps)?

Accepted Answer

Formyl-THF ( N10-formyl-THF )

Question 13

How many ATP are spent to go from ribose 5-phosphate to IMP in purine de novo synthesis?

Accepted Answer

6 ATP

Question 14

What is the pivotal branch-point nucleotide of purine synthesis, from which both AMP and GMP diverge?

Accepted Answer

Inosine monophosphate (IMP)

Question 15

Converting IMP to AMP requires which energy nucleotide (and which amino acid)?

Accepted Answer

GTP (plus aspartate )

Question 16

Converting IMP to GMP requires which energy nucleotide (and which amino acid)?

Accepted Answer

ATP (plus glutamine )

Question 17

Which enzyme carries out AMP + ATP → 2 ADP?

Accepted Answer

Adenylate kinase

Question 18

Pyrimidine synthesis starts with carbamoyl phosphate made by which enzyme?

Accepted Answer

Carbamoyl phosphate synthetase II (CPS-II)

Question 19

CPS-II synthesizes carbamoyl phosphate from which three inputs?

Accepted Answer

Glutamine, CO2, and 2 ATP

Question 20

CPS-II is activated by ___ and inhibited by ___.

Accepted Answer

Activated by PRPP ; inhibited by UTP

Question 21

Which CPS isoform is cytosolic, uses glutamine, and serves pyrimidine synthesis?

Accepted Answer

CPS-II

Question 22

CPS-I differs from CPS-II how (location, nitrogen source, activator, pathway)?

Accepted Answer

Mitochondrial ; uses NH4+ (ammonium) ; activated by N-acetylglutamate ; serves the urea cycle

Question 23

Carbamoyl phosphate reacts with aspartate and cyclizes to form which pyrimidine ring intermediate?

Accepted Answer

Orotic acid (orotate)

Question 24

Orotate monophosphate is decarboxylated to which 'mother' pyrimidine nucleotide?

Accepted Answer

UMP

Question 25

UTP is converted to CTP by which enzyme, using which nitrogen donor?

Accepted Answer

CTP synthetase , using glutamine

Question 26

How many ATP does it cost to go from ribose 5-phosphate to UMP ?

Accepted Answer

4 ATP (less than the 6 ATP for purines)

Question 27

Which enzyme converts all ribonucleoside diphosphates to deoxyribonucleoside diphosphates for DNA synthesis?

Accepted Answer

Ribonucleotide reductase (RNR)

Question 28

At which phosphorylation level does ribonucleotide reductase act (NMP, NDP, or NTP)?

Accepted Answer

The NDP level (reduces ADP, GDP, CDP, UDP to dADP, dGDP, dCDP, dUDP )

Question 29

What supplies the reducing power for ribonucleotide reductase (delivered via thioredoxin )?

Accepted Answer

NADPH

Question 30

At RNR 's activity site, which nucleotide turns the enzyme OFF and which keeps it ON?

Accepted Answer

dATP turns it OFF ; ATP keeps it ON

Question 31

Adenosine deaminase (ADA) deficiency causes which deoxynucleotide to accumulate?

Accepted Answer

dATP

Question 32

In ADA deficiency , accumulated dATP inhibits which enzyme, producing what disease?

Accepted Answer

Inhibits ribonucleotide reductase → blocks all dNDP synthesis → SCID (severe combined immunodeficiency)

Question 33

Which enzyme converts dUMP to dTMP , and using which one-carbon cofactor?

Accepted Answer

Thymidylate synthase , using N5,N10-methylene-THF

Question 34

In the thymidylate synthase reaction, the THF cofactor is oxidized to what, and which enzyme regenerates active THF?

Accepted Answer

Oxidized to dihydrofolate (DHF) ; regenerated to THF by dihydrofolate reductase (DHFR) using NADPH

Question 35

5-Fluorouracil (5-FU) is converted to which active metabolite that inhibits thymidylate synthase ?

Accepted Answer

F-dUMP

Question 36

What is the inhibition mechanism of F-dUMP on thymidylate synthase ?

Accepted Answer

Forms a covalent, irreversible ternary complex with the enzyme and methylene-THF ( suicide inhibitor )

Question 37

Methotrexate inhibits which enzyme, and how tightly relative to the natural substrate?

Accepted Answer

DHFR (dihydrofolate reductase) , binding ~1000x more tightly than DHF

Question 38

Why does methotrexate (DHFR inhibitor) block BOTH dTMP synthesis and de novo purine synthesis?

Accepted Answer

Without THF regeneration there is no methylene-THF for thymidylate synthase AND no formyl-THF for the purine ring steps

Question 39

Exam shortcut — which enzyme does each drug target: 5-FU vs methotrexate ?

Accepted Answer

5-FU → thymidylate synthase ; methotrexate → DHFR

Question 40

Which two phosphoribosyltransferases run the purine salvage pathway?

Accepted Answer

HGPRT and APRT

Question 41

HGPRT salvages which two bases, and into which nucleotides?

Accepted Answer

Hypoxanthine → IMP and guanine → GMP

Question 42

APRT salvages which base into which nucleotide?

Accepted Answer

Adenine → AMP

Question 43

Why does the brain depend heavily on purine salvage?

Accepted Answer

It expresses very low PRPP amidotransferase , so it cannot run de novo synthesis efficiently

Question 44

Lesch-Nyhan syndrome is caused by a complete deficiency of which enzyme?

Accepted Answer

HGPRT (hypoxanthine-guanine phosphoribosyltransferase)

Question 45

In Lesch-Nyhan , why does uric acid skyrocket?

Accepted Answer

Hypoxanthine/guanine can't be salvaged → PRPP accumulates → PRPP amidotransferase overactivated → de novo synthesis unchecked → excess urate

Question 46

Name the classic neurologic triad of Lesch-Nyhan syndrome .

Accepted Answer

Self-mutilation, choreoathetosis, and intellectual disability

Question 47

Which enzyme cleaves purine nucleosides into free base plus ribose 1-phosphate during degradation?

Accepted Answer

Purine nucleoside phosphorylase (PNP)

Question 48

In purine degradation, the adenine pathway passes through which intermediates to hypoxanthine ?

Accepted Answer

Adenosine → inosine → hypoxanthine

Question 49

In purine degradation, the guanine pathway passes through which intermediates to xanthine ?

Accepted Answer

Guanosine → guanine → xanthine

Question 50

Which enzyme oxidizes hypoxanthine to xanthine and then xanthine to uric acid (acting twice)?

Accepted Answer

Xanthine oxidase

Question 51

Xanthine oxidase requires which three cofactors?

Accepted Answer

FAD, molybdenum (Mo), and non-heme iron

Question 52

Why is uric acid the terminal purine catabolite in humans (unlike most mammals)?

Accepted Answer

Humans lack uricase , so uric acid cannot be degraded further

Question 53

At physiological pH 7.4, uric acid exists predominantly as which salt?

Accepted Answer

Monosodium urate (MSU)

Question 54

Under polarized light microscopy, monosodium urate crystals appear how?

Accepted Answer

Negatively birefringent

Question 55

Which joint is most classically affected in acute gout (podagra)?

Accepted Answer

The first metatarsophalangeal joint (big toe) — ~50% of attacks

Question 56

What is the single most common cause of hyperuricemia (~90% of gout cases)?

Accepted Answer

Decreased renal excretion of uric acid

Question 57

Lesch-Nyhan syndrome is the prototype of which mechanistic category of hyperuricemia?

Accepted Answer

The overproduction type (excess de novo purine synthesis)

Question 58

Allopurinol is a structural analogue of which purine base?

Accepted Answer

Hypoxanthine

Question 59

Xanthine oxidase oxidizes allopurinol to which metabolite, which then inhibits the enzyme?

Accepted Answer

Alloxanthine (oxypurinol) — a suicide inhibitor of xanthine oxidase

Question 60

Why does inhibiting xanthine oxidase with allopurinol reduce crystal formation?

Accepted Answer

Hypoxanthine and xanthine accumulate instead of uric acid, and they are more soluble (don't crystallize)

Question 61

Allopurinol slows the metabolism of which class of drugs (e.g., 6-mercaptopurine ), requiring what action?

Accepted Answer

Purine base analogues — their dose must be reduced to avoid toxicity

Question 62

Roughly what fraction of dietary purine bases is incorporated into tissue nucleic acids?

Accepted Answer

About 5%

Question 63

How does the fate of parenterally injected nucleotides/nucleosides differ from dietary ones?

Accepted Answer

Injected nucleotides/nucleosides ARE incorporated efficiently into body nucleic acids (dietary ones are mostly degraded in the gut)

Question 64

At ribonucleotide reductase 's SPECIFICITY site, which nucleotide directs reduction of ADP (to dADP )?

Accepted Answer

dGTP