Purine/pyrimidine

Question 1

Overview of purine metabolism

Answer 1

Required for RNA/DNA, metabolism of carb/lipid/protein, regulatory, etc

Usu synthesized from glut, gly, asp, N5-formylTHF
- main regulation at PRPP amidotransferase (first committed step)
Ribose5P -(inhibited by IMP, AMP, GMP)> PRPP
PRPP -(inhibited by IMP, AMP, GMP, + by PRPP) -> 5-phosphoribosylamine ->&raquo_space;>
IMP -> AMP, GMP (each inhibit their own pathway)

Can be salvaged from dietary

Excess is degraded to uric acid and excreted

Question 2

Origin of purine base

Answer 2

Glycine -> central C-C-N
Glutamine -> neighboring N(x2)
N5-formyl-THF -> lateral C(x2)
Asp -> lateral N
CO2 -> medial C

Question 3

PRPP synthetase

Answer 3

PRPP = 5-phosphoribosyl 1-pyrophos
- used in both purine and pyrimidine biosynthesis
(and salvage pathways)

Ribose-5-phos -> PRPP
(+) Pi
(-) purines (IMP, AMP, GMP)

Question 4

PRPP amidotransferase

Answer 4

First committed step in purine biosynthesis

PRPP + glutamine -> 5-phosphoribosylamine + glutamate

(-) IMP, AMP, GMP
(+) PRPP

Question 5

Conversion of IMP

Answer 5

Regulation to keep purine pools balanced

IMP -adenylosuccinate synthease> -> AMP
(via adenylosuccinate, uses GTP, releases fumarate)
(-) by AMP

IMP -IMP dehydrogenase> -> GMP
(via xanthosine monophos, ATP->ADP, NAD->NADH, glutamine->glutamate
(-) by GMP

Question 6

Salvage pathways

Answer 6

Free base + PRPP -> nucleoside

• from diet
• from normal turnover
• same for purine and pyrimidine!

Hypoxanthine -HGPRT> IMP
Guanine -HGPRT> GMP
 (hypoxanthine-guanine phosphoribosyl trans)
Adenine -APRT> AMP
 (adenine phosphoribosyl trans)
Pyrimidine -PPRT> pyrimidine MP
 (pyrimidine phosphoribosyl trans)

Question 7

Purine degradation

Answer 7

AMP -> IMP and adnosine -> inosine ->
hypoxanthine -xanthine oxidase> xanthine
Guanine -aminohydrolase> xanthine

Xanthine -xanthine oxidase> uric acid

Uric acid normally secreted in urine

Question 8

Overview of pyrimidine metabolism

Answer 8

Most de novo from Glut, CO2, Asp
 - key regulation at CPSII 
  (-) UTP
  (+ ATP, PRPP)
 -> orotate -> uridine, cytidine, thymidine

Components:
Asp -> C-C-C-N
Glut (R group) -> N
CO2 -> C

Synthesize free (without ribose phosphate attached vs purine)
 -> PRPP adds ribose5P after ring is formed

Question 9

Orotate metabolism

Answer 9

Orotate + PRPP -> OMP
OMP -decarboxylase> uridine MP

Nucleotide monophos kinase -> UDP
Nucleotide diphos kinase -> UTP
UTP -> CTP (via synthase)

Question 10

Pyrimidine degradation

Answer 10

Ring opened -> soluble -> citric acid cycle

Uracil -> beta-alanine
Thymine -> beta-amino-iso-butyrate

Question 11

Ribonucleotide reductase

Answer 11

Converts ribonucleotide diphos:
NDP + thioredoxin (reduced = 2SH) -> dNDP + thioredoxin (oxidized = S-S)
(thioredoxin reduced again via NADPH)

Highly regulated (imbalance = mutagenic, lethal)
via expression (cell division)
specific substrates (ATP, dATP, dTTP, dGTP)
(-) dATP
(+) ATP

Question 12

Thymidylate synthase

Answer 12

UMP -> UDP -> dUDP -> dUMP
dUMP + N5N10-methylene-THF -> dTMP + DHF
(requires dihydrofolate reductase)

dTMP -> dTDP -> dTTP