Lecture 31 - Purines and Pyrimidines

What are the major purine and pyrimidine bases of nucleic acids?

Purine - Adenine and GuaninePyrimidine - Cytosine, Thymine, and Uracil

What links a base and a sugar?

N-glycosidic bond

What is the first step in the purine synthesis pathway?

Synthesis of PRPP using PRPP synthetase.

What is the committed step in purine synthesis?

Glutamine-PRPP amidotransferase enzyme

How is Gln-PRPP amidotransferase regulated?

Positive activators: cellular PRPP (major) and Glutamine (minor)Inhibitors: end products - AMP, GMP, and IMP (not XMP)

What are the differences between monomerization and dimerization for Gln-PRPP amidotransferase enzyme?

Monomerization leads to activation by PRPPDimerization leads to deactivation; product (AMP, GMP, IMP) inhibition

What enzyme takes IMP to XMP?

IMP dehydrogenase

What is the enzyme that converts IMP to adenylosuccinate and how is it catalyzed?

Adenylosuccinate synthetase and with the hydrolysis of GTP as a source of energy

What are the energy sources for IMP's conversions?

-IMP to AMP requires GTP (inhibited by AMP)-IMP to GMP requires ATP-XMP to GMP requires ATP

What are the major tissues in purine biosynthesis?

Liver and RBC (salvage)

What does pyrimidine biosynthesis result in?

Orotic acid

What is Orotic Aciduria? How is it treated?

UMP-synthase deficiency. (UMP synthase produces UMP from orotic acid)That can be treated oral feeding of uridine

What are the clinical features of UMP-synthase deficiency?

Orotic acid crystals in urine*, abnormal growthmegaloblastic anemia (cannot be corrected by foliac acid or vit B12)

What enzyme utilizes N5,N10-methylene tetrahydrofolate as a co-factor?

Thymidylate synthase(used in synthesis of deoxyribonucleotides)

What is ribonucleotide reductase?

Enzyme that catalyzes the formation of deoxyribonucleotides from ribonucleotides

What cofactor does ribonucleotide reductase use?

Thioredoxin (and NADPH as reductant)

What substrates are involved in the biosynthesis of deoxyribonucleotides from ribonucleotides?

ADP, GDP, CDP, and UDP (not TDP)

What do the different sites of ribonucleotide reductase do?

S - substrate specificity siteA - overall activity site. ATP activates overall activity and dATP inhibits overall activity here.C - catalytic site. Where enzymatic reduction of of ribonucleoside diphosphate to deoxyribonucleoside phosphate occurs.

How is the catalytic site, C controlled in ribonucleotide reductase?

Hydroxyurea by acting as a free radical and inhibiting it.

What are purine and pyrimidine salvage enzymes?

Purine: hypoxanthine-guanine phosphoribosyltransferase - HGPRT(Purine: adenine phosphoribosyltransferase)(Pyrimidine: pyrimidine phosphoribosyltransferase)

What causes Lesch- Nyhan syndrome?

When there is a purine salvage enzyme deficiency: HGPRT Clinical features: neurological, self mutilation, involuntary movements, and mental retardation.

What does xanithine oxidase do?

Catalyze the oxidation of Hypoxanthine (HX) to Xanthine (X) to Uric acid (UA)

What is allopurinol?

xanthine oxidase inhibitor (and reduces uric acid)

What is the mechanism of allopurinol?

Inhibits: Hypoxanthine -> xanithine -> uric acid

What enzymes are the primary causes of gout (excess UA)?

-Superactive PRPP synthetase-Superactive Gln: PRPP amidotransferase-Deficient HGPRT

When suffering from gout, what will most likely be the cause of hyperuricemia?

Von Gierkes (glycogen storage) renal deficiency A mutation in PRPP that: causes the enzyme to be feedback inhibited-increases enzyme Vmax-decreases Km for ribose-5-phosphate -a deficient HGPRT