BIOCHEM L9 (KINGWAL) (PURINE, PYRIMIDINE, AND SINGLE-CARBON METABOLISM)

The addition of a pentose sugar to a base through a glycosidic bond produces a ___________

nucleoside

The addition of one or more phosphate groups to a nucleoside produces a ___________

nucleotide

T/F: Purine and pyrimidine bases found in nucleotides can be synthesized de novo or can be obtained through salvage pathways that allow the reuse of the preformed bases resulting from normal cell turnover.

True

Purines and pyrimidines fun facts

1. High-energy substrates for many anabolic reactions

2. Precursors for DNA and RNA synth

3. Function in intracellular signaling (cAMP)

4. Contribute to the structure of several coenzymes such as NAD+, FAD, CoA

T/F: Both purines and pyrimidines are produced in adequate amounts from de novo synthesis, no dietary requirement exists.

True

Salvage pathways do what?

Recycle unnecessary purines/pyrimidines acquired from diet; allow them to be used for resynthesis of nucleotides

Purine synthesis is mediated by what enzyme

PRPP "Activated pentose"

Synthesis of PRPP

From ATP and Ribose 5-phosphate, catalyzed by PRPP synthetase

PRPP is ______-linked, activated by _________ and inhibited by _______

X-linked, activated by inorganic phosphate, inhibited by purine nucleotides

Formation of IMP from?

Amino acids (Gly, Glutamine, Asp)

CO2

Single-carbon Folate

T/F: Inosine monophosphate is the first purine intermediate with an intact ring.

True

T/F: Increased throughput from PhosphoRibosylamine to IMP is characteristic of gout.

True

GTP is required for _____ synthesis

AMP synthesis

ATP is required for _________ synthesis

GMP

The end products in GMP and AMP synthesis act to _________ (slow, catalyze) their own synthesis.

Slow their own synthesis

Hypoxanthine and Guanine salvage pathways share what enzyme?

HGPRT (hypoxanthine-guanine phosphoribosyl transferase)

IMP, AMP and GMP inhibit

PRPP

Synthetic inhibitors of purine synthesis?

Sulfonamides

Folic acid analogues

Folic acid analogues example and purpose

Methotrexate; control the spread of cancer by interfering with nucleotide synthesis and therefore of DNA and RNA

Sulfonamides MOA

inhibit the growth of rapidly dividing microorganisms without interfering with human cell functions

T/F: The active form of folic acid, tetrahydrofolic acid (THF) is produced by dihydrofolate reductase.

True

Folate deficiency can inhibit purine synthesis how?

Megaloblastic anemia (folate deficiency) causes a decreased availability of purines and TMP required for DNA synthesis.

Methotrexate MOA

inhibits dihydrofolate reductase via competitive inhibition

End product for the degradation of all purines is?

Uric acid, excreted via urine

Adenine degradation can occur through multiple routes, which all converge into the formation of

inosine

Inosine (nucleoside) is converted to what by the removal of the ribose sugar

hypoxanthine

Hypoxanthine is acted on by what enzyme to form xanthine

xanthine oxidase

Xanthine can also be formed from guanine via what

deamination

Xanthine is acted on by xanthine oxidase to form what

uric acid

Xanthine oxidase reaction produces hydrogen peroxide which is converted to water and oxygen by what enzyme

catalase

Von Gierke's disease

Buildup of PRPP due to an increase in nonoxidative branch of PPP

Glycogen storage disease type I (GSD I)

Inherited disorder caused by deficiencies in g6p (in GSD Ia) and g6t (in GSD Ib) in the glycogen metabolism pathway

Increase in concentration of PRPP leads to what?

Overproduction of purines -> elevation of uric acid and gout

Tissue damage in gout is caused by?

Precipitation of needle-shaped Na+ urate crystals

Treatment of gout?

Colchicine: anti-inflammatory; prevents neutrophil migration and phagocytosis

Allopurinol: xanthine oxidase inhibitor

Lesch-Nyhan syndrome

Deficiency in salvage enzyme HGPRT

Gout, self-mutilation, mental retardation

Elevated uric acid levels

Adenosine deaminase (ADA) deficiency

ADA deficiency produces severe combined immunodeficiency disease (SCID)

Lymphocytes undergo cell division during a normal immune response, slowed synthesis of DNA leads to cell death and immune deficiency

Pyrimidine synthesis rate limiting step

formation of carbamoyl phosphate;

*Rate limiting enzyme; carbamoyl phosphate synthetase

NAG stimulates the mitochondrial form of what enzyme

Carbamoyl phosphate synthetase (CPS) of the urea cycle

NAG _______ (is, is not) an allosteric activator of cytoplasmic CPS involved in pyrimidine synthesis.

IS NOT

THF contributes __________ carbons to the Purine ring structure as well as the methyl functional group on Thymine.

Single carbons

Can UMP be converted to TMP by thymidine kinase?

Nope

Deoxyribose is produced by _________ of ribonucleoside diphosphatases

reduction

Ribonucleotide diphosphatases are recognized by what enzyme

ribonucleotide reductase

Cofactor for ribonucleotide reductase?

Thioredoxin

Oxidized thioredoxin is recycled back to its reduced form by

thioredoxin reductase; requires NADPH

NADPH generated by the PPP is used to regenerate

reduced thioredoxin

Purine analog examples

Methylxanthines (caffeine, theobromine, theophylline)

6-thioguanine

6-mercaptopurine

Pyrimidine analogs

5-fluorouracil

Bromodeoxyuridine

Anti-HIV drugs