Nucleic Acid Synthesis & Metabolism

Question 1

How do sulfa antibiotics work?

Answer 1

Competitively inhibit bacterial synthesis of folic acid, which is required for purine synthesis; does not affect human purine synthesis pathways because humans cannot synthesize folic acid and must take it in from the environment

Question 2

How does methotrexate work?

Answer 2

Inhibits the reduction of dihydrofolate to tetrahydrofolate catalyzed by dihydrofolate reductase; inhibits purine synthesis and thus slows down DNA replication in all mammalian cells (i.e. cancer as well as all dividing cells)

Question 3

PRPP Synthetase

Answer 3

Catalyzes the conversion of ribose-5-phosphate to PRPP in the purine synthesis pathway

Question 4

Glutamine phosphoribosyl pyrophosphate amidotransferase

Answer 4

Adds the first nitrogen group to PRPP during the purine synthesis pathway using glutamine as a nitrogen donor

Question 5

What is the first base produced in the purine synthesis pathway?

Answer 5

IMP

Question 6

Carbamoyl phosphate synthetase II

Answer 6

Catalyzes the first step of pyrimidine base synthesis, which is conversion of 2 ATP, CO2 and Glutamine to Carbamoyl Phosphate and Glutamate

Question 7

What is the first nucleotide produced in the pyrimidine synthesis pathway?

Answer 7

UMP

Question 8

CTP Synthase

Answer 8

Catalyzes the conversion of UTP to CTP in the pyrimidine synthesis pathway; Gln is the nitrogen donor, generating Glu

Question 9

Thymidylate synthase

Answer 9

Catalyzes the conversion of dUMP to dTMP

Question 10

Fates of dCDP

Answer 10

Can either be phosphorylated to dCTP in the DNA pyrimidine synthesis pathway or can be dephosphorylated to dCMP and then converted to dUMP for eventual production of dTTP via thymidylate synthase

Question 11

Ribonucleotide reductase

Answer 11

Catalyzes the production of dNDPs from NDPs; redox reaction requires thioredoxin and NADPH

On/off switch is active in the presence of ATP and inactive in the presence of dATP

Substrate specificity site changes specificity as concentration of one dNTP builds up; ensures equal amounts of NDP are converted to dNDP

Question 12

Severe Combined Immunodeficiency Syndrome (SCID)

Answer 12

Caused by enzymatic deficiency of adenosine deaminase (ADA) in the purine metabolism pathway, which normally functions to break down adenosine base for conversion to uric acid

Results in large build-up of dATP in blood cells which inhibits ribonucleotide reductase and therefore DNA synthesis; rapidly dividing cells (i.e. immune cells) are most affected

Question 13

Gout

Answer 13

Caused by under-excretion of uric acid resulting in hyperuricemia with recurrent attacks of acute, arthritis joint inflammation caused by deposition of mono-sodium urate crystals

Treated with allopurinol, which inhibits xanthine oxidase in the purine metabolism pathway, reducing uric acid production; causes accumulation of xanthine and hypoxanthine which are more soluble intermediates in the breakdown of purines

Question 14

Lesch-Nyhan Syndrome

Answer 14

Caused by deficiency in the hypoxanthine-guanine phophoribosyltransferase (HGPRT) enzyme of the purine salvage pathway which normally shunts hypoxanthine and guanine back into purine synthesis; results in hyperuricemia and higher rates of de novo purine synthesis

Patients have severe intellectual disability including self-mutilating behavior; also associated with gout

Question 15

Pyrimidine Breakdown Pathway

Answer 15

Pyrimidine is broken down into ribose and base ring; the base ring is then opened up and can be converted to molecules that can be recycled for use in other pathways, i.e. Succinyl coA, Malonyl coA, and Acetyl coA

Question 16

5-fluorouracil

Answer 16

Competitive inhibitor of thymidylate synthase; blocks tne enzymatic conversion of dUMP to dTMP and therefore inhibits nucleotide synthesis; used as anti-cancer chemotherapy

Question 17

Purine metabolism pathway

Answer 17

Guanosine and adenosine are degraded via a common xanthine intermediate; xanthine oxidase converts xanthine to uric acid, which is excreted in the urine

Question 18

Key differences in purine and pyrimidine nucleotide de novo synthesis (3)

Answer 18

1. Purine base is made on the ribose sugar; pyrimidine ring is synthesized then attached to ribose sugar
2. Initial nucleotide product in purine synthesis is IMP; initial nucleotide product in pyrimidine synthesis is UMP
3. In purine synthesis, IMP is converted to GMP and AMP as monophosphates; in pyrimidine synthesis, UTP is converted to CTP as a triphosphate

Question 19

Hypoxanthine-guanine phosphoribosyl transferase (HGPRT)

Answer 19

Catalyzes the salvage reaction of hypoxanthine + PRPP to produce IMP, and guanine + PRPP to produce guanylate

Defect leads to Lesch-Nyhan Syndrome, characterized by increased uric acid production and increased reliance on de novo purine synthesis