Nucleotide Metabolism

Question 1

Purine Precursors

Answer 1

Aspartate, Glutamate, Glycine, THF, CO2

Question 2

Purine synthesis activation

Answer 2

Formation of PRPP from Ribose phosphate via the enzyme PRPP synthetase (ribose phosphate pyrophosphokinase)

Question 3

Phosphoribosyl Pyrophosphate

Answer 3

PRPP

Involved in the synthesis of purines pyrimidines, in the salvage of purines and pyrimidines,in the synthesis of NAD+, histidine biosynthesis, and the conversion of guanine to GMP

Question 4

Glutamine PRPP Amidotransferase

Answer 4

Enzyme in the first and rate limiting step of purine de novo synthesis. Adds an amine group via glutamine.

Forms Phosphoribosylamine from PRPP. requires glutamine and Mg++ as cofactors

Requires 4 ATP

Activated by PRPP
Inhibited by AMP, GMP, IMP

Question 5

Parent Purine nucleotide

Answer 5

Inosine Monophosphate (IMP) formed in purine de novo synthesis

Question 6

Key points in Purine de novo synthesis

Answer 6

Activation take place first forming PRPP

Rate limiting enzyme is Glutamine PRPP Amidotransferase

Ultimately forms the primary/ parent purine nucleotide IMP

Multiple reactions forming IMP. Requires 2 THF and 4 ATP

Question 7

Sulfonamides

Answer 7

Competitive inhibits the synthesis of folic acid in microorganisms thereby decreasing the synthesis of nucleotides needed for DNA replicatioin

Question 8

Methotrexate

Answer 8

Competitively inhibits Dihydrofolate reductase.

a folic acid analogue used to treat psoriasis, arthritis, and neoplastic diseases

most common nonspecific anti-cancer teatment

Question 9

Trimethoprim

Answer 9

Inhibits THF synthesis in prokaryotes

Question 10

Conversion of IMP to AMP and GMP

Answer 10

AMP pathway:

IMP to adenylosuccinate via adenylosuccinate synthetase. requires GTP

adenylosuccinate then forms AMP with the release of fumarate

GMP Pathway:
IMP forms Xanosine monophosphate via IMP dehydrogenase (requires NAD+)

Then forms GMP via GMP synthase requiring ATP

Question 11

Mycophenolic Acid

Answer 11

Reversible noncompetitive inhibitor of IMP dehydrogenase (GMP formation)

Used in graft rejection to deprive T and B cells

Question 12

Salvage Pathway of Purines

Answer 12

Purines from normal cellular turnover (de novo synthesis) are added to sugars in a one step process.

Involves 2 enzymes dependant on the Nucleotide formed.

HGPRT forms IMP and GMP from Hypoxanthine and glutamine respectively

APRT forms AMP from Adenine

Question 13

HGRPT

Answer 13

hypoxanthine-guanine phosphoribosyltransferase

Forms IMP from hypozathine in salvage pathway of purines

Forms GMP from glutamine in salvage pathway of purines

Question 14

APRT

Answer 14

adenine phosphoribosyltransferase

forms AMP from adenine in salvage pathway of purines

Question 15

Lesch-Nyhan Syndrome

Answer 15

Deficiency of HGPRT in purine salvage pathway

Degradation of hypoxanthine and guanine to uric acid. (excess uric acid)

causes severe mental retardation, self-mutilation, and involuntary movements, gout.

causes increased PRPP levels which increases de novo purine synthesis.

X-Linked

Orange crystals often found in baby’s diapers

Question 16

Degradation of Purines

Answer 16

Purines from diet and those not used for salvage are ultimately degraded to uric acid.

Important reactions:
Adenosine forms Inosine via adenosine deaminase

Inosine ultimately forms Xanthine that is broken down to Uric acid via Xanthine oxidase

Guanine forms Xanthine via Guanase and PNP

Question 17

SCIDS

Answer 17

Severe Combined Immunodeficiency Syndrome

Deficiency in Adenosine Deaminase (in adenosine degradation)

T and B cell dysfunction caused by buildup of dATP which inhibits ribonucleotide reductase therefore inhibiting DNA synthesis

Question 18

PNP Deficiency

Answer 18

Purine nucleoside phosphorylase deficiency in purine degradation. (IMP and GMP)

characterized by decreased uric acid production and increased purine nucleosides and nucleotides. Impairment of T-Cells

Question 19

Gout

Answer 19

Characterized by hyperuricemia, arthritic inflammation caused by urate crystals

Primary gout results in problems in excreting uric acid.

Secondary gout caused by leukemias, increaed RBC mass, HGPRT deficiency, chronic renal insufficiency

Question 20

Colchicine

Answer 20

Treatment of Gout

microtubule inhibitor

Does produce clinical improvement but does not
decrease serum uric acid levels.

Inhibits migration of white cells to joints where
uric acid crystals are phagocytized

Question 21

Allopurinol

Answer 21

Gout treatment

Noncompetitive inhibitor of Xanthine Oxidase. results in secretion of xanthine and hypoxanthine instead of urate.

Question 22

Pyrimidine precursors

Answer 22

Aspartic Acid, Glutamine

Question 23

Carbamoyl Phosphate Synthetase II

Answer 23

CPS II

First and regulatory step in Pyrimidine synthesis

Forms Carbamoyl Phosphate from Glutamate and CO2

Activated by ATP and PRPP
Inhibited by UTP

Question 24

Key points in Pyrimidine synthesis

Answer 24

Glutamate and CO2 form Carbamoyl Phosphate via CPS II

Carbamoyl ultimately forms Orotate

Orotate forms Orotidine monophosphate (OMP) via orotate phosphoribosyl-transferase (orotate added to a ribose sugar)

OMP forms UMP via OMP decarboxylase (CO2 removed from the pyrimidine)

Deficiency in the last two enzymes results in Orotic aciduria - treated by dietary uridine

Question 25

CPS I vs CPS II

Answer 25

Carbamoyl phosphate synthetase I

• involved in urea synthesis
• uses free ammonia as the source of nitrogen
• occurs in liver mitochondria
• activated by N-acetylglutamate

Carbamoyl phosphate synthetase II

• involved in pyrimidine synthesis
• uses glutamine as the source of nitrogen
• occurs in the cytosol of all nucleated cells
• is inhibited by UDP and UTP

Question 26

Megaloblastic Anemia

Answer 26

Results from inhibition of DNA synthesis during red blood cell production.

Marked appearance of enlarged and fewer RBCs.

Caused by:
Vitamin B12 and folate deficiency
Orotic Aciduria
Drugs - methotrexate, AraC, etc.

Question 27

Synthesis of CTP from UTP

Answer 27

UTP is converted to CTP by CTP synthase. requires glutamine (as the nitrogen donor) and ATP.

Question 28

Ribonucleotides to Deoxyribonucleotides

Answer 28

Catalized by ribonucleotide reductase

ATP increases activity

Inhibited by dATP and hypoxyurea (cancer treatment drug)

Question 29

Synthesis of Thymidine

Answer 29

UMP is fist converted to dUMP

dUMP is converted to dTMP (thymidine) via Thymidylate synthase requiring THF

inhibited by methotrexate and 5-flurouracil

Question 30

Flurouracil

Answer 30

Inhibits Thymidylate synthase in dTMP synthesis.

Used as an anticancer drug

Question 31

Pyrimidine phosphoribosyltransferase

Answer 31

Enzyme in pyrimidine salvage where PRPP is the ribose phosphate source