Nucleotide Metabolism Flashcards
Purine Precursors
Aspartate, Glutamate, Glycine, THF, CO2
Purine synthesis activation
Formation of PRPP from Ribose phosphate via the enzyme PRPP synthetase (ribose phosphate pyrophosphokinase)
Phosphoribosyl Pyrophosphate
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
Glutamine PRPP Amidotransferase
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
Parent Purine nucleotide
Inosine Monophosphate (IMP) formed in purine de novo synthesis
Key points in Purine de novo synthesis
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
Sulfonamides
Competitive inhibits the synthesis of folic acid in microorganisms thereby decreasing the synthesis of nucleotides needed for DNA replicatioin
Methotrexate
Competitively inhibits Dihydrofolate reductase.
a folic acid analogue used to treat psoriasis, arthritis, and neoplastic diseases
most common nonspecific anti-cancer teatment
Trimethoprim
Inhibits THF synthesis in prokaryotes
Conversion of IMP to AMP and GMP
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
Mycophenolic Acid
Reversible noncompetitive inhibitor of IMP dehydrogenase (GMP formation)
Used in graft rejection to deprive T and B cells
Salvage Pathway of Purines
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
HGRPT
hypoxanthine-guanine phosphoribosyltransferase
Forms IMP from hypozathine in salvage pathway of purines
Forms GMP from glutamine in salvage pathway of purines
APRT
adenine phosphoribosyltransferase
forms AMP from adenine in salvage pathway of purines
Lesch-Nyhan Syndrome
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
Degradation of Purines
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
SCIDS
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
PNP Deficiency
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
Gout
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
Colchicine
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
Allopurinol
Gout treatment
Noncompetitive inhibitor of Xanthine Oxidase. results in secretion of xanthine and hypoxanthine instead of urate.
Pyrimidine precursors
Aspartic Acid, Glutamine
Carbamoyl Phosphate Synthetase II
CPS II
First and regulatory step in Pyrimidine synthesis
Forms Carbamoyl Phosphate from Glutamate and CO2
Activated by ATP and PRPP
Inhibited by UTP
Key points in Pyrimidine synthesis
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
CPS I vs CPS II
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
Megaloblastic Anemia
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.
Synthesis of CTP from UTP
UTP is converted to CTP by CTP synthase. requires glutamine (as the nitrogen donor) and ATP.
Ribonucleotides to Deoxyribonucleotides
Catalized by ribonucleotide reductase
ATP increases activity
Inhibited by dATP and hypoxyurea (cancer treatment drug)
Synthesis of Thymidine
UMP is fist converted to dUMP
dUMP is converted to dTMP (thymidine) via Thymidylate synthase requiring THF
inhibited by methotrexate and 5-flurouracil
Flurouracil
Inhibits Thymidylate synthase in dTMP synthesis.
Used as an anticancer drug
Pyrimidine phosphoribosyltransferase
Enzyme in pyrimidine salvage where PRPP is the ribose phosphate source
