Nucleotide metabolism Flashcards
Ribose phosphate pyrophosphokinase
ribose-5-phosphate+ATP–>5’-phosphoribosyl-1-pyrophosphate (PRPP)+AMP
requires Mg2+
PRPP used for de novo synthesis and salvage of purines and pyrimidines, synthesis of NAD+, histidine biosynthesis, and conversion of guanine to GMP
Activated by Pi
Inibited by Purine ribonucleotides (A/G/I M/D/T P)
Glutamine:phosphoribosyl pyrophosphate amidotransferase
PRPP (5’-phsophoribosyl-1-pyrophosphate) + Glutamine + H2O—>5’-phosphoribosylamine + Glutamate + PPi
Inhibitors: AMP, GMP, IMP
Activator: PRPP
First step of denovo purine synthesis; committed step; rate-limiting step
De Novo Purine synthesis
Constructing a purine ring on a preformed ribose-5-phosphate (as PRPP)
Overall uses 4 ATPs
Uses Aspartate, CO2, Glycine, Glutamine, N10-Formyl-tetrahydrofolate as sources of Carbons and Nitrogens
Tetrahydrofolate is especially important
End product is IMP
PABA analogues
aka Sulfonaminde/Sulfa drug
structural analoge of para-aminobezonic acid, an intermediate in bacterial synthesis of folic acid–>competitvely inhibits synthetic pathway (Dihdropeteroate synthetase)
Folic acid (N10-Formyl-tetrahydrofolate) is a required cofactor in purine synthesis so de novo purine synthesis is slowed in these bacteria.
Humans cannot synthesize folic acid, so purine synthesis is not affected
Folic Acid analogs
Including methotrexate and aminopterin (less common)
Inhibit reduction of dihydrofolate to tetraydrofolate catalyzed by dihydrofolate reductase
LImit amount of tetrahydorfolate (N10-Formyl-tetrahydrofolate) aroudn for purine synthesis thus slowing DNA replication
Used as an anti-cancer drug (toxic to all dividing cells), often used for leukemias
side effects: anemia, scaly skin, GI symptoms, respiratory tract disturbances, baldness
*Cancer cells can become resistant to MTX by amplification (duplication) of dihydrofolate reductase gene
Trimethoprim
Inhibits tetrahydrofolate synthesis in prokaryotes
Inhibits bacterial dihydrofolate reductase
used as prophalatic and for UTIs
Most common vitamin deficiency
Folic acid (B9)
-first seen as megaloblastic anemia–>Hb levels are low while blood marrow shows abnormally high number of megaloblastic cells (immature erythrocytes)
AMP synthesis
used to synthesize AMP from IMP, requires GTP
Adenylosuccinate Synthetase:
IMP + Aspartate + GTP—>Adenylosuccinate + GDP + Pi
Inhbited by AMP
Adenylosuccinase: Adenylosuccinate—>AMP + fumarate
Adenylosuccinate Synthetase:
Adenylosuccinate Synthetase:
IMP + Aspartate + GTP—>Adenylosuccinate + GDP + Pi
Inhbited by AMP
Adenylosuccinase
Adenylosuccinate—>AMP + fumarate
GMP synthesis
IMP dehydrogenase:
IMP + H2O + NAD+ ––>NADH + Xanthosine monophosphate (XMP)
inhibited by GMP and Mycophenolic Acid (deprives rapidly dviding B and T cells of nucleid acid components, used to prevent graft rejection)
GMP Synthetase:
XMP + ATP + Glutamine—>GMP + AMP + Glutamate
IMP dehydrogenase:
IMP + H2O + NAD+ ––>NADH + Xanthosine monophosphate (XMP)
inhibited by GMP and Mycophenolic Acid (deprives rapidly dviding B and T cells of nucleid acid components, used to prevent graft rejection)
Mycophenolic Acid
Mycophenolic Acid–>Inibits IMP dehydrogenase; deprives rapidly dviding B and T cells of nucleid acid components, used to prevent graft rejection
reversible, uncompetitive inhibitor
IMP + H2O + NAD+ ––>NADH + Xanthosine monophosphate (XMP)
also inhibited by GMP normally
GMP Synthetase:
GMP Synthetase:
XMP + ATP + Glutamine—>GMP + AMP + Glutamate
Purine Salvage:
2 enzymes:
Hypoxanthine-Guanine Phoshoribosyltransferase (HGPRT): Catalyzes 2 different rxns
Hypoxanthine + PRPP–>IMP + PPi
Guanine + PRPP–>GMP + PPi
(Lesch-Nyhan Syndrome)
Adenine Phosphoribosyltransferase (APRT)f
Lesch-Nyhan Dyndrome
- X-linked recessive, near complete defciency of HGPRT
- inability to salvage hypoxanthine and guanine
- accumulation of uric acid (elevated in urine), end product of hypoxanthine and guanine degredation
- Orange crystals (uric acid) in diaper
- increased PRPP, decreased IMP and GMP; Increased de novo purine synthesis
- symptoms include severe mental retardation, self-mutilation, ivoluntary movements, gout
Degredation of Purines
- Purines (and primidiens) are not essential components of the diet, most mononucleotides are synthesized de novo
- most purines degraded to uric acid rather than salvaged
- pyrimidies absorbed in intestinal mucosa dn returned to circulation (as are ribose and deoxyribose)
Adenosice Deaminase (ADA)
Adenosine (nucleoSide) + H2O––>Inosine + NH3
Part of purine degredation (Inosine is a better substrate for purine nucleotide phosphorylase than adenosine)
Implicated in SCIDS
SCIDs
Severe Combined Immunodeficiency
Deficiency of Adenosine Deaminase (ADA):
Adenosine–>Inosine
dATP accumulates in T-Cells and B-cells inhibiting DNA synthesis (inhibitor of Ribonucleotide reductase)
usually death before age 2 from infection
Purine Nucleoside Phosphorylase
PNP or PNP(ase)
- Prefers Inosine or guanosine as substrates but can act on andenosine
- Purine nucleoside + Pi—>Purine + ribose-1-phosphate
- deficieny causes T-cell related immunodeficiencies; decreased uric acid production; increased purine nucleosides and nucleotides
Xanthine Oxidase
2 reactions:
Hypoxanthine + O2 + H2O –––> Xanthine + H2O2
Xanthine + O2 + H2O –––> Uric Acid + H2O2
Can be inhibited by allopurinol
Gout
- Hyperuricemia–caused most commonly by impaired elimination in urine but also by increased production due to errors in purine metabolism–leads to deposition of uric acid crystals in joints
- Uric acid crystals lead to acute arthritic joint inflammation
- Treated with Allopurinol: xanthine oxidase non-competitive inhibitor; accumulation and excretionof hypoxanthine and xanthine which are more soluble than uric acid
- Colchicine-microtubule inhibitor; produces clinical improvement but does not decres serum uric acid; probably inhibits migrations of white cells to joints
- 1° gout is genetic anf affects mainly males over 30
- 2° gout brought on by disorders including leukemia, polycythemia, HGPRT deficiency treatment of cancer with antimetabolites, chronic renal insufficiency; affects men and women of any age
Pyrimidine synthesis
- pyrimidines synthesized before being attached to ribose-5-P (PRPP)
- Rate-limiting/committed step is CPS-II (Mammals: Inhibited by UTP, activated by ATP; Prokaryotes: aspartate transcarbamoylase is regulated step, inhibited by CTP)
- C’s and N’s come from Glutamine, Aspartic acid and CO2 (no folic acid derivatives)
- Final product is UMP
Carbamoyl Phosphate synthetase II (CPS II)–>Aspartate transbarbamoylase–>dihydroorotase–>dihydroorotate dehydrogenase (uses NAD+)–>orotate phosphoribosyl-transferase–>OMP decarboxylase
Carbamoyl Phosphate Synthetase II
2 ATP + CO2 + Glutamine—->Carbamoyl Phosphate + 2 ADP + 2 Pi + Glutamate
Mammals: Inhibited by UTP, activated by ATP
Prokaryotes:
CPS I vs CPS II
CPS I vs II
•urea synthesis vs pyrimidine synthesis
•free NH3 vs glutamine for N source
•liver mitochondria vs cytosol of all nucleated cells
•activated by N-acetylglutamate vs is not affected
•no affect vs inhibited by UDP/UTP
Orotic Aciduria
- Deficiency in orotate phosphoribosyltransferase and OMP decarboxylase (2 domains of a single polypeptide)
- abnormal growth, megaloblastic anemia, excretion of large amounts of orotic acid in urine
- (also caused by deficiency of OTC ins urea cycle (type II hyperammonemia) as excess carbamoyl phosphate gets shunted through pyramidine synthesis)
- rare genetic form can be treated with dietary uridine despite utilization of dietary nucleotides, enough is absorbed
- aquired form may appear in patients being treated for cancer with pyrimidine analog
UMP–>UTP
Uses NMP kinase and NDP kinase
CTP Synthase
UTP + Glutamine + ATP—->CTP + Glutamate + ADP + Pi
Ribonucleotide Diphosphate Reductase
converts adenosine, guanosine, cytosine, and uridine ribonucleotides to deoxyribonucleotides; Inhibited by dATP and hydroxyurea
NDP (except T) + Thioredoxin(2 S-H)—>dNDP + thioredoxin(S-S) + H2O
Thioredoxin reductase:
Thioredoxin(S-S; oxidized) + NADPH—>Thioredoxin(2 S-H; oxidized) + NADP+
Enzyme contains a single active site
2 activitiy sites may bind ATP (activates) or dATP (inhibits)
2 substrate specificity sites bind NTPs and increase conversion of different species of ribonucleotieds to maintain proper balance (ATP, dATP, dTTP, dGTP)
Thymidylate synthase
dUMP——>dTMP
N5, N10-methylTHF
(B9)
indirectly inhibited by MTX (methotrexate)
Directly inhibited by 5-fluorouracil–>(5-FdUMP) in cancer treatments
Purine and Pyrimidine analogs used in treatment of cancer
- 5-fluorouracil: makes uracil analog; usually given with thymidine (protects normal, non-cancerous cells and cancer cells too kinda); converted to 5-fluorodeoxyridylic acid (dFUMP) which inhibits thymidilate synthase (affects cancer and non-cancer equally); in cancer cells 5-fluorocells is incorporated into RNA which is more detrimental to cancer cells than normal cells–>the RNA is what really gets them
- bromodeoxyuridine: thymidine analog (large group)
- trifluorothyidine: thymidine analog
- AraC
Degradation fo Pyrmidines
- unlike purines, pyrimidines can be degraded to soluble precursors for other biomolecules
- Can be salvaged by pyrimidine phosphoribosyltransferase (usese PRPP)
