Purine Metabolism Flashcards
Are nucleotides synthesized in the oxy or deoxy form?
oxy
if cell needs DNA synthesis, then oxy is converted to deoxy
Purine Nomenlature
example: adenine
adenine -> adenosine -> adenosine monophosphate
purine nucleosides end in “osine”
Why are drugs/antiviral analogues administered as nucleosides?
They can pass the plasma membrane. Once phosphorylated (nucleotide) have a negative charge and cannot.
What are the active form of nucleotides?
di and tri phosphates
nucleoside monophosphates can be converted to di/triphosphates via nucleoside monophosphate kinase (turn mono=>diphosphate)
AND
nucleoside diphosphate kinase (turn di=>triphosphate)
What is the purine proteome?
purine binding proteins comprise a family of 3-4,000 proteins (27-30,000 genes). 10% of proteins in body and 50% of druggable targets in biology.
Ex: kinases, helicases, reductases, transferases etc.
kinase inhibitor in leukemia- imatinib- 95% remission rate
What are the common purine bases?
Adenine 6-aminopurine
Guanine 2-amino, 6-oxy
Hypoxanthine 6-oxy
Xanthine 2,6-dioxy
nitrogen for all these inside ring are 1,3,7,9 positions.
what is a competitive inhibitor of adenosine?
caffiene
What are two pathways to make purines and pyrimidines?
de novo biosynthetic pathways: from non base molecules
salvage pathways: reutilization of bases from dietary or catabolic sources
In de novo synthesis of purines A & G, how is ribose-5-phosphate activated?
step 1: Ribose-5-phosphate (PRPP synthase + ATP) => PRPP + ATP
PRPP= 5-phosphoribosyl-1-pyrophosphate; inhibited by AMP, GMP, IMP
step 2: PRPP (amidophosphoribosyl transferase + glutamine) => 5-phosphoribosylamine + PPI+(glutamate)
rate limiting step* inhibited by AMP, GMP, IMP
Where do the atoms of IMP come from?
inosine monophosphate
glycine, 10 formyl THF, amide of glutamine, aspartate, CO2
What is the precursor for AMP and GMP?
IMP
inosine monophospate
How is AMP synthesized from IMP?
step 1: IMP (adenylosuccinate synthetase + aspartic acid + GTP) => adenylosuccinate
step 2: (adenylosuccinase) => AMP + fumarase
How is GMP synthesized from IMP?
IMP (IMP dehydrogenase + H2O + NAD) => xanthosine monophosphate
(GMP synthetase + Glutamine + ATP) => GMP + glutamate
How are AMP and GMP maintained at balanced stoichiometric levels?
When AMP is made, GTP is required. Therefore, is not enough GTP, you cannot make AMP.
When GMP is made, ATP is required…
What are the nitrogen sources for Adenine and Guanine?
hypoxanthine-IMP + ASPARTATE (N source) forms adenine
xanthine-XMP + GLUTAMINE (N source) forms guanine
common mechanism is conversion of carbonyl oxygen to amino group in A & G synthesis
What inhibits the synthesis of AMP and GMP?
Themselves (final product AMP and GMP respectively)
What is the mechanism behind the salvage pathway?
just one step to make A and G from old bases
PRPP + Adenine (APRT)=> adenylate/AMP
APRT- adenine phosphoribosyl transferase
PRPP + Guanine (HGPRT)=> guanylate/AMP
HGPRT- hypoxanthine guanine phosphoribyl transferase
What are the stages of nucleotide metabolism?
Nucleic acids can be broken down to endonucleotides and then to nucleoside monophosphates which can be converted back to nucleosides and nucleic acid synthesis
how are monophosphate nucleosides broken down?
First, nucleotidase cleaves and removes the phosphate
requires: H2O and ATP; A Pi is released
form nucleoside
Then, phosphorylase cleaves the glycosidic linkage
Pi from first reaction combines with free ribose=> ribose-1-phosphate
free nucleobases can either be degraded to uric acid OR enter the salvage pathway where PRPP is added back onto bases to form their respective nucleoside triphosphate
How are purines degraded to urate?
GMP undergoes cleavage by nucleotidase followed by phosphorylase:
GMP -> Guanosine -> GUANINE loses NH4+ -> Xanthine
*Note Adenine => IMP via adenosine deaminase (ADA)
AMP (adenylate deaminase)-> IMP (nucleotidase) -> inosine (phosphorylase) -> hypoxanthine (xanthine oxidase) -> Xanthine
Xanthine (xanthine oxidase) => Urate
*xanthine oxidase requires H2O and O2; hydrogen peroxide is a byproduct
*deaminase: removal of NH4+
What happens to Uric Acid? What happens if there is a build up of Uric Acid?
normally, most of it gets excreted in the urine.
If too much: Hyperuricemia: urate is not soluble in plasma or tissues. Precipitates as crystals which bind and activate macrophages. Cytokines produced -> sterile inflammation
What are two causes of hyperuricemia?
decreased excretion of urate: 80% gout cases
related to idiopathic, renal disease, diabetes insipidus, hypertension, down syndrome etc.
increased productino of urate: 20% gout cases
due to PRPP synthase overactivity, hemolytic diseases
also- HGPRT deficiency - Less Nychan disease which is exaccerbated by alcoholism
What happens when you are HGPRT deficient?
lose salvage pathway!
build up of PRPP and this pushes against inhibition by IMP AMP GMP
Therefore, shift more towards synthesis pathway
initial increase in synthesis- then increase degradation
*HGPRT has polymorphisms in general population allowing people to have different levels of it
What is the treatment for gout?
Allopurinol- “suicide inhibitor” for xanthine oxidase
similar structure to xanthine but N moved from 7 to 8 position
xanthine oxidase binds allopurinol- cannot form uric acid
What happens when you are VERY HGPRT deficient?
Lesch-Nyhan Syndrome
X-linked recessive
practically have no HGPRT, decreased IMP GMP, increased PRPP
severe hyperuremia: gouty arthritis, kidney stones, tophi (deposits)
neurological impairment: spasticity, overactive reflex
behavioral problems: cognitive dysfunction, agression, self-harm
What is SCID?
Severe Combined Immunideficiency Syndrome
Autosomal Recessive
due to deficiency in ADA (adenine -> IMP)
large buildup of deoxyATP toxic because they inhibit ribonucleotide reductase. They are especially toxic to T and B cells which die off=> decreased immune response
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