SYLLABUS 18: Nucleotide Metabolism Flashcards
functions of purines and pyrimidines?
- active precursors of DNA, RNA
- activated intermedaites in CH (UDP) & lipid (CDP) metabolism & methylation (SAM)
- high energy intermediates per ATP and GTP
- metabolic regulators per cAMP and cGMP
- part of coenzymes, NAD, FAD, CoA, B12
difference between a nucleoside and nucleotide?
base + sugar = nucleoside
nucleoside + P = nucleotide
how does purine synthesis begin?
what regulates this step?
with PRPP, 5-phosphoribosyl-1-pyrsophosphate
this is synthesized from ribose 5-P, which was synthesized from PPP G6P, and ATP, via PRPP synthase, aka ribose P pyrophosphokinase
reaction is feedback inhibited by IMP, AMP, GMP
what is PRPP classification
an activated form of ribose 5 phosphate, as it’s created via ATP -> AMP
what is the rate limiting step of purine biosynthesis
PRPP + glutamine -> 5-phosphoribosyl-1-amine catalyzed by amido phosphoribosyltransferase
glutamine becomes glutamate
this enzyme is product inhibited by IMP, AMP, GMP
what’s the firstpurine nucleotide?
what is its structure
IMP, inosinate
produced by the de novo purine synthesis pathway
does not have the amino group that AMP/GMP has
what base does IMP contain
hypoxanthine
how does IMP become AMP?
- inosinate uses Asp + GTP to become adenylsuccinate
- adenylsuccinate becomes AMP, adenylate, releasing fumarate
* GMP can inhibit step 2 in its synthesis and this drives IMP to AMP*
how does IMP become GMP?
- Inosinate becomes Xanthylate by reducing NAD+ to NADH + H+
- Xanthylate becomes Guanylate, GMP, by using ATP -> AMP + PPi and Gln -> Glu
* AMP can inhibit step 1 of its synthesis to drive the reaction to GMP production *
how is there a balance between AMP and GMP synthesis
regulatory considerations by end product inhibition - AMP inhibits step 1 of its synthesis, and GMP inhibits step 2 of its synthesis, driving the reaction to produce the other purine
what does this show
AMP and GMP regulate their own synthesis by inhibiting their own synthesis if they are in high concentration
what happens to the AMP and GMP produced?
1) can degrade these purine bases as uric acid
2) can salvage these purine bases via salvage pathways
how do interconversions btwn forms of purines / prymidines occur?
reversible interconversions btwn mono, di, and tri P forms of purines and pyrimidines
and XMP -> XDP via nucleoside mono P kinase
and any XDP -> XTP via nucleoside di P kinase
what are salvage pathways
RNA is constantly degraded to free nitrogenous bases - i.e. adenine, guanine
if purines are degraded to free bases, they can be salvaged back to nucleotides rather than excreted
this saves energy because don’t need to synthesize more purines via energetically expensive de novo synthesis pathway
what are the salvage enzymes
- adenine phosphoribosyl transferase which catalyzes adenine -> AMP
- hypoxanthine-guanine phosphoribosyl transferase which catalyzes hypoxanthine -> IMP and guanine -> GMP
what causes Lesch-Nyhan syndrome?
how does it manifest?
absence of the salvage enzyme hypoxanthine-guanine phosphoribosyl transferase
thus cannot salvage either guanine (-> GMP) or hypoxynthine (-> IMP)
compulsive self-mutilation, mental retardation, aggressiveness, spasticity
what happens to non-salvaged purines?
excreted as uric acid
how does purine degredation -> excretion occur? describe mechanism
adenine -> hypoxanthine -> xanthine by xanthine oxidase
guanine -> xanthine also
xanthine is oxidized to uric acid by xanthine oxidase
uric acid is excreted into the urine
how does xanthine oxidase work?
when in its oxidase form, it produces superoxide radical and H2O2 during its oxidating reactions of purine degredfation
what chemically gout?
excess uric acid in the serum which forms crystalline, insoluble deposits of calcium and sodium urate in the joints
this is very painful, causes inflammation
what is gout associated with aka what causes it but not chemically speaking?
- high purine diets - eg liver
- deficiency in salvage enzymes b/c then get excess of purines undergoing breakdown to uric acid
- exacerbated by alcohol which increases lactic acid and competes w/ uric acid for excretion
- kidney disease - b/c cannot excrete uric acid
- ketoacidosis
how can you treat gout?
- decrease purine and alcohol intake in diet
- treat with a drug like allopurinol, colchicine, or methotrexate
how does allopurinol work?
gout drug treatment
it inhibits xanthine oxidase
this causes build up of hypoxanthine and xanthine, so these are excreted instead of uric acid, and these are much more soluble than uric acid
how do the drugs colchine and methotrexate work
they block inflammatory reactions and the immune system which become activated in response to high levels of uric acid caused by gout
this reduces inflammation
what is a nucleoside? nucleotide?
A nucleoside = base + sugar. Nucleotide = nucleoside + P
what is hypoxathine? inosine? are these normally found in RNA or DNA?
Hypoxanthine is the base for IMP. Inosine is the first purine nucleotide produced via the
de novo purine synthesis pathway. Hypoxanthine and Inosine are not found in RNA or
DNA – they are precursors to Adenine and Guanosine which are found in RNA and DNA.
why is folate important for purine synthesis?
Folate is important for purine synthesis because C2 and C8 of the purine ring come from
folic acid biochemistry, the N5 N10 methenyl compound. These C thus are from serine,
glycine, or histidine.
where does ribose sugar of nucleosides come from?
purine synthesis starts with an activated form of ribose called
5- phosphoribosyl-1-pyrophosphate (PRPP), which is synthesized from ribose 5-P and ATP as catalyzed by PRPP synthase (or ribose P pyrophosphokinase).
Recall that ribose 5-P is produced from G6P in the pentose cycle. PRPP synthase is feedback inhibited by IMP, AMP, GMP
what regulates the conversion of IMP to either AMP or GMP?
The conversion of IMP to either AMP or GMP is end-product inhibited by AMP and GMP
themselves; AMP inhibits its own production, and GMP inhibits its own production.
what is basis for the lesh-nyhan syndrome?
Lesch-Nyhan syndrome is caused by a lack in the purine salvage enzyme hypoxanthineguanine phophoribosyl transferase, which is responsible for purine salvage of guanosine (-> GMP) and hypoxanthine (-> IMP).
what are causes of gout and how is it treated
Gout is caused by excess uric acid in serum, which forms crystalline, insoluble deposits
of calcium and sodium urate in the joints. This is very painful and causes inflammation.
This is caused by several factors: high purine diet; deficiency in salvage enzymes; alcohol intake; kidney disease; and ketoacidosis. It is treated by: decreasing purine intake; decreasing alcohol intake; and treatment with a xanthine oxidase inhibitor, alluprinol or an anti-inflammatory drug such as colcihne or methotrexate.
what is orle of xanthine oxidase in repuerfusion injury
ATP makes hypoxanthine when it breaks
down. Hypoxanthine gets metabolized by xanthine dehydrogenase, which normally is
oxidized by an NAD-dependent enzyme, X-DH, the products being NADH + Uric acid.
However, in ischemic tissue, ATP isn’t being made, and there’s no oxygen. Thus lots of
ATP and GTP break down to these X and HX compounds. Additionally, Xanthine-DH is
converted to Xanthine Oxidase during ischemia. Xanthine Oxidase will oxidize Xanthine
or Hypoxanthine, but it uses Oxygen to do so, rather than NAD in the case of the DH.
This produces Uric Acid + Superoxide Radical. This causes a burst of superoxide radical,
thus causing a reperfusion injury.
