25 - Purine Metabolism Flashcards
Describe the life cycle of a nucleotide monophosphate.
- Nucleobases are converted to nucleotides and then phosphorylated to become nucleotide monophosphate OR they are converted directly to nucleotide monophosphates with salvage enzymes and PRPP
- Nucleotide monophosphates are phosphorylated to nucleotide triphosphates, which can be incorporated into nucleic acid
- Nucleic acid is degraded by endonucleases to yield oligonucleotides
- Oligonucleotides are degraded into nucleotide monophosphates by phosphodiesterases
- Nucleotide monophosphates can then be converted back to nucleobases
- Nucleobases can be degraded to uric acid (purines) or beta-ureidopropionate (pyrimidine)
What is PRPP?
5-phosphoribosyl-α-pyrophosphate
- It is the product of teh first step in purine synthesis via PRPP synthetase and is required in pyrimidine biosynthesis.
- It is also involved in the salvage of free base. Eg:
guanine (or hypoxanthine or adenine) + PRPP = GMP + PPi
What is PRPP?
5-phosphoribosyl-α-pyrophosphate
- It is the product of teh first step in purine synthesis via PRPP synthetase and is required in pyrimidine biosynthesis.
- It is also involved in the salvage of free base. Eg:
guanine (or hypoxanthine or adenine) + PRPP = GMP + PPi
Five carbon sugars become PRPP. PRPP becomes inosinate (IMP), aka the purine ring. Describe the de novo biosynthesis of IMP
Give four enzymes in this pathway that can be inhibited (and what inhibits them)
- 11 Enzymatically catalyzed reactions build up the purine residue on a ribose ring.
Five carbon sugars become PRPP. PRPP becomes inosinate (IMP), aka the purine ring. Describe the de novo biosynthesis of IMP
Give four enzymes in this pathway that can be inhibited (and what inhibits them)
- 11 Enzymatically catalyzed reactions build up the purine residue on a ribose ring.
alpha-D-ribose-5-phosphate (RSP)
- Inhibited by allopurinol
PRPP
- allopurinol
- AMP
- GMP
GAR transformylase (THF cofactor) - Sulfonamide
AICAR transformylase (THF CF) - Sulfonamide
How is the formation of GMP and AMP from IMP regulated?
- AMP inhibits the formation of AMP by inhibiting adenylosuccinate
- GMP inhibits the formation of AMP by inhibiting IMP dehydrogenase
How is AMP and GMP produced from IMP
AMP
- IMP is converted to adenylosuccinate by aspartate via adenylosuccinate synthetase (requires GTP)
- Adenylosuccinate is converted to AMP via adenylosuccinate lyase
- AMP is phosphorylated to 2ADP with one ATP
GMP
- IMP is dehydrogenated to xanthosine monophosphate (XMP) via IMP dehydrogenase (requires NAD+)
- XMP is converted to GMP via GMP synthetase (requires ATP and glutamate)
- GMP is then phosphorylated to GDP via ATP
- GDP is then phosphorylated to GTP by another ATP
How is AMP and GMP produced from IMP
AMP
- IMP is converted to adenylosuccinate by aspartate via adenylosuccinate synthetase (requires GTP)
- Adenylosuccinate is converted to AMP via adenylosuccinate lyase
- AMP is phosphorylated to 2ADP with one ATP
GMP
- IMP is dehydrogenated to xanthosine monophosphate (XMP) via IMP dehydrogenase (requires NAD+)
- XMP is converted to GMP via GMP synthetase (requires ATP and glutamate)
- GMP is then phosphorylated to GDP via ATP
- GDP is then phosphorylated to GTP by another ATP
Purines are broken down to uric acid. What is a common metabolite to all purines that is the precursor to uric acid?
Give examples of how purine breakdown inhibition can be used to treat gout and how deficiency of an enzyme (name it) can cause severe combined immunodeficiency syndrome (SCID)
AMP = adenosine = inosine = hypoxanthine = xanthine
IMP = inosine = hypoxanthine = xanthine
XMP = xanthosine = xanthine
GMP = guanosine = guanine = xanthine
xanthine (common metabolite to all) is converted to uric acid via xanthine oxidase. Inhibition of xanthine oxidase by allopurinol is a treatment for gout. This will also lead to the accumulation of hypoxanthine from AMP/IMP metabolism.
The conversion of adenosine to inosine is catalyzed by adenosine deaminase, which is deficient in SCID.
What is allopurinol?
- Structural analog of hypoxanthine
- Xanthine oxidase hydroxylates allopurinol (as it does HX) to alloxanthine
- Alloxanthine binds tightly to the reduced form of xanthine oxidase, inhibiting it
- THis inhibition causes accumulation of hypoxanthine and xanthine (both are more soluble than uric acid and more readily excreted)
- Decreases uric acid formation
- Also used in conjunction with cancer chemotherapy (to combat increased uric acid production from cell breakdown)
Discuss adenosine deaminase deficiency
- Gives rise to severe combined immunodeficiency syndrome (SCID)
- Adenosine increases (along with dAMP, dADP and dATP)
Increase in dATP inhibits ribonucleotide reductase (RNR), thus decreasing synthesis of dGTP, dCTP and dTTP. This causes an overall decrease in DNA synthesis, particularly in T and B lymphocytes (causing SCID)
Several mutations in the breakdown of purines can lead to enzyme abnormalities in 3 types of gout. Give the three causes
- Elevated levels of PRPP synthetase (increased Vmax) leads to elevated purine production. Treated with allopurinol (inhibits PRPP synthetase)
- Loss of feedback inhibition of PRPP amidotransferase increases PRPP, AMP and GMP. Also inhibited by allopurinol.
- Decreased levels of HGPRT, which is an enzyme that can convert hypoxanthine and guanine to IMP and GMP, respectively, so that they are not converted to uric acid. Lesch Nyhan syndrome results (x linked, gout, arthritis, mental retardation, high PRPP, self mutilation etc.).
What does excess production of uric acid cause?
Discuss how different animals deal with the products of purine degradation.
Gout
- In humans and other primates, the final product of purine degradation is uric acid. Uric acid is excreted in urine.
- Terrestrial reptiles and many insects degrade purines to uric acid, but these organisms do not exrete urea, they also catabolize excess amino acid nitrogen to uric acid via purine biosynthesis.
- In all orther organisms, uric acid is further processed before excretion. Mammals oxidize it to allantoin. Dalmations lack normal urate oxidase (uricase) gene
- Bony fish oxidize uric acid to allantoic acid. Cartilaginous fish and amphibia degrade allantoic acid to urea.
- Marine invertebrates decompose urea to NH4
