PURINES
Discuss the two reactions involved in the GMP Biosynthesis
- IMP dehydrogenase uses IMP and NAD - GMP synthetase uses glutamine and ATP to produce GMP
Describe the second and third reaction of the purine pathway
Second reaction (GAR synthetase), glycine is activated at carboxy group with ATP and transferred to NH2 group on PRA forming 5- phosphoribosylglycinamide (GAR) • Third reaction (GAR transformylase), activated formate (from N10- formyl-H4folate) attached to GAR forming 2 5'phosphoribosylformylglycinamide (FGAR)
Discuss the two reactions involved in the AMP Biosynthesis
• 2 reactions: - Adenylsuccinate synthase uses IMP, aspartate and GTP - Adenylsuccinase uses adenylsuccinate and H2O to produce AMP
Describe the Purine Nucleotide Cycle
• AMP normally synthesized from IMP (adenylosuccinate synthetase) and IMP can be salvaged from AMP (AMP deaminase) • Combining these 2 enzymes gives purine nucleotide cycle • Cycle has net effect of deaminating aspartate to fumarate • Very important in muscle cells • Allows RNA to act as metabolic reserve to replenish TCA cycle with fumarate under heavy metabolic demand
Describe Purine Catabolism
• Catabolism of purines ultimately leads to formation of uric acid • At physiological pH Uric acid is soluble and is secreted in urine as urate
Describe how the Abnormal catabolism of purines can cause disease
• Disorders vary from having mild symptoms to causing death • Clinical manifestations are due to build- up of purine degradation product uric acid as insoluble crystals of sodium urate • Excess accumulation of urate leads to deposition of crystals in joints • Disease is called GOUT
Describe the eighth and ninth reaction of the purine pathway
• Eighth reaction (adenylosuccinatelyase), fumarate is eliminated producing 5'-phophoribosyl-5-aminoimidazole-4-carboxamide (AICAR) • Ninth reaction (AICAR transformylase), The exo NH2 group is formylated by N10-formyl-H4folate giving 5'-phosphoribosyl-5-formamido imidazole-4- carboxamide (FAICAR)
Describe the final reaction of the purine pathway
• Final reaction (IMP cyclohydrolase), cyclisation occurs with loss of water, first purine formed Inosine-5'-monophosphate (IMP)
Describe the Nucleoside Salvage for purines
• For purines 2 distinct transferase enzymes - Hypoxanthine-guanine phosphoribosyl transferase (HGPRTase) Hypoxanthine + PRPP<---> IMP + PPi - Adenine phophoribosyltransferase (APRTase) Adenine + PRPP<----> AMP + PPi
Describe the Nucleoside Salvage for pyrimidines
• For pyrimidines only one transferase - Pyrimidinephosphoribosyl transferase pyrimidine + PRPP <--->pryrimidine-MP + PPi
Describe the fourth and fifth reaction of the purine pathway
• Fourth reaction (FGAM synthetase), amine group of glutamate condensed (using ATP) to form aminidine producing 5'phosphoribosylformylglycinamidine (FGAM) • Fifth reaction (AIR synthase), cyclisation occurs (using ATP) to produce imidizole ring producing 5'phosphoribosyl-5-aminoimidazole (AIR)
Describe the Severe Combined Immunodeficiency Disease (SCID)
• High concentration of dATP inhibits ribonucleotide reductase and prevents other dNTPs from being synthesized • DNA synthesis is inhibited, preventing rapid proliferation of lymphocytes • Immune responses seriously impaired • Disease is fatal in infancy unless special precautions are taken • Disease associated with loss of adenine deaminase and nucleoside phosphorylase activity
Discuss the Causes of Gout (Hyperuricemia)
• Kidney excretion problems • Elevated levels of blood sodium urate levels occur when: • Liver produces more uric acid than the body can excrete due to excess purine production • Diet of rich foods produces more uric acid than the body can excrete • Partial defect in scavenging enzyme hypoxanthine:guaninephosphoribosyl transferase (HGPRT)
Describe the Treatment of Gout
• Many gouts can be cured by administration of Allopurinol • Allopurinol is structural analog of hypoxanthine • Strongly inhibits Xanthine oxidase
Describe the Conversion of AMP & GMP to ATP >P
• PRPP synthase inhibited by purine 5'-nucleotides (mainly - AMP and GMP) Ribose 5-phosphate - - Low energy charge • Amidotransferase stimulated by PRPP - High energy charge • Amidotransferase inhibited by IMP, AMP and GMP (prevents excess purine biosynthesis) • Adenylsuccinate synthetase inhibited by AMP (prevents excess AMP) • IMP dehydrogenase inhibited by GMP (prevents excess GMP) • Excess GTP stimulates synthesis of ATP and visa versa (maintains ATP:GTP)
Summarise purines
• Purine biosynthesis involves 10 steps and feedback regulation is central to controlling this pathway • The purine nucleotide cycle allows RNA to act as an energy reserve • Salvage pathways involve the synthesis of nucleosides and nucleotides from their respective bases • Defects in purine catabolism can lead to disease
Describe the characteristics of the Lesch-Nyhan Syndrome
• Results from dysfunction of HGPRT gene • HGPRT located on X-chromosome and Lesch-Nyhan is virtually limited to males • Symptoms include severe gout and neurological problems (spasticity, mental retardation and self-mutilation) • Death occurs before age of 20
Describe the Nucleoside/Nucleotide Salvage pathway
• Salvage pathways involve synthesis of purine and pyrimidine nucleotides from their respective bases (nucleosides) • Free bases derived obtained from hydrolysis of cellular nucleic acids and from dietary intake
Describe the sixth and seventh reaction of the purine pathway
• Sixth reaction (AIR carboxylase), bicarbonate carboxylates the imidizole ring giving 5'phosphoribosyl-5-aminoimidizole-4-carboxylic acid (CAIR) • Seventh reaction (SAICAR synthetase), Aspartate is added to carboxyl group in reaction driven by ATP hydrolysis producing 5'-phosphoribosyl- 5-aminoimidazole-4-N-succinocarboxamide (SAICAR)
Describe the Symptoms of Gout
• Urate in blood crystallises in: • joint spaces • As stones in kidney and ureter • Swelling, inflammation, stiffness, intense arthritic pain and back pain • Gout usually effects the first metatarsal phalangeal joint of big toe (hallux), ankle joints, the carpometacarpal and intercarpal joints
Describe the characteristics of the von Gierke's Disease
• von Gierke's is glycogen storage disease resulting from lack of glucose-6- phosphatase • Excessive uric acid is produced • Increased levels of glucose-6- phosphate increases flux through pentose phosphate pathway • Ribulose-5-phosphate and PRPP levels are elevated • Increased PRPP activates purine biosynthesis ultimately leading to excess uric acid
How many reactions are involved in the purine biosynthesis pathway
10 reactions in purine biosynthesis pathway, unlike pyrimidine biosynthesis purines are assembled around the ribose •N1 from aspartate •C8 and C2 from N10-formyl- H4folate •N3 and N9 from glutamine •C4, C5 and N7 from glycine •C6 from CO2 (as bicarbonate)
Describe the first reaction of the purine pathway
First step, PRPP (from pentose phosphate pathway) is aminated using glutamine as amine donor by glutamine PRPP amidotransferase
