Aminotransferases methyltransferase reactions

Fig. 20.3 Pathway of methionine metabolism. The numbers represent the following enzymes or sequences (1) methionine adenosyltransferase (2) S-adenosylmethionine-dependent transmethylation reactions (3) glycine methyltransferase (4) S-adenosylhomocysteine hydrolase (5) betaine-homocysteine methyltransferase (6) 5-methyltetrahydrofolate homocysteine methyltransferase (7) serine hydroxymethyltransferase (8) 5,10-methylenetetrahydrofolate reductase (9) S-adenosylmethionine decarboxylase (10) spermidine and spermine synthases (11) methylthio-adenosine phosphorylase (12) conversion of methylthioribose to methionine (13) cystathionine P-synthase (14) cystathionine y-lyase (15) cysteine dioxygenase (16) cysteine suplhinate decarboxylase (17) hypotaurine NAD oxidoreductase (18) cysteine sulphintite a-oxoglutarate aminotransferase (19) sulfine oxidase. MeCbl = methylcobalamin PLP = pyridoxal phosphate...

Later experimental work provided evidence that the 8-carbon polyketoacid intermediate in the synthesis of y-coniceine is derived from octanoic acid since this acid was shown to be readily incorporated into coniine. Further work indicated that 5-keto-octanoic acid and 5-keto-octanal were produced during the biosynthesis of y-coniceine. A transaminase (L-alanine 5-keto-octanal aminotransferase) was obtained from C. maculatum. This transaminase catalyzes the reaction between 5-keto-octanal with L-alanine as the amino group donor to form the piperidine ring and the propyl side chain. Another C. maculatum alkaloid, A-methylconiine, was shown to be produced by another enzyme from the plant a coniine methyltransferase which acts as a transmethylator utilizing 5-adenosyl-L-methionine as a methyl group donor. 

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