The microbial biosynthesis of methionine.
1. The enzymes leading to the methylation of homocysteine have been examined in three micro-organisms: a cobalamin-producing bacterium, Bacillus megaterium; a yeast, Candida utilis; and a basidiomycete fungus, Coprinus lagopus. The yeast and the fungus contain negligible endogenous cobalamin. 2. Extracts of each organism catalyse C(1)-transfer from serine to homocysteine with a polyglutamate folate coenzyme. 3. The enzymes generating the methyl group of methionine from C-3 of serine have similar properties in each case, but different mechanisms of homocysteine transmethylation from 5-methyltetrahydrofolates were found. 4. B. megaterium contains an enzyme with properties suggestive of a vitamin B(12)-dependent homocysteine transmethylase, whereas Cand. utilis and Cop. lagopus transfer the methyl group by a reaction characteristic of the cobalamin-independent mechanism established for Escherichia coli. 5. The specificity of each transmethylase for a 5-methyltetrahydropteroylpolyglutamate is consistent with the results of analyses of endogenous folates in these organisms, which showed only conjugated forms. 6. None of the extracts catalysed methionine production from S-adenosylmethionine and homocysteine. 7. These results are compared with results now available for methionine synthesis in other organisms, which show a considerable diversity of mechanisms.
D001410:Bacillus megaterium / Q000201:enzymology / Q000378:metabolism*; D001487:Basidiomycota / Q000201:enzymology / Q000378:metabolism*; D002175:Candida / Q000201:enzymology / Q000378:metabolism*; D002247:Carbon Isotopes; D002374:Catalase; D004926:Escherichia coli / Q000378:metabolism; D006710:Homocysteine; D008715:Methionine / Q000096:biosynthesis*; D008745:Methylation; D008780:Methyltransferases / Q000378:metabolism; D012694:Serine; D013762:Tetrahydrofolate Dehydrogenase / Q000378:metabolism
D002247:Carbon Isotopes; D006710:Homocysteine; D012694:Serine; D008715:Methionine; D002374:Catalase; D013762:Tetrahydrofolate Dehydrogenase; D008780:Methyltransferases