Biosynthesis of the 7-mercaptoheptanoic acid subunit of component B of methanogenic bacteria

Deuterium- and 13C-labeled precursors were used to establish the pathway for the biosynthesis of the 7-mercaptoheptanoic acid moiety of component B in methanogenic bacteria. The extent and position of the label incorporated into 7-mercaptoheptanoic acid were measured from the molecular and fragment ions in the mass spectrum of the methyl ester methylthiol derivative of the 7-mercaptoheptanoic acid. Deuterium from [2,2,2-2H3]acetate was found to be incorporated into four separate positions of 7-mercaptoheptanoic acid. One deuterium was equally distributed between the C-2 and the C-3 of the 7-mercaptoheptanoic acid, and the remaining three were at carbons 4-6. The extent of incorporation of the C-2 and C-3 positions was the same as that observed for the incorporation of [2,2,2-2H3]acetate into the α-ketoglutarate produced by the cells. [1,2-13C2]Acetate was incorporated into four separate sites of the 7-mercaptoheptanoic acid molecule. On the basis of this and additional information, it is concluded that 7-mercaptoheptanoic acid is biosynthesized from α-ketosuberate, which arises from α-ketoglutarate by repeated α-keto acid chain elongation. The mechanism for the conversion of α-ketosuberate to a thiol appears to be analogous to that for the conversion of sulfopyruvate to coenzyme M (2-mercaptoethanesulfonic acid)