Biosynthesis of the 7-mercaptoheptanoic acid subunit of component B [(7-mercaptoheptanoyl)threonine phosphate] of methanogenic bacteria

Abstract

Deuterium- and {sup 13}C-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-{sup 2}H{sub 3})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-{sup 2}H{sub 3})acetate into the {alpha}-ketoglutarate produced by the cells. (1,2-{sup 13}C{sub 2})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 {alpha}-ketosuberate, which arises from {alpha}-ketoglutarate by repeated {alpha}-keto acid chain elongation. The mechanism for the conversion of {alpha}-ketosuberate to a thiol appears to be analogous to that for the conversion of sulfopyruvate to coenzyme M (2-mercaptoethanesulfonic acid).