The biosynthesis of spermidine. Part 3: the stereochemistry of the formation of the N-CH2 group in the biosynthesis of spermidine
Abstract
The stereochemistry of the formation of spermidine from butane-1,4-diamine and decarboxylated adenosylmethionine in Escherichia coli is shown to be inversion. That is to say, spermidine synthase mediates an SN2 displacement in which the nucleophile is an amino group of butane-1,4-diamine, the carbon centre undergoing substitution with inversion is C-l of the 3-aminopropyl group of decarboxylated adenosylmethionine and the leaving group is 5-methylthioadenosine. This conclusion is based on the analysis, with the help of high-field 1H n.m.r. spectroscopy, of the relative stereochemistries of derivatives of the [1′,2′-2H2]spermidines derived from [3,4-2H2] methionines of known relative configuration.