Studies on the biosynthesis of the Chlorobium chlorophylls

Abstract

The Chlorobium chlorophylls (660) from Chloropseudomonas ethylicum are shown by ¹³ C n.m.r. spectroscopy and certain chemical transformations to be meso -methylated at the δ-position. Earlier work, which proposed that the meso -alkyl group was present at the α or β positions, is shown to be experimentally correct, but incorrectly interpreted. On the basis of ¹⁴ C and ¹³ C feeding experiments, the novel methyl groups in the (660) chlorophylls are shown to be derived from methionine in all cases. For most of the homologous mixture of (660) chlorophylls, the branch point from the biosynthetic pathway to chlorophyll a appears to lie between uroporphyrinogen III and coproporphyrinogen III; earlier workers had suggested that Chlorobium chlorophyll biosynthesis proceeded through magnesium protoporphyrin IX mono-methyl ester and possibly also via bacteriochlorophyll a or one of its immediate precursors. Evidence against this proposal, and a working hypothesis explaining feeding results, is presented. On the basis of this hypothesis, proposals for the structures of certain fractions of the (660) chlorophylls which are currently in dispute, are presented.