Chlorophyll synthetase cannot synthesize chlorophyll a'

Chlorophyll synthetase catalyzes the last step of chlorophyll biosynthesis, namely prenylation (esterification) of chlorophyllide with phytyl diphosphate or geranylgeranyl diphosphate. During investigation of various chlorophyllide derivatives as potential substrates we observed lower esterification with increasing percentages of chlorophyllide a' in epimeric mixtures of chlorophyllides a and a'. To avoid epimerization during esterification, we studied the reaction in detail with model compounds [zinc-13(2)(R)-methoxy-pheophorbide a and zinc-13(2)(S)-methoxy-pheophorbide a, zinc-13(2)(R)-methoxy-pyropheophorbide a and zinc-chlorine6-13(1), 15(2)-dimethylester]. We conclude that compounds which have the 13(2)-carbomethoxy group at the same side of the macrocycle as the propionic side chain of ring D are neither substrates nor competitive inhibitors. Only compounds having the 13(2)-carbomethoxy group at the opposite site are substrates for the enzyme. Naturally occurring chlorophyll a' must be formed by epimerization after esterification.