Chloroplast biogenesis: biosynthesis of protochlorophyll(ide) via acidic and fully esterified biosynthetic branches in higher plants.

The operation of two parallel biosynthetic pathways during the biosynthesis of the protochlorophyll(ide) pool of cucumber cotyledons was probed with the use of several [14C]tetrapyrroles in vitro. A comparison of the ratio (R) of delta-amino[14C]levulinic acid incorporation into protochlorophyllide/14C incorporation into protochlorophyllide ester with other incorporation ratios, RX, where X represents any of several [14C]tetrapyrrole substrates, allowed us to determine which exogenous 14C-labeled substrate was common precursor of protochlorophyllide and protochlorophyllide ester and which one was not. From such comparisons, a biosynthetic pathway of protochlorophyll(ide) biosynthesis is inferred. According to this pathway, protochlorophyllide is formed via an acidic (mono/dicarboxylic) biosynthetic branch, while protochlorophyllide ester is formed via a fully esterified (neutral) biosynthetic branch. The two biosynthetic branches are linked together by porphyrin ester synthetases at the level of protochlorophyllides and possibly at the level of the photoporphyrin IX and the magnesium protoporphyrin monoester pools.