Chlorophyll Biosynthesis.

A leaf with 70 million cells houses .v5 x 109 chloroplasts, each containing m600 million molecules of chlorophyll (Simpson and Knoetzel, 1995). These 10’8 chlorophyll molecules, all of which are bound to proteins of the photosynthetic membranes, harvest the sunlight. Approximately 250 to 300 of them transfer the absorbed light energy through neighboring pigments to the “special pair” chlorophylls in a reaction center. These special pair chlorophylls in photosystems I and II are the primary electron donors that drive the conversion of light into chemical energy to be conserved in NADPH2 and ATf? In chlorophyll, four pyrrole rings (designated I to IV) are ligated into a tetrapyrrole ring with a magnesium atom in the center (Figure 1). Ring IV is esterified with phytol. Chlorophyll a has a methyl group in position 3, but higher plants and algae use for light harvesting an additional form of chlorophyll, chlorophyll b, that has a formyl group instead of a methyl group at this position. The porphyrin ring with its conjugated double bonds is assembled in the chloroplast from eight molecules of 5-aminolevulinic acid, a highly reactive nonprotein amino acid (5-amino, 4-keto pentanoic acid). The bonds highlighted in red in the chlorophyll structure of Figure 1 delineate the locations of the atoms derived from the eight 5-aminolevulinic acid carbon skeletons. An intact 5-aminolevulinate molecule can be recognized in pyrrole ring IV: the nitrogen coordinated with the magnesium atom originates from the amino group; it is followed by four carbon atoms and the carboxyl-group, which is esterified to phytol. Biosynthesis of chlorophyll comprises a number of challenging topics. Because mutants play an increasing role in exploring the biochemistry and molecular biology of this pathway and its regulation, we first highlight recent results on the use of mutants. The subsequent sections deal with the biosynthesis of 5-aminolevulinate, the synthesis of the pyrrole ring, the synthesis of the tetrapyrrole chain, and the formation of the tetrapyrrole ring. We then discuss trimming of the acetate and propionate side chains and establishment of the conjugated double bond system; insertion of the magnesium, formation of ring V, and synthesis of protochlorophyllide; and reduction to chlorophyllide and phytylation. Next, we discuss chlorophyll synthesis in chloroplast development and end with a discussion of chlorophyll binding proteins.