Plant Molecular Biology 2

Volume 212 of the series NATO ASI Series pp 449-459

Chlorophyll Biosynthesis

  • Diter von WettsteinAffiliated withDepartment of Physiology, Carlsberg Laboratory

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Chlorophyll is bound to proteins of the photosynthetic membranes. It harvests sunlight and carries out the first reactions in the conversion of light energy to chemical energy, which is conserved in NADPH2 and ATP. The chemical formula of chlorophyll a is reproduced in Fig. 1. Four pyrrole rings (I-IV) are bound into a tetrapyrrole ring with a magnesium atom in the center. Ring IV is esterified with a higher alcohol, phytol. For light harvesting higher plants use an additional form of chlorophyll, chlorophyll b which contains in position 3 a formyl group (CHO) instead of a methyl group. The ring system with its characteristic conjugated double bonds is assembled in the chloroplast from 8 molecules of 5-aminolevulinic acid (Fig. 2) which contains 5 carbon atoms and as functional groups besides the carboxyl group an amino and a ketogroup. The following experiments have shown that 5-aminolevulinate is the precursor of chlorophyll: If seeds of higher plants are germinated in the dark, the seedlings have yellow leaves due to lack of chlorophyll. If these are placed in a solution of 5-aminolevulinate in darkness they will green in the course of a few hours due to the accumulation of protochlorophyllide, a late precursor of chlorophyll which in higher plants requires light for conversion into chlorophyll. If radioactively labeled 5-aminolevulinate is used with detached leaves or isolated plastids, the label is found in protochlorophyllide. This experiment reveals that all the enzymes building protochlorophyllide from 5-aminolevulinate are present in the plastid of dark-grown leaves and that it is the synthesis of 5-aminolevulinate which is limiting in the dark.