The Characteristics of Specific Chlorophylls and Their Roles in Biogenesis of the Photosynthetic Apparatus

Summary

This chapter promotes a concept that provides insight into unique biological functions of the accessory chlorophylls b and c. The basis for the concept is the nature of atoms and molecules as malleable clouds of electrons. The distribution of electron density in the chlorophyll macrocycle, which is readily probed by absorption of light energy, is affected by functional groups on the periphery of the structure. Spectroscopic and molecular orbital data support the proposal that the central Mg ion in chlorophylls b and c is less shielded by the molecular electron cloud than in chlorophylls a and d. Consequently, the extent to which the Mg ion is exposed causes it to interact differently with ligands. Chlorophylls a and d are versatile Lewis acids and form coordination bonds with a range of Lewis bases, from electron-rich neutral structures such as the imidazole group of histidine and the charge-compensated ion-pair of the carboxyl group of glutamate with the guanidinium group of arginine, to those with a strong dipole moment such as amide groups and water. In contrast, chlorophyll b (and probably chlorophyll c) is found only with “hard” Lewis bases that have a strongly electronegative, oxygen-containing dipole moment. For a coordination bond to form between chlorophyll b and a protein, the potential ligand apparently must have a sufficiently strong dipole to displace a strongly bound water molecule. Chlorophyll b is required for accumulation of light-harvesting complexes in chloroplasts, a role that is attributed to interaction with a backbone peptide bond carbonyl group near the N-terminus of the apoproteins. Experiments on chloroplast development with the alga Chlamydomonas reinhardtii show that synthesis of chlorophyll b in the chloroplast envelope inner membrane is required to retain the proteins in this membrane during import into the organelle.