An Overview of Chlorophylls and Bacteriochlorophylls: Biochemistry, Biophysics, Functions and Applications

The structure of the light-harvesting chlorophyll a/b-protein complex, an integral membrane protein, has been determined at 3.4 A resolution by electron crystallography of two-dimensional crystals. Two of the three membrane-spanning alpha-helices are held together by ion pairs formed by charged residues that also serve as chlorophyll ligands. In the centre of the complex, chlorophyll a is in close contact with chlorophyll b for rapid energy transfer, and with two carotenoids that prevent the formation of toxic singlet oxygen.

Oxygenic photosynthesis is the principal producer of both oxygen and organic matter on Earth. The conversion of sunlight into chemical energy is driven by two multisubunit membrane protein complexes named photosystem I and II. We determined the crystal structure of the complete photosystem I (PSI) from a higher plant (Pisum sativum var. alaska) to 4.4 A resolution. Its intricate structure shows 12 core subunits, 4 different light-harvesting membrane proteins (LHCI) assembled in a half-moon shape on one side of the core, 45 transmembrane helices, 167 chlorophylls, 3 Fe-S clusters and 2 phylloquinones. About 20 chlorophylls are positioned in strategic locations in the cleft between LHCI and the core. This structure provides a framework for exploration not only of energy and electron transfer but also of the evolutionary forces that shaped the photosynthetic apparatus of terrestrial plants after the divergence of chloroplasts from marine cyanobacteria one billion years ago.