H-Bonds and Functional Properties of Bacteriochlorophyll Cofactors in Photosynthetic Proteins

Abstract

In photosynthetic bacteria, algae and higher plants, the early events of photosynthesis involve two types of chlorophyll-containing, integral membrane proteins. The reaction centers (RCs) perform the photoinduced charge separation and the first steps of electron migration. Antenna or light-harvesting (LH) proteins increase the light absorption cross sections of the RCs by absorbing light and transferring singlet excitation energy to the primary electron donor of the RC. All of the above-mentioned functions of LH and RC proteins involve chlorophylls. These cofactors generally are strictly positioned within their host proteins through remarkably light networks of bonding interactions, which never are of covalent type. Many of these interactions are H-bonds engaged between carbonyl functions of the chlorophylls and a variety of electron-accepting sites of the proteins. Some of these H-bonds are highly conserved among photosynthetic organisms, such as that formed on the C9 keto carbonyl of the primary electron acceptors of all known types of reaction centers (1).