SHARED THEMATIC ELEMENTS IN PHOTOCHEMICAL-REACTION CENTERS

The structural, functional, and evolutionary relationships between photosystem II and the purple non-sulfur bacterial reaction center have been recognized for several years. These can be classified as ''quinone type'' (type II) photosystems because the terminal electron acceptor is a mobile quinone molecule. The analogous relationship between photosystem I and the green sulfur bacterial (and heliobacterial) reaction centers has only recently become clear. These can be classified as ''iron-sulfur type'' (type I) photosystems because the terminal electron acceptor consists of one or more bound iron-sulfur clusters. At a fundamental level, the quinone type and iron-sulfur type reaction centers share a common photochemical motif in the early process of charge separation, leading to the speculation that all photochemical reaction centers have a common evolutionary origin. This review summarizes the current state of knowledge in comparative reaction center biochemistry between prokaryotic bacteria, cyanobacteria, and green plants.