ABSTRACT
Background In plants and algae, the reaction centers of photosystem I and II are enclosed within core complexes that contain a precisely dened set of proteins – essentially all encoded in the chloroplast genome. e primary cofactor for the photochemical reactions in these complexes, chlorophyll (Chl) a, is also required for assembly of these complexes. e end-product of the Chl biosynthetic pathway in plants in the dark, protochlorophyllide (Pchlide), is unable to support the assembly processes, which suggests that the light-dependent reduction of the double bond between C17 and C18 of Pchlide (see legend to Fig. 1) has a profound eect on the properties of the molecule. Plants and green algae (Chlorophyta) contain in addition Chl b, an accessory Chl found only in peripheral light-harvesting complexes (LHCs). ese complexes usually contain three xanthophyll molecules, two luteins and one neoxanthin, and nearly equal amounts of Chl a and Chl b (7 or 8 Chl a and 5 or 6 Chl b molecules for the major LHCII, with an a/b ratio of 1.4) bound to proteins (LHCPs) that are encoded in the nuclear genome and imported into the plastid after synthesis in the cytosol. Chl b-less mutant plants are decient in Chl and, although containing fully functional reaction centers, have a relatively low photosynthetic capacity and greater sensitivity to high-intensity light because of a deciency in LHCs [1]. Algal species in the family Chromophyta contain Chl c (Fig. 1) instead of Chl b, which is also restricted to LHCs and seems to serve the same function in these organisms that Chl b provides in the green plants [2]. A large volume of data exists in the literature on these Chl derivatives. In this article we propose a mechanism for the important auxiliary roles these Chls play in photosynthesis.
