Chlorophyll Reconstitution of Photosynthetic Light-Harvesting Complexes

Abstract

Light-harvesting complexes (LHCs) play crucial roles in efficient photoenergy conversion and photoprotection of photosynthetic systems. In LHCs, functional pigments such as chlorophylls (Chls), bacteriochlorophylls (BChls), and carotenoids are sophisticatedly assembled with the help of polypeptides. The pigment assemblies in LHCs control the site-energy of each pigment, excitonic interactions among pigments, and excitation energy gradient in the protein matrix, as well as the formation and stability of the protein structure. In vitro reconstitution of LHCs is promising in understanding these structural and functional mechanisms of LHCs. In this review, we summarize two strategies of pigment reconstitution of LHCs; one is the formation of LHCs from a mixture of photosynthetic pigments and denatured polypeptides by their self-assembly, and the other is pigment substitution by the insertion of exogenous pigments into apoproteins partially lacking bound pigments. Next, we overview reconstitution studies of major LHC II derived from oxygenic photosynthetic organisms and core and peripheral antenna proteins of purple photosynthetic bacteria. Here, we focus on substituting Chls and BChls, key pigments in photosynthesis, in LHCs by the reconstitution. (B)Chl reconstitution of LHCs has allowed us to change essential parameters for the pigment-protein interactions and photofunctions, deepening our understanding of the molecular basis of the efficient light-harvesting functions. Reconstitution of LHCs will also be helpful for the modification and design of pigment-protein complexes toward utilization of sunlight energy for global problems on agricultural productivity and bioenergy production.

Keywords: Bacteriochlorophyll; Chlorophyll; Light-harvesting protein; Photosynthesis; Pigment exchange.