Supramolecular complexes in photosynthetic bacteria.

Abstract

The pioneering experiments of Emerson and Arnold (1) gave rise to the concept of a photosynthetic unit in which many antenna pigments could efficiently pass excited state energy on to a reaction center (RC) where the first photochemistry occurred. The experimental data indicated that the antenna must contain a large set of chlorophyll molecules relative to the reaction center and implied a close geometric relationship between the antenna molecules and the RC that would allow for efficient delivery of excited state energy resulting from a quantum of light absorbed anywhere in the antenna chlorophyll. With the development of membrane biochemistry came the realization that all chlorophyll and bacteriochlorophyll molecules were specifically bound to proteins that held them in a fixed relationship to each other (2–4). In photosynthetic bacteria, the antenna pigments were found to be specifically bound in protein complexes referred to as light-harvesting (LH) complexes whereas the RC cofactors were bound to a separate protein. The isolation and crystallization of these complexes has led to the determination of the structure of the bacterial RC at high resolution (5–7), of a bacterial light-harvesting complex (LH2) at high resolution (8, 9), and of Photosystem I of Synechococcus elongatus (which contains the RC and core LH) at intermediate resolution (10). In addition, electron cryomicroscopy has provided lower resolution structures for a major light-harvesting complex (LHCII) of oxygenic organisms (11), the core light-harvesting complex of Rhodospirillum rubrum (12), and Photosystem II RC of spinach, which also contained part of the core LH (13). As a result of these studies, the early concept of a photosynthetic unit consisting of a kind of “funnel” of antenna pigments that could pass excitation energy to an RC has been transformed into the framework of specific integral membrane protein complexes that form a two-dimensional mosaic in which the RC is at the center and is surrounded by LH complexes. A “core” complex contains the RC and a set of light-harvesting pigments (LH1) that are intimately and specifically associated with each other and the RC. Among the challenges now being addressed in photosynthetic bacteria research is to describe the structure of this supramolecular photosynthetic core complex and its relationship to the bc1 complex, and to determine the structure-function relationships that are critical for the capture of light energy.