Photosynthetic Reaction Centers

Primary processes in the photosynthetic RC. Here, P represents the charge-separating (bacterio) chlorophyll pigments (the “primary electron donor”) and A represents the first “stable” acceptor. Energy transfer from the antenna pigments leads to photoexcitation of P on the fs-ps time scale (left). Charge separation produces oxidized P+ and A– on the ps-ns scale (center). The recombination of P+A– to produce PA, heat, and potentially damaging chemical species is efficiently prevented by further forward electron transfer that is now proton coupled. These more complex chemical processes ultimately produce stable photosynthetic products and occur, initially, on the ns and μs time scales (right). The first notions of the operation of a photosynthetic RC originated with the photosynthetic unit experiments of Emerson and Arnold (9), which demonstrated that approximately 2,500 chlorophyll molecules were involved in the release of just one molecule of O2. Thus, a photosynthetic unit contains numerous pigments but the photochemically active chromophores are present in much lower concentration. This pioneering concept led to the distinction of two types of pigments: the light-harvesting, but photochemically inactive, antenna chromophores; and the photochemically active RC pigments. The antenna pigments physiologically increase the absorption cross section of the RC dramatically. Moreover, they ensure that the potentially reactive intermediates containing unpaired electron spins (e.g. semiquinones) generated by single photon photochemistry are efficiently converted by a second photochemical event to products (e.g. hydroquinones) that contain only paired spins. For efficient energy transfer between the antenna and the RC, the RC absorbs at longer wavelengths, effectively forming a trap for excitation energy. Despite these conceptual advances, more than 35 years passed before the first physical isolation of a pigment protein RC complex was reported (17). Since that time, many other RCs have been isolated and characterized biophysically and biochemically. Structure of the purple bacterial RC (Rhodobacter sphaeroides from MH. Stowell et al; Protein Data Band file no. 1AIJ). The heterodimeric RC (left) is comprised of a C2-symmetrical heterodimer of the L and M polypeptides shown in orange and blue respectively. A third subunit, H, is also shown in green. The pseudo-C2symmetric cofactor arrangement and the active pathway of electron transfer are indicated on the right. Charge separation from the RC Chl dimer (P) to the BChl monomer to B-Phe occurs in approximately 3 ps down the active L branch. This is followed by charge stabilization with electron transfer to the quinones. The phytyl and isoprenoid chain have been removed for visualization. A remarkable aspect of the RC structures is the occurrence of two almost identical electron acceptor pathways arranged along the C2 axis relative to the primary charge-separating dimer (bacterio) chlorophyll (Fig. 2). This finding posed a key question: Does electron transfer involve both branches? In the purple bacterial RC, only one branch is active although the inactive branch can be forced into operation with modification of amino acid side chains on the active branch (1). The strong asymmetry imposed on primary charge separation photo-chemistry in the purple bacterial RC results from two homologous polypeptides that function as a heterodimer. A heterodimer is also involved in the core of the RCs of PSI and PSII. However, some RCs, such as heliobacteria (2) and green sulfur bacteria (6, 18), contain two identical homodimeric polypeptides, and electron transfer is potentially bifurcated. Genetic sequence information has greatly improved the understanding of the origin of the RC proteins. From the sequence analysis, it became clear that the purple bacteria RC is remarkably similar to that of PSII, and PSI was also discovered to have similarity with that of the green sulfur bacteria (6, 10). Further elaboration with 16S-rRNA phylogenic trees (5) and broader homology comparison (14) revealed a close interrelationship among many RCs. Recent structural comparisons between PSI and PSII, for example, show a distinct structural homology, which suggests that even these two RCs likely share a common ancestor (13). Electron-transfer pathways for the two different types and six classes of RCs shown according to the midpoint potentials of key redox components. In Type I RCs (left), iron-sulfur clusters are used as the electron acceptors. Type I is subdivided into three classes: PSI, green sulfur bacteria, and heliobacteria (see text). In Type II RCs, quinones are used as the first “stable” electron acceptors (left). Type II is also subdivided into three classes: PSII, purple bacteria, and green filamentous bacteria. Intermediates in the scheme have the following designations: the RC primary donor, P; transient initial (bacterio) pheophytin acceptor, (B) Ph; “stable” quinone acceptors, QA and QB; transient initial chlorophyll (A0) and quinone (A1) acceptors; “stable” iron-sulfur cluster acceptors, FX, FA, and FB; and final NADP acceptor (NADP). The electron donors are a Tyr residue, TyrZ, and a cluster of 4 manganese ions for PSII, a plastocyanin molecule (PC) for PSI, and cytochrome c for the bacterial RCs. The intermediate electron transfer complexes, cytochromebc1 and b6f, are boxed. Further differentiation in photosynthetic organisms is found in the structure and arrangement of the antenna pigments associated with each RC. The RC from heliobacteria features a very simple organization with a core containing approximately 40 chlorophyll g and no additional auxiliary peripheral antenna proteins (2). Building on this organizational theme are RCs from PSI and green sulfur bacteria, which contain large numbers (∼100) of pigments attached directly to the polypeptides that bind the RC components (13), as well as an extensive external antenna array with which the RCs communicate in a controlled way. At the other extreme are the RCs from purple bacteria and PSII, which contain only six to eight pigments arranged along the C2 symmetry axis and are fundamental to the charge separation process. These RCs rely on a substantial antenna system as conduits of excitation energy. This antenna system is bound to polypeptides distinct from the RC polypeptides. The charge-stabilizing reactions that occur following primary charge separation in the RC are coming into sharper focus. A key realization has been that these reactions are often proton coupled; that is, the motion of the electron must be coupled in some fashion to the motion of a proton (Fig. 1). This marks the conversion from pure photon and electron chemistry to chemistry that involves nuclei as well. Okamura and his coworkers have produced seminal results on electron/proton coupling on the reducing side in the bacterial RC (16). Results that incorporate these and other thoughts concerning proton/electron coupling in PSII are emerging as the underlying mechanism that drives water oxidation to produce O2. To oxidize water, potentials upwards of 1 V must be generated in PSII; moreover, the observations by Joliot and Kok (12) that O2 evolution follows a four-flash oscillatory pattern necessitates that the oxidizing equivalents produced by P680+ must be stored. To accomplish this, P680+ (midpoint reduction potential at pH 7, approximately 1.2 V) is intimately coupled to a redox-active Tyr (TyrZ) and an inorganic Mn4Ca1ClXcluster where the water oxidation reaction is catalyzed. Current insights into the cluster structure have been largely driven by x-ray absorption spectroscopy, which predicts that the catalytic manganese complex is organized as pair of manganese-oxo dimers (20). The water oxidation reaction is more difficult to access. Recent H2 16O/H2 18O exchange measurements (11) show rapid exchange of substrate water (ms time regime), supporting the notion that substrate water is bound terminally to manganese. A number of other measurements concerning the water oxidation reaction have led to a metalloradical model for PSII water oxidation. In this proposal, TryZ is directly involved in hydrogen atom abstraction from the substrate water terminally bound at the manganese cluster (19). This proposal, as with most hypotheses, has attracted both supporters and detractors! It is certain that in the next few years the structure of PSII and the oxygen-evolving complex will be resolved at high resolution, as will be the structures of many other photosynthetic RCs. Already the last 25 years have seen tremendous advances in the understanding of photosynthetic RCs. Long-held concepts have been challenged, reinvestigated, and changed as the results of new structural, dynamic, biochemical, and molecular biological insights. These have been stimulating and exciting developments; undoubtedly they will continue.