Shared thematic elements in photochemical reaction centers.

Abstract

The structural, functional, and evolutionary relationships between photosystem II where P is a chlorophyll primary electron donor, I is a chlorophyll primary electron acceptor, and Q is a quinone secondary electron acceptor. In this generalized RC, the absorption of a photon results in charge separation between the chlorophyll donor and acceptor molecules. The electron is rapidly transferred to the quinone, which acts to stabilize against the rapid charge recombination between the primary reactants.As depicted in Table 1, the identities of the components and the kinetics of the initial forward electron transfer reactions are remarkably similar in the purple bacterial RC, PSII, and PSI. A further shared feature is that the photoactive components are located on a chlorophyllcontaining, homo-or heterodimer that is predicted to possess an overall C2 axis of symmetry (this is only known for certain in the purple bacterial RC). The differences between the various RCs lie largely in the details of the protein scaffold that modulates the redox potentials of the photoactive components and in the identities of the secondary electron donors and acceptors that have evolved to provide the specialized functions of water oxidation in PSII and nicotinamide adenine dinucleotide phosphate (NADP+) reduction in PSI. It is the purpose of this article to review recent developments in the area of comparative RC biochemistry and examine the evolutionary implications offound similarities in structure and function. PSI and the Chlorobium RCThe functional relationship between the purple bacterial RC and PSII in green plants and cyanobacteria has been apparent for nearly a decade: both are considered "quinone-type" photosystems in which the bound primary quinone, QA, donates its electron across the heterodimeric protein boundary to a mobile secondary quinone, QB (reviewed in refs.1 and 2). Due to the overwhelming similarity in structure and function, it is well accepted that the purple bacterial RC and PSII share a common ancestor. In contrast, there has been no definitive bacterial analog for PSI. However, a series of recent publications have provided excellent evidence that the RC in green sulfur bacteria of the genus Chlorbiaceae (e.g., Chlorobium limicola f. thiosulfatophilum, Chlorobium phaeobacteroides, Chlorobium vibriforme) contains bound iron-sulfur clusters (3-6). The discovery of iron-sulfur clusters in Gram-positive bacteria of the genus Heliobacteriaceae (e.g., Heliobacterium chlorum, Heliobacterium gestii, and Heliobacillus mobilis) has further stimulated interest in these organisms as evolutionary precursors of PSI (ref. 7; see also ref. 8).PSI is considered an "iron-sulfurtype" RC in which the bound primary quinone, A1, donates its electron to an iron-sulfur cluster (probably Fx) instead of to the secondary quinone (reviewed in ref. 9). Fx is a rare example of an interpolypeptide [4Fe-4S] cluster that occupies the same relative position as the non-heme iron in the purple bacterial RC and in PSII. However, it may s...

Radicals and Radical Pairs in Photosynthesis

“…The primary donor is a Chl a species, presumably a dimer. From its excited state an electron is transferred to a monomeric Chl a acceptor (termed An), then on to a phylloquinone secondary acceptor (A,) and three iron-sulfur clusters (Fx, FA and FB) (143,145). Structural studies have been performed on PS !…”

Isolation and Characterization of Homodimeric Type-I Reaction Center Complex from Candidatus Chloracidobacterium thermophilum, an Aerobic Chlorophototroph

“…After absorbing a photon, a BChl dimer bound to the RC near the periplasmic surface of the membrane achieves a long lived, charge-separated state by transferring an electron through a series of bound cofactors to a terminal acceptor, which is bound to the RC near the cytoplasmic surface of the membrane. Based on their terminal electron acceptors, RC complexes are classified into two types (1). Type-I RCs utilize Fe-S clusters as terminal electron acceptors, whereas type-II RCs use quinones as terminal electron acceptors.…”

Organization of Photosystem I Polypeptides (Identification of PsaB Domains That May Interact with PsaD)

“…This domain contains the F,-binding loop and may be important in interactions between PsaA and PsaB (Golbeck, 1993;Smart et al, 1993). The resistance of the C-terminal domain of PsaB to in vitro proteolysis may be due to the folded structure of PsaB or may result from shielding by smaller PSI subunits.…”

“…Deduced amino acid sequences of PsaA and PsaB from cyanobacteria to higher plants show a high degree of similarity and many common structural features. Among them, the conserved motif FPCDGPGRGGTC serves as the binding site for the [4Fe-4Sl cluster F, (Golbeck, 1993). Based on hydropathy analyses, PsaA and PsaB have been structure at 6-K resolution (Krauss et al, 1993).…”

“…PSI of cyanobacteria and chloroplasts is a multisubunit membrane-protein complex that catalyzes electron transfer from reduced plastocyanin (or Cyt c, ) to oxidized Fd (or flavodoxin) Bryant, 1992;Golbeck, 1993;Chitnis et al, 1995). PsaA and PsaB, the 82-kD transmembrane subunits of PSI, form a heterodimeric core that harbors approximately 100 antenna Chl u molecules, the primary reaction center P700, and a chain of electron acceptors A, , , A,, and F, .…”