Electron transfer in the Rhodobacter sphaeroides reaction center assembled with zinc bacteriochlorophyll

Abstract

The cofactor composition and electron-transfer kinetics of the reaction center (RC) from a magnesium chelatase (bchD) mutant of Rhodobacter sphaeroides were characterized. In this RC, the special pair (P) and accessory (B) bacteriochlorophyll (BChl) -binding sites contain Zn-BChl rather than BChl a. Spectroscopic measurements reveal that Zn-BChl also occupies the H sites that are normally occupied by bacteriopheophytin in wild type, and at least 1 of these Zn-BChl molecules is involved in electron transfer in intact Zn-RCs with an efficiency of >95% of the wild-type RC. The absorption spectrum of this Zn-containing RC in the near-infrared region associated with P and B is shifted from 865 to 855 nm and from 802 to 794 nm respectively, compared with wild type. The bands of P and B in the visible region are centered at 600 nm, similar to those of wild type, whereas the H-cofactors have a band at 560 nm, which is a spectral signature of monomeric Zn-BChl in organic solvent. The Zn-BChl H-cofactor spectral differences compared with the P and B positions in the visible region are proposed to be due to a difference in the 5th ligand coordinating the Zn. We suggest that this coordination is a key feature of protein-cofactor interactions, which significantly contributes to the redox midpoint potential of H and the formation of the charge-separated state, and provides a unifying explanation for the properties of the primary acceptor in photosystems I (PS1) and II (PS2). magnesium chelatase mutant ͉ photosynthetic bacterial reaction center ͉ photosystems I and II ͉ protein-cofactor interaction T he purple bacterial reaction center (RC) is a pigmentprotein complex that is capable of converting light energy to chemical energy with quantum yield approaching 1 (1-3). Electron transfer (ET) in this RC has been extensively studied; the structure and spectroscopic features of the complex are well known, the complex is very stable, and a large variety of mutants is available. This RC also serves as a model system for understanding protein-cofactor interactions and the role that protein plays in ET (4).The RC from Rhodobacter (Rb.) sphaeroides comprises 3 protein subunits, H, M, and L. As shown in Fig. 1, the RC complex binds 9 cofactors that form 2 potential ET chains (referred to as A and B) in a C2 symmetric arrangement. The ''special pair'' (P) is a dimer of bacteriochlorophyll (BChl) a molecules and is located on the periplasmic side of the cytoplasmic membrane. Two monomeric BChls (B A and B B , with the subscripts denoting which chain the cofactor belongs to) are present on either side of P. These are followed by 2 bacteriopheophytin (BPhe) molecules (H A and H B ). A nonheme iron and 2 quinones (Q A and Q B ) are near the cytoplasmic side of the RC (5, 6). When P is excited, an electron is transferred through the A branch cofactors, and then to Q B . In the WT RC, the times for ET from P* to H A to Q A to Q B are 3 ps, 200 ps, and 200 s, respectively. The transfer from P* to H A is thought to be via B A .The ET reactions P...

Demetalation kinetics of natural chlorophylls purified from oxygenic photosynthetic organisms: effect of the formyl groups conjugated directly to the chlorin π-macrocycle

“…Recent results on the reaction center containing Zn-BChl from a magnesium chelatase-deleted mutant of Rhodobacter sphaeroides indicated that Zn-BChl occupied the BPhe site and was involved in the electron transfer. 44 This unique feature might originate from difference in demetalation properties between Mg-BChl and Zn-BChl, namely, Zn-BChl was more tolerant to demetalation than Mg-BChl. 35 In contrast, Acidiphilium rubrum, containing Zn-BChl as a major photosynthetic pigment, which grows under acidic conditions, possesses BPhe, not Zn-BChl, in the site of the electron acceptor in the reaction center.…”

Pushing the limits of flash photolysis to unravel the secrets of biological electron and proton transfer

“…But many research groups turned to picosecond spectroscopy because the field seemed promising and more open. Among the contributors, I wish to mention Jean-Louis Martin, with Jacques Breton and Marten Vos, at ENSTA; Vlad Shuvalov in Puschino; Rienk van Grondelle in Amsterdam; Dewey Holten in St Louis; Maurice Windsor in Pullman; Alfred Holzwarth in Mülheim; Jim Allen and Neal Woodbury in Tempe, and others [32][33][34][35][36][37][38][39].…”

Highly Sensitive Method to Isolate Photocurrent Signals from Large Background Redox Currents on Protein‐Modified Electrodes

“…Attenuated electron transfer rates may be caused by the presence of a molecular blocking layer because the distance between HQ and electrode is increased or the increased surface coverage on the electrode surface decreases surface accessibility for redox species, for example. Under these conditions in PBS pH 7.2 the first electron transfer of HQ is favorable in reducing P + , which has a midpoint potential of approximately 0.5 V. [29] The coverage of the molecular blocking layer on the gold electrode was estimated using cyclic voltammetry (CV) measured in the dark. On an unmodified bare gold surface, the presence of 10 mM HQ resulted in reversible reductive and oxidative peaks at 0.2 and 0.36 V, respectively, corresponding to the first of the two-electron step redox reaction of HQ with an approximate reductive midpoint potential of E m,1 = 0.28 V. The midpoint potential of the second of the two-electron step redox reaction was estimated at E m,2 = 0.54 V, with reductive and oxidative peaks at 0.54 and 0.6 V, respectively (Figure 2, red).…”

“…On an unmodified bare gold surface, the presence of 10 mM HQ resulted in reversible reductive and oxidative peaks at 0.2 and 0.36 V, respectively, corresponding to the first of the two-electron step redox reaction of HQ with an approximate reductive midpoint potential of E m,1 = 0.28 V. The midpoint potential of the second of the two-electron step redox reaction was estimated at E m,2 = 0.54 V, with reductive and oxidative peaks at 0.54 and 0.6 V, respectively (Figure 2, red). Under these conditions in PBS pH 7.2 the first electron transfer of HQ is favorable in reducing P + , which has a midpoint potential of approximately 0.5 V. [29] The coverage of the molecular blocking layer on the gold electrode was estimated using cyclic voltammetry (CV) measured in the dark. The immersion of a freshly prepared clean gold electrode into a solution of mercaptohexanol (MCH) resulted in the formation of a self-assembled alkylthiol monolayer (SAM), which led to a decrease in interfacial capacitance and significantly impeded electron transfer, resulting in attenuated HQ redox peaks A) The spatial arrangement of redox-active cofactors in the wild type RC.…”