PHOTOSYNTHETIC ELECTROGENIC EVENTS IN NATIVE MEMBRANES OF CHLOROFLEXUS-AURANTIACUS - FLASH-INDUCED CHARGE DISPLACEMENTS WITHIN THE REACTION-CENTER CYTOCHROME C(554) COMPLEXстатья

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[1] Photosynthetic electrogenic events in native membranes of chloroflexus-aurantiacus - flash-induced charge displacements within the reaction-center cytochrome c(554) complex / A. MULKIDJANIAN, G. VENTUROLI, A. HOCHKOEPPLER et al. // Photosynthesis Research. — 1994. — Vol. 41, no. 1. — P. 135–143. The thermophilic phototroph Chloroflexus aurantiacus possesses a photosynthetic reaction center (RC) containing a pair of menaquinones as primary (Q(A)) and secondary (Q(B)) electron accepters and a bacteriochlorophyll dimer (P) as a primary donor. A tetraheme cytochrome c(554) with two high(H)- and two low(L)-potential hemes operates as an immediate electron donor for P. The following equilibrium E(m,7) values were determined by ESR for the hemes in whole membrane preparations: 280 mV (H1), 150 mV (H2), 95 mV (L1) and 0 mV (L2) (Van Vliet et al. (1991) fur. J. Biochem. 199: 317-323). Partial electrogenic reactions induced by a laser flash in Chl. aurantiacus chromatophores adsorbed to a phospholipid-impregnated collodion film were studied electrometrically at pH 8.3. The photoelectric response included a fast phase of Delta psi generation (tau < 10 ns, phase A). It was ascribed to the charge separation between p(+) and Q(A)(-) as its amplitude decreased both at high and low E(h) values (E(m,high) = 360 +/- 10 mV, estimated E(m,low) similar to -160 mV) in good agreement with E(m) values for P/P+ and Q(A)/Q(A)(-) redox couples. A slower kinetic component appeared upon reduction of the cytochrome C-554 hemes (phase C). With H1 reduced before the flash the amplitude of phase C was equal to 15-20% of that of phase A and its rise time was 1.2-1.3 mu s: we attribute this phase to the electrogenic electron transfer from H1 to P+. Pre-reduction of H2 decreased the tau value to about 700-800 ns and increased the amplitude of phase C to 30-35% of that of phase A. Pre-reduction of L1 further accelerated phase C (up to tau of 500 ns) and induced a reverse electrogenic phase with tau of 12 mu s and amplitude equal to 10% of phase A. Upon pre-reduction of L2 the rise time of phase C was decreased to about 300 ns and its amplitude decreased by 30%. The acceleration in the onset of phase C is explained by the acceleration of the rate-limiting H1 double right arrow P electrogenic reaction after reduction of the other hemes due to their electrostatic influence; a P-H1-(L1-L2)-H2 alignment of redox centers with an approximately rhombic arrangement of the cytochrome c(554) hemes is proposed. The observed reverse phase is ascribed to the post-flash charge redistribution between the hemes. Redox titration of the amplitude of phase C yielded the E(m,8.3) values of H1, H2 and L2 hemes: 340 +/- 10mV for H1, 160 +/- 20 mV for H2 and -40 +/- 40 mV for L2. [ DOI ]

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