Exciton Interactions Between Hemes bn and bp in the Cytochrome b6f Complex

Abstract

Circular dichroism spectra have been previously utilized (1) to analyze heme-heme interactions of the two b-hemes in the mitochondrial bc1 complex that were predicted to bridge the ‘B’ and ‘D’ trans-membrane helices on the n- and p-sides of the cytochrome bc complexes (2). It was of interest in the context of the 3.0 A structure of the b6f complex (3-6) and its unique bound chromophoric prosthetic groups, Chl a that is 12 A from heme bp, and heme cn that shares electrons with heme bn, to analyze CD spectra of the heme Soret bands in crystallization-quality b6f complex. Sources of the cytochrome b6f complex were the cyanobacteria, M. laminosus and Nostoc PCC sp. 7120, and spinach thylakoids. In the crystal structures, the oxidized b hemes are separated by 20-21A center-center and 7-8A, edge to edge, and rotated relative to each other by approximately 55° about an axis almost normal to the membrane plane. A bi-lobed dithionite minus ascorbate-reduced CD difference spectrum, qualitatively similar to that seen in the mitochondrial bc1 complex, was obtained from all three sources. Positive and negative bands on the blue and red sides of a 431 nm node, the peak of the absorbance difference spectrum, are diagnostic of excitonic heme-heme interactions. There is no significant contribution to these difference spectra from the Chl a, heme cn, or the heme of cytochrome f.∗deceased; 1, Palmer and Degli-Esposti, 1994; 2, Widger et al. 1984; 3, Kurisu et al. 2003; 4, Stroebel et al. 2003; 5, Yamashita et al. 2007; 6, Baniulis et al. 2009.