PARENT SESSION
Plenary Lectures 4
Thursday September 2nd, 2004 8:30 AM Room 210A

Structure of the Cytochrome b₆f complex: new prosthetic groups, cyclic electron transfer, and trans-membrane quinone transfer. William Cramer¹, Huamin Zhang^*,1, Jiusheng Yan¹, Genji Kurisu⁴, Janet Smith¹, ¹ Dept. of Biological Sciences, West Lafayette, IN, USA⁴ Department of Life Sciences, Meguro, Tokyo, Japan

ABSTRACT- Structures of the cytochrome b6f complex obtained at 3.0-3.1 angstrom resolution from the thermophilic cyanobacterium, Mastigocladis laminosus, (Zhang et al., Science, 302: 1009-1014, 2003) and the green alga, Chlamydomonas reinhardtii (Stroebel et al., Nature, 426: 413-418, 2003): whose appearance in evolution is separated by a billion years, are almost identical. Two monomers of molecular weight 110,000, containing eight subunits and seven natural prosthetic groups, are separated by a large 30 x 25 x 15 angstrom lipid-containing quinone exchange cavity. Three prosthetic groups, Chl a, beta-carotene, and heme x are unique to the b6f complex compared to bc1. The unique heme, heme x, which is 5-coordinate and ferric high spin (see Zhang, Kramer, Bowman, and Cramer, this Conference), with no strong field ligand, occupies a position close to the intra-membrane heme bn.This position is filled by the n-side bound quinone, Qn, in the cytochrome bc1 complex of the mitochondrial respiratory chain. The structure and position of heme x suggests that it can function in cyclic electron transport as well as being an intermediate in a quinone cycle mechanism. However, the function of the high spin character of heme x is not explained by any hypothesis. The significant differences between the cyanobacterial and algal structures are: (i) on the n-side, a plastoquinone molecule is present in the quinone exchange cavity in the cyanobacterial complex, and a sulfolipid is bound in the algal complex at a position corresponding to a synthetic DOPC lipid molecule in the cyanobacterial complex; (ii) on the p-side, in both complexes a quinone analogue inhibitor, TDS, passes through an 11 x 12 angstrom portal that separates the large cavity from a niche containing the iron-sulfur cluster. In the cyanobacterial complex, TDS is oriented oppositely to its position in the algal structure and bc1 complexes, so that its head group in the M. laminosus structure is 20 angstrom from the iron-sulfur cluster (see Yan and Cramer, this Conference). [supported by NIH GM38323 and a Fellowship from the Japanese Ministry of Science and Education (GK)].

KEY WORDS: Cyclic electron transfer, Cytochrome b6f complex, Quinone analogue inhibitor, Heme x