The membrane protein complex, fumarate reductase, couples the reduction of fumarate to succinate to the oxidation of quinol to quinone, in a reaction which is opposite to that catalysed by succinate dehydrogenase (SDH) in the related complex II of the aerobic respiratory chain. SDH is also the only membrane-bound enzyme in the citric acid cycle (Krebs' cycle). The structure of fumarate reductase from the anaerobic bacterium Wolinella succinogenes has been recently determined [1], using data collected at beamline BM14. The phases were determined by multiple isomorphous replacement with anomalous scattering (MIRAS). All data were collected at 277 K on only one crystal per data set. Since most derivatives contained only one heavy atom bound per fumarate reductase complex of 130 kDa, the reliable incorporation of anomalous differences was essential in order to include the contribution of the endogenous iron sites (11 Fe per fumarate reductase complex) in the calculation of phases. The phases were improved using a series of density modification methods, including solvent flattening, histogram matching, NCS averaging and phase extension to a resolution of 2.20 Å. Representative electron density maps are shown in Figure 7. The excellent quality of the experimental map allowed rapid model building for most of the structure. The two fumarate reductase complexes in the asymmetric unit of the crystal form a dimer, with each monomer containing the three different subunits A, B, and C (see Figure 8). The large hydrophilic subunit A contains the site of fumarate reduction and the covalently bound prosthetic group flavin adenine dinucleotide (FAD). The smaller hydrophilic subunit B contains three iron-sulphur centres. The quinol-oxidizing membrane-embedded subunit C binds two haem b groups. On the basis of the structure, a pathway of electron transfer can be traced from the two haem groups of subunit C via the three iron-sulphur centres and the FAD to the site of fumarate reduction. The structure has also enabled the suggestion of a catalytic mechanism for the reduction of fumarate to succinate with general relevance also to succinate dehydrogenases.

[1] C.R.D. Lancaster, A. Kröger, M. Auer, H. Michel, Nature, 402, 377-385 (1999).

C.R.D. Lancaster (a), A. Kröger (b), M. Auer (a), H. Michel (a).

(a) Max-Planck-Institut für Biophysik, Frankfurt am Main (Germany).
(b) Institut für Mikrobiologie, J.W. Goethe-Universität, Frankfurt/M. (Germany).