A third crystal form of Wolinella succinogenes quinol : fumarate reductasereveals domain closure at the site of fumarate reduction

Quinol:fumarate reductase (QFR) is a membrane protein complex that couples the reduction of fumarate to succinate to the oxidation of quinol to quinon e. Previously, the crystal structure of QFR from Wolinella succinogenes was determined based on two different crystal forms, and the site of fumarate binding in the flavoprotein subunit A of the enzyme was located between the FAD-binding domain and the capping domain [Lancaster, C.R.D., Kroger, A., Auer, M., & Michel, H. (1999) Nature 402, 377-385]. Here we describe the st ructure of W. succinogenes QFR based on a third crystal form and refined at 3.1 Angstrom resolution. Compared with the previous crystal forms, the cap ping domain is rotated in this structure by approximately 14 degrees relati ve to the FAD-binding domain. As a consequence, the topology of the dicarbo xylate binding site is much more similar to those of membrane-bound and sol uble fumarate reductase enzymes from other organisms than to that found in the previous crystal forms of W. succinogenes QFR. This and the effects of the replacement of Arg A301 by Glu or Lys by site-directed mutagenesis stro ngly support a common mechanism for fumarate reduction in this superfamily of enzymes.