Crystal Structure of the Active Site Mutant Form of Soluble Fumarate Reductase, Osm1

Soluble fumarate reductase is essential for survival under anaerobic conditions. This enzyme can maintain the redox balance in the cell by catalyzing the reduction of fumarate to succinate. Although the overall reaction mechanism of soluble fumarate reductase in yeast, Osm1, has been proposed by a previous structural study, the details of the underlying mechanism are not completely elucidated. The present study provides the structural information regarding the active site mutant form of Osm1 (R326A), thus, revealing that R326A mutation does not affect the substrate binding. Structural alterations of the residues surrounding the active site, and the missing 2nd flavin adenine dinucleotide (FAD) in the previously defined 2nd FAD binding site, were observed as characteristic features of the Osm1 R326A crystal structure. Based on these findings, we provided a clue that can explain the loss of activity of Osm1 R326A.