Abstract
The reduction of elemental sulfur (S0) or polysulfide to sulfide constitutes a significant part of the natural geobiological sulfur cycle. However, little is known about the enzymes catalyzing sulfur reduction in sulfur-reducing bacteria. Recently, we found a novel multiheme-containing selenoprotein (MHSEP) from a sulfur-reducing deltaproteobacterium, Geobacter sulfurreducens PCA. MHSEP catalyzed the reduction of polysulfide and selenite by using methyl viologen as an artificial electron donor. Kinetic analysis revealed that polysulfide was an efficient substrate for the enzyme. MHSEP gene disruption in G. sulfurreducens led to significant decreases in sulfur reduction and sulfide production in an S0-supplemented medium. The 1.98 Å crystal structure of MHSEP revealed that MHSEP formed a tetramer, in which each protomer contained five c-type hemes. Four of the five hemes were arranged so that electrons were transferred from the solvent-accessible heme 1, through heme 2 and heme 3, to heme 4. The heme 4 was in close vicinity to the catalytic Sec325 and was axially coordinated by His and Cys unlike the other four bis-His ligated hemes, suggesting that heme 4 could be the active-site cofactor.
