Living on Sulfate : Three-Dimensional Structure and Spectroscopy of Adenosine 5'-Phosphosulfate Reductase and Dissimilatory Sulfite Reductase

Abstract

The reduction of sulfate to sulfide and the reverse reaction are widespread biological processes. Hereby, microorganisms play a central role. Plants also reduce sulfate for the purpose of biosynthesis, and both plants and animals convert reduced sulfur compounds to sulfate. Sulfate respiration is used for energy conservation by strictly anaerobic bacteria and archaea. The redox equivalents generated by the oxidation of organic compounds are transferred to sulfate as the terminal electron acceptor. There are three key enzymes localized in the cytoplasm or at the cytoplasmic aspect of the inner membrane: ATP sulfurylase (ATPS), adenosine 5´-phosphosulfate reductase (APSR), and dissimilatory sulfite reductase (SIR). Sulfate (S6+) cannot be directly reduced by dihydrogen or organic acids, it has to be activated to adenosine 5´-phosphosulfate (APS) catalyzed by ATPS. The enzyme APSR (cofactors flavin adenine dinucleotide, [4Fe4S]) catalyzes the conversion of APS to sulfite (S4+) and AMP, followed by the complex multicomponent enzyme SIR (cofactors siroheme, [4Fe4S]) which catalyzes the reduction of sulfite (S4+) to sulfide (S2?). In this contribution we present the three-dimensional structures of APSR from Archaeoglobus fulgidus and of catalytically relevant reaction intermediates. In addition, we discuss spectroscopic and structural data of SIR purified from this organism.