Abstract
Adenylylsulfate reductase (APSR or APS reductase) plays a key role to catalyze APS to sulfite at the dissimilatory sulfate reduction. The 500-kDa APS reductase is isolated and purified directly from the D. gigas for crystallization. The alignment of APSR sequence from D. gigas and A. fulgidus exhibits a largest difference at C-terminal of β subunit. The overall structure of APS reductase consists of six αβ-heterodimers to form a hexamer structure. The α subunit shows no-covalent bonds with FAD, and two [4Fe-4S] clusters are enveloped by cluster-binding motifs. The difference of reduced potentials at two clusters is due to the number of polar interactions between the clusters and the protein matrixes. The entrance of substrates-binding channel on α subunit shows larger space comparing with the structure from A. fulgidus because of the shift of the loop (A326-A332) and the α helix (A289-A299) in the α subunit. The C-terminal of β subunit wraps around α-subunit to form a functional unit, and the loop of C-terminal of β subunit inserts into the active channel of α subunit from another αβ-heterodimer. The interactions between the substrate-binding residue (Arg-282) and the residues (Asp-159) on the C-terminal of β subunit affect binding of the substrate. The dynamic light scattering experiment shows the multiple forms of APS reductase changes from hexamer to dimmer after adding AMP. From structure of APSR and the feature of polymerization, the hypothetic model is suggested that APSR may self regulate the activity by the C-terminal of β subunit blocking active site. Keywords: adenylylsulfate reductase, APS reductase, dissimilatory sulfate reduction