Abstract
Adenylyl sulfate reductase (APSR or APS reductase, E.C.1.8.99.2), which was purified from the anaerobic sulfate-reducing bacterium Desulfovibrio gigas, played a key role to catalyze APS to sulfite in the dissimilatory sulfate reduction. Reduction of ferricyanide was used as reversed reaction of APSR for determining the formation of APS from AMP and sulfite. The overall structure of APSR contains six αβ-heterodimers that comprise one α-subunit and one β-subunit to form a hexamer structure. The C-terminus of the β-subunit (B105-B152) wrapped around α-subunit formed a functional unit, and the loop of the β-subunit C-terminus (B153-B167) inserted into the active channel of the α-subunit from another αβ-heterodimer. The hypothetic mode was suggested that APSR might self regulate the activity by the β-subunit C-terminus blocking the active site because the dynamic light scattering (DLS) data showed that the average diameter of APSR decreased from 12.81 to 8.11 nm after adding AMP, corresponding the size change from the hexamer to dimer structure. The sole disulfide of the structure is positioned at the tail of C-terminus of the β-subunit (Cys 156 and Cys 16), which was the key connection to keep the shape of the loop of the β-subunit C-terminus. After treating with 2-mercaptoethanol (β-me) to break the disulfide, the average diameter of APSR decreased from 12.81 to 8.47 nm. The results suggest that the dimer form could be the functional structure.