CRYSTAL-STRUCTURE OF PHOSPHOADENYLYL SULFATE (PAPS) REDUCTASE - A NEWFAMILY OF ADENINE-NUCLEOTIDE ALPHA-HYDROLASES

Background: Assimilatory sulphate reduction supplies prototrophic orga nisms with reduced sulphur for the biosynthesis of all sulphur-contain ing metabolites. This process is driven by a sequence of enzymatic ste ps involving phosphoadenylyl sulphate (PAPS) reductase, Thioredoxin is used as the electron donor for the reduction of PAPS to phospho-adeno sine-phosphate (PAP) and sulphite. Unlike most electron-transfer react ions, there are no cofactors or prosthetic groups involved in this red uction and PAPS reductase is one of the rare examples of an enzyme tha t is able to store two electrons. Determination of the structure of PA PS reductase is the first step towards elucidating the biochemical det ails of the reduction of PAPS to sulphite. Results: We have determined the crystal structure of PAPS reductase at 2.0 Angstrom resolution in the open, reduced form, in which a flexible loop covers the active si te. The protein is active as a dimer, each monomer consisting of a cen tral six-stranded beta sheet with alpha helices packing against each s ide. A highly modified Version of the P loop, the fingerprint peptide of mononucleotide-binding proteins, is present in the active site of t he protein, which appears to be a positively charged cleft containing a number of conserved arginine and lysine residues, Although PAPS redu ctase has no ATPase activity, it shows a striking similarity to the st ructure of the ATP pyrophosphatase (ATP PPase) domain of GMP synthetas e, indicating that both enzyme families have evolved from a common anc estral nucleotide-binding fold. Conclusions: The sequence conservation between ATP sulphurylases, a subfamily of ATP PPases, and PAPS reduct ase and the similarities in both their mechanisms and folds, suggest a n evolutionary link between the ATP PPases and the PAPS reductases. To gether with the N type ATP PPases, PAPS reductases and ATP sulphurylas es are proposed to form a new family of homologous enzymes with adenin e nucleotide alpha-hydrolase activity. The open, reduced form of PAPS reductase is able to bind PAPS, whereas the closed oxidized form canno t. A movement between the two monomers of the dimer may allow this swi tch in conformation to occur.