Molecular cloning, expression, and characterization of human bifunctional 3'-phosphoadenosine 5'-phosphosulfate synthase and its functional domains

The universal sulfonate donor, 3'-phosphoadenosine 5'-phosphosulfate (PAPS), is synthesized by the concerted action of ATP sulfurylase and adenosine 5'-phosphosulfate (APS) kinase, which in animals are fused into a bifunctional protein. The cDNA for human PAPS synthase (hPAPSS) along with polymerase chain reaction products corresponding to several NH_2- and COOH- terminal fragments were cloned and expressed in COS-1 cells. A 1-268-amino acid fragment expressed APS kinase activity, whereas a 220-623 fragment evinced ATP sulfurylase activity. The 1-268 fragment and full-length hPAPSS (1-623) exhibited hyperbolic responses against APS substrate with equivalent K(m) values (0.6 and 0.4 μM, respectively). The 1-268 fragment demonstrated Michaelis-Menten kinetics against ATP as substrate (K(m) 0.26 mM); however, full-length hPAPSS exhibited a sigmoidal response (apparent K(m) 1.5 mM) suggesting cooperative binding. Catalytic efficiency (V(max)/K(m)) of the 1- 268 fragment was 64-fold higher than full-length hPAPSS for ATP. The kinetic data suggest that the COOH-terminal domain of hPAPSS exerts a regulatory role over APS kinase activity located in the NH₂-terminal domain of this bifunctional protein. In addition, the 1-268 fragment and full-length hPAPSS were overexpressed in Escherichia coli and column purified. Purified full- length hPAPSS, in contrast to the COS-1 cell-expressed cDNA construct, exhibited a hyperbolic response curve against ATP suggesting that hPAPSS is perhaps modified in vivo.