Sedimentation equilibrium studies on arylsulfate sulfohydrolase II (EC 22.214.171.124) from Aspergillus oryzae under nondissociating conditions have resulted in a revised molecular weight of 94,900 ± 7100. Sedimentation equilibrium and gel electrophoresis data collected in the presence of the dissociating agents, urea and sodium dodecylsulfate demonstrate that the native enzyme is composed of two identical subunits as suggested by previous studies employing an irreversible inhibitor. The pH dependencies of the kinetic parameters V and V Km for the enzymic hydrolysis of 4-nitrophenyl sulfate indicate that two groups of pKa 4.7 and 6.0 control the activity of the enzyme. The product inorganic sulfate was shown to be a linear competitive inhibitor of the enzyme at pH 4.0, implying that it is a last released product along the reaction pathway. Inhibition by the phenol product was not observed. Enzymic hydrolysis of 4-nitrophenyl sulfate in 18O enriched water revealed that one atom of solvent oxygen is incorporated per molecule of inorganic sulfate, which is consistent with a mechanism featuring sulfur-oxygen bond cleavage. Evidence is presented based on stopped-flow kinetics, partitioning experiments in the presence of amine nucleophiles, and 18O exchange studies that collectively suggest that the breakdown of a covalent sulfuryl enzyme intermediate probably is not the rate-limiting step along the reaction pathway. The substrate specificity of the enzyme was examined by testing a variety of sulfate and phosphate esters as inhibitors of the hydrolysis of 4-nitrophenyl sulfate. The Cbz-l-Phe-l-Tyrosine-O-sulfate methyl ester serves as a substrate for the enzyme. Apparently substrate activity requires an aromatic sulfate ester whose binding is enhanced by incorporating the aromatic moiety in a hydrophobic matrix.
All Science Journal Classification (ASJC) codes
- Molecular Biology