1.3 angstrom structure of arylsulfatase from Pseudomonas aeruginosa establishes the catalytic mechanism of sulfate ester cleavage in the sulfatase family

Background: Sulfatases constitute a family of enzymes with a highly conserv ed active site region including a C alpha -formylglycine that is posttransl ationally generated by the oxidation of a conserved cysteine or serine resi due. The crystal structures of two human arylsulfatases, ASA and ASB, along with ASA mutants and their complexes led to different proposals for the ca talytic mechanism in the hydrolysis of sulfate esters. Results: The crystal structure of a bacterial sulfatase from Pseudomonas ae ruginosa (PAS) has been determined at 1.3 Angstrom. Fold and active site re gion are strikingly similar to those of the known human sulfatases. The str ucture allows a precise determination of the active site region, unequivoca lly showing the presence of a C alpha -formylglycine hydrate as the key cat alytic residue. Furthermore, the cation located in the active site is unamb iguously characterized as calcium by both its B value and the geometry of i ts coordination sphere. The active site contains a noncovalently bonded sul fate that occupies the same position as the one in para-nitrocate-cholsulfa te in previously studied ASA complexes. Conclusions: The structure of PAS shows that the resting state of the key c atalytic residue in sulfatases is a formylglycine hydrate. These structural data establish a mechanism for sulfate ester cleavage involving an aldehyd e hydrate as the functional group that initiates the reaction through a nuc leophilic attack on the sulfur atom in the substrate. The alcohol is elimin ated from a reaction intermediate containing pentacoordinated sulfur. Subse quent elimination of the sulfate regenerates the aldehyde, which is again h ydrated. The metal cation involved in stabilizing the charge and anchoring the substrate during catalysis is established as calcium.