CRYSTAL-STRUCTURE OF HUMAN ARYLSULFATASE-A - THE ALDEHYDE FUNCTION AND THE METAL-ION AT THE ACTIVE-SITE SUGGEST A NOVEL MECHANISM FOR SULFATE ESTER HYDROLYSIS

Human lysosomal arylsulfatase A (ASA) is a prototype member of the sul fatase family, These enzymes require the posttranslational oxidation o f the -CH2SH group of a conserved cysteine to an aldehyde, yielding a formylglycine. Without this modification sulfatases are catalytically inactive, as revealed by a lysosomal storage disorder known as multipl e sulfatase deficiency. The 2.1 Angstrom resolution X-ray crystal stru cture shows an ASA homooctamer composed of a tetramer of dimers, (alph a(2))(4). The alpha/beta fold of the monomer has significant structura l analogy to another hydrolytic enzyme, the alkaline phosphatase, and superposition of these two structures shows that the active centers ar e located in largely identical positions. The functionally essential f ormylglycine is located in a positively charged pocket and acts as lig and to an octahedrally coordinated metal ion interpreted as Mg2+. The electron density at the formylglycine suggests the presence of a 2-fol d disordered aldehyde group with the possible contribution of an aldeh yde hydrate, -CH(OH)(2), with gem-hydroxyl groups. In the proposed cat alytic mechanism, the aldehyde accepts a water molecule to form a hydr ate. One of the two hydroxyl groups hydrolyzes the substrate sulfate e ster via a transesterification step, resulting in a covalent intermedi ate, The second hydroxyl serves to eliminate sulfate under inversion o f configuration through C-O cleavage and reformation of the aldehyde. This study provides the structural basis for understanding a novel mec hanism of ester hydrolysis and explains the functional importance of t he unusually modified amino acid.