A complex iron-calcium cofactor catalyzing phosphotransfer chemistry

Abstract

Alkaline phosphatases play a crucial role in phosphate acquisition by microorganisms.To expand our understanding of catalysis by this class of enzymes, we have determined thestructure of the widely occurring microbial alkaline phosphatase PhoX. The enzymecontains a complex active-site cofactor comprising two antiferromagnetically coupledferric iron ions (Fe3+), three calcium ions (Ca2+), and an oxo group bridging three of themetal ions. Notably, the main part of the cofactor resembles synthetic oxide-centeredtriangular metal complexes. Structures of PhoX-ligand complexes reveal how theactive-site metal ions bind substrate and implicate the cofactor oxo group in thecatalytic mechanism. The presence of iron in PhoX raises the possibility that ironbioavailability limits microbial phosphate acquisition.