A complex iron-calcium cofactor catalyzing phosphotransfer chemistry

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Science  05 Sep 2014:
Vol. 345, Issue 6201, pp. 1170-1173
DOI: 10.1126/science.1254237

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Cofactors linked to nutrient limitation

Microbes require inventive ways to acquire scarce nutrients from the environment. Enzymes that catalyze the acquisition of phosphorus from dissolved organic matter, for example, rely on complex metal cofactors in the active site. Yong et al. determined the crystal structure of the PhoX alkaline phosphatase from Pseudomonas fluorescens (see the Perspective by Moore). The metal centers arrange themselves in a triangular structure of two iron atoms and one calcium atom, bridged together by an oxide ion. The presence of iron, which itself is a trace nutrient in most environments, suggests that it limits phosphorus acquisition.

Science, this issue p. 1170; see also p. 1120


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 the structure of the widely occurring microbial alkaline phosphatase PhoX. The enzyme contains a complex active-site cofactor comprising two antiferromagnetically coupled ferric iron ions (Fe3+), three calcium ions (Ca2+), and an oxo group bridging three of the metal ions. Notably, the main part of the cofactor resembles synthetic oxide-centered triangular metal complexes. Structures of PhoX-ligand complexes reveal how the active-site metal ions bind substrate and implicate the cofactor oxo group in the catalytic mechanism. The presence of iron in PhoX raises the possibility that iron bioavailability limits microbial phosphate acquisition.

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