Acquiring a Trace Element

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Science  28 Sep 2007:
Vol. 317, Issue 5846, pp. 1835
DOI: 10.1126/science.317.5846.1835a

Iron, as the central element in heme cofactors or as part of metal clusters, endows enzymes with the capacity to carry out a much wider range of redox reactions (such as those in respiration and photosynthesis) than is supported by the functional groups of the genetically encoded amino acids. Hence, the acquisition of iron is a highly competitive endeavor, and as ocean supplementation experiments have shown, iron can be a limiting nutrient for the growth of plankton. Nevertheless, marine organisms face a special challenge because iron in an aqueous and aerobic environment of neutral pH is present mostly in insoluble forms. The bacterial solution has been the manufacture and secretion of siderophores, small molecules that chelate Fe(III). Following on their previous identification of a borate-siderophore interaction, Harris et al. provide a fuller characterization of the equilibria in the reaction of B(OH)3 and vibrioferrin, a siderophore of Marinobacter spp. The tetrahedral coordination of B(III) by the pair of α-hydroxycarboxylate moieties in vibrioferrin is highly pH-dependent, and accounting for the protons contributed by the hydroxyls as well as one donated by solvent allowed the authors to assemble the formal binding constants for the multiple borate-vibrioferrin complexes. Extending this analysis to the other two types of siderophores— the catecholates and the hydroxamates—revealed that the former are also competent to bind boron whereas the latter are not. Whether any of these capabilities are in fact used by the siderophore producers is as yet unclear, though low-pH environments may be one place to look. — GJC

J. Am. Chem. Soc. 129, 10.1021/ja073788v (2007).

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