Putative nickel-binding sites of microbial proteins

Nickel is biologically important because of its catalytic role in the mechanisms of action of metalloenzymes, and also because of its toxic cellular effects. There exist at least 3 groups of nickel-binding proteins in microorganisms: nickel-specific transporters, accessory proteins involved in nickel incorporation and nickel-containing enzymes. The differences in their physiological functions determine the nature of the ligands and the structures of the nickel-binding sites. The homology among the accessory proteins HypB, ORF4 and UreG suggests that the mechanism of nickel incorporation into hydrogenases in Escherichia coli is the same as or similar to that into hydrogenases of Rhodobacter capsulatus and into urease of Klebsiella aerogenes.