A robust metallo-oxidase from the hyperthermophilic bacterium Aquifex aeolicus

The gene, Aquifex aeolicus AAC07157.1, encoding a multicopper oxidase (McoA) and localized in the genome as part of a putative copper-resistance determinant, has been cloned, over-expressed in Escherichia coli and the recombinant enzyme purified to homogeneity. The purified enzyme shows spectroscopic and biochemical characteristics typical of the well-characterized multicopper oxidase family of enzymes. McoA presents higher specificity (k _cat/K_m) for cuprous and ferrous ions than for aromatic substrates and is therefore designated as a metallo-oxidase. Addition of copper is required for maximal catalytic efficiency. A comparative model structure of McoA has been constructed and a striking structural feature is the presence of a methionine-rich region (residues 321-363), reminiscent of those found in copper homeostasis proteins. The kinetic properties of a mutant enzyme, McoAΔP321-V363, deleted in the methionine-rich region, provide evidence for the key role of this region in the modulation of the catalytic mechanism. McoA has an optimal temperature of 75°C and presents remarkable heat stability at 80 and 90°C, with activity lasting for up to 9 and 5 h, respectively. McoA probably contributes to copper and iron homeostasis in A. aeolicus.