The highly conserved methionine of subunit I of the heme-copper oxidases is not at the heme-copper dinuclear center: mutagenesis of M110 in subunit I of cytochrome bo3-type ubiquinol oxidase from Escherichia coli.

A common feature within the heme-copper oxidase superfamily is the dinuclear heme-copper center. Analysis via extended X-ray absorption fine structure (EXAFS) has led to the proposal that sulfur may be bound to CuB, a component of the dinuclear center, and a highly conserved methionine (M110 in the E. coli oxidase) in subunit I has been proposed as the ligand. Recent models of subunit I, however, suggest that this residue is unlikely to be near CuB, but is predicted to be near the low spin heme component of the heme-copper oxidases. In this paper, the role of M110 is examined by spectroscopic analyses of site-directed mutants of the bo3-type oxidase from Escherichia coli. The results show that M110 is a non-essential residue and suggest that it is probably not near the heme-copper dinuclear center.