Assignment of the CO-sensitive carboxyl group in mitochondrial forms of cytochrome c oxidase using yeast mutants

► CO photolysis from yeast CcO induces the same carboxyl change seen in bovine CcO. ► E243D and I67N mutants were analysed to identify the responsible amino acid. ► Changes in the IR spectra definitively identify E243 as the CO-sensitive amino acid. ► The I67N mutation inhibits the enzyme by interfering with E243 function.