Abstract
Cytochrome c oxidase accepts electrons from cytochrome c and transfers them to oxygen to form water. Electrons enter the complex through the Cu(A) site, formed by two copper atoms bound to mitochondrially encoded subunit II. The effect of amino-acid alterations in one of the Cu(A) ligands and in an amino acid adjacent to another of the ligands in the yeast enzyme is examined. Substitution of tyrosine for the Cu(A) ligand, cysteine 221, completely abolishes enzyme activity. In addition, 19 independent revertants of this mutant yeast strain recover function by restoring the cysteine codon. Replacement of a non-conserved glycine at position 228 by valine, adjacent to the Cu(A)-ligand histidine 229, virtually blocks enzyme activity. Activity is restored by inserting alanine or phenylalanine at position 228 or by amino-acid substitutions at nearby codons. Our results demonstrate that the Cu(A) ligand appears to be essential for enzyme function while other residues in the copper-binding region are less functionally constrained.
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