Some contemporary issues relevant to the chemistry of mammalian cytochrome c oxidase are discussed. These include the optical properties of heme A and the spectroscopic consequences of the differences in side-chain substitution compared to heme B; a common fallacy concerning the electrostatic exchange interaction between cytochrome a3 and CuB; the question of the number and location of the copper components of the enzyme; and the mode of binding of ligands such as cyanide and azide.
