Cytochrome oxidase preparations are heterogeneous, so ligand binding to the purified enzyme is multiphasic. The usual strategy when analyzing cyanide binding kinetics is to fit a multiexponential expression to the progress curve. However, a more satisfying approach is to fit a stretched exponential function from which the distribution of the rate constants can be estimated. By analyzing the data in which a preparation of the `fast' cyanide binding cytochrome oxidase is converted to the `slow' form of the enzyme, it is shown that the use of the multiexponential function, rather than the stretched exponential, is barely justified at best. Moreover, the stretched exponential approach shows that the relatively narrow, almost normal distribution of small time constants (t) observed with the `fast' form of the enzyme becomes a broad, long-tailed distribution in the `slow' form of the enzyme. The enzyme heterogeneity inferred from the multiexponential model can be characterized unambiguously using the stretched exponential model.

Cytochrome oxidase (E.C. 1.9.3.1) is the terminal enzyme of the respiratory electron transfer chain of many organisms. It oxidizes ferrocytochrome c reducing oxygen to water and generates a proton electrochemical potential. Many bacteria have homologous enzymes such as cytochrome bo (E.C. 1.10.3._) that utilize quinol rather than ferrocytochrome c as the electron donor, but these enzymes are otherwise functionally and structurally similar (Saraste et al., 1991). In each case, the enzyme has a binuclear haem-copper center at which the oxygen chemistry takes place. In the eukaryotic enzyme, this center is made up of a five-coordinate haem a₃ atoms and a copper atom (Cu₈).

Cytochrome oxidase (like its homologues) binds cyanide, azide, sulphide, carbon monoxide, nitric oxide and various other ligands (Nicholls et al., 1976) in the vicinity of the binuclear center of the enzyme. The binding of ligands to the binuclear center is usually monitored spectroscopically because the spin state of the haem changes on ligand binding (Brill and Williams, 1961; and Smith and Williams, 1968) which is associated with a change in the wavelengths of the haem absorbance maxima. For example, the cyanide-binding spectrum of oxidized cytochrome oxidase has features in the Soret (around 420 nm) and in the visible (around 600 nm) regions (Vanneste, 1966).

Cytochrome Oxidase, Kinetics of Cyanide Binding Cyanide, Cytochrome oxidase, Ligand binding, haem a₃ atoms, Copper atom, Cu₈, Electrochemical potential, Enzyme cyanide complex, Standard exponential models, High-Resolution Exponential Analysis, Sperm-Whale Ferrimyoglobin Derivatives.