Heme a and Copper Environments in Cytochrome Oxidase

Abstract

In 1928, Warburg and Negelein obtained the action spectrum of a respiratory enzyme by utilizing the recovery of a carbon monoxide (CO)-inhibited respiration of yeast suspensions by illumination, and predicted the heme-like nature of the active site of this enzyme (Warburg and Negelein, 1928). Ten years later, from spectral examinations of the effect of several respiratory inhibitors on the heart muscle preparation, Keilin and Hartree postulated the existence of cytochrome au, and considered the identity of this cytochrome with cytochrome oxidase or the respiratory enzyme. However, they reserved the final conclusion, since they failed to observe spectral changes inducible by illumination (Keilin and Hartree, 1939). This discrepancy, however, was reconciled by the finding of Chance that under the atmosphere of CO and O₂ in a 1:1 ratio, instead of 100% CO as employed by Keilin and Hartree, the CO complex was photodissociated easily at room temperatures (Chance, 1953). Chance et al. also found that at 77°K CO was photodissociated irreversibly from its complex (Chance et al., 1965), and later the recombination of CO was shown to occur with the midpoint temperature of around 180°K as the sample temperature was raised (Yonetani et al., 1973).