Paracoccus denitrificans cytochrome c oxidase: a kinetic study on the two-and four-subunit complexes

Cytochrome c oxidase from Paracoccus denitrificans has been purified in two different forms differing in polypeptide composition. An enzyme containing polypeptides I–IV is obtained when the purification procedure is performed in β-d-dodecylmaltoside. If, however, Triton X-100 is used to purify the enzyme under otherwise identical conditions, an enzyme is obtained containing only subunits I–II. The two enzymes are undistinguishable by optical spectroscopy but show significant differences in the transient and steady-state time regimes, as studied by stopped-flow spectroscopy. The observed differences, however, are not due to removal of subunits III and IV, but rather to a specific effect of Triton X-100 which appears to affect cytochrome c binding. From these results it is not expected that subunits III and IV play any significant role in cytochrome c binding and, possibly, in the subsequent electron transfer processes. The results also suggest that both electrostatic and hydrophobic interactions may be important in the initial electron transfer process from cytochrome c.