Accommodation of NO in the active site of mammalian and bacterial cytochrome c oxidase aa3

Following different reports on the stoichiometry and configuration of NO binding to mammalian and bacterial reduced cytochrome c oxidase aa(3) (CcO), we investigated NO binding and dynamics in the active site of beef heart CcO as a function of NO concentration, using ultrafast transient absorption and EPR spectroscopy. We find that in the physiological range only one NO molecule binds to heme a(3), and time-resolved experiments indicate that even transient binding to Cu(B) does not occur. Only at very high (approximately 2 mM) concentrations a second NO is accommodated in the active site, although in a different configuration than previously observed for CcO from Paracoccus denitrificans E. Pilet, W. Nitschke, F. Rappaport, T. Soulimane, J.-C. Lambry, U. Liebl and M.H. Vos. Biochemistry 43 (2004) 14118-14127], where we proposed that a second NO does bind to Cu(B). In addition, in the bacterial enzyme two NO molecules can bind already at NO concentrations of approximately 1 microM. The unexpected differences highlighted in this study may relate to differences in the physiological relevance of the CcO-NO interactions in both species.