Interaction of Carbon Monoxide with the Apoptosis-Inducing Cytochrome c−Cardiolipin Complex

Abstract

The interaction of mitochondrial cytochrome (cyt) c with cardiolipin (CL) is involved in the initial stages of apoptosis. This interaction can lead to destabilization of the heme-Met80 bond and peroxidase activity [Basova, L. V., et al. (2007) Biochemistry 46, 3423-3434]. We show that under these conditions carbon monoxide (CO) binds to cyt c, with very high affinity ( approximately 5 x 10(7) M(-1)), in contrast to the native cyt c protein involved in respiratory electron shuttling that does not bind CO. Binding of CO to the cyt c-CL complex inhibits its peroxidase activity. Photodissociated CO from the cyt c-CL complex shows <20% picosecond geminate rebinding and predominantly bimolecular rebinding, with a second-order rate constant of approximately 10(7) M(-1) s(-1), an order of magnitude higher than in myoglobin. These findings contrast with those of Met80X mutant cyt c, where picosecond geminate recombination dominates due to the rigidity of the protein. Our data imply that CL leads to substantial changes in protein conformation and flexibility, allowing access of ligands to the heme. Together with the findings that (a) approximately 30 CL per cyt c are required for full CO binding and (b) salt-induced dissociation indicates that the two negative headgroup charges interact with approximately 5 positive surface charges of the protein, these results are consistent with a CL anchorage model with an acyl chain impaled in the protein [Kalanxhi, E., and Wallace, C. J. A. (2007) Biochem. J. 407, 179-187]. The affinity of CO for the complex is high enough to envisage an antiapoptotic effect of nanomolar CO concentrations via inhibition of the cyt c peroxidase activity.