Nitric oxide binds to the proximal heme coordination site of the ferrocytochrome c/cardiolipin complex: Formation mechanism and dynamics

Mammalian mitochondrial cytochrome c interacts with cardiolipin to form a complex (cyt. c/CL) important in apoptosis. Here we show that this interaction leads to structural changes in ferrocytochrome c that leads to an open coordinate site on the central iron, resulting from the dissociation of the intrinsic methionine residue, where NO can rapidly bind (k = 1.2 × 10 ⁷ M^-1 s^-1). Accompanying NO binding, the proximal histidine dissociates leaving the heme pentacoordinate, in contrast to the hexacoordinate nitrosyl adducts of native ferrocytochrome c or of the protein in which the coordinating methionine is removed by chemical modification or mutation.Wepresent the results of stopped-flow and photolysis experiments that show that following initial NO binding to the heme, there ensues an unusually complex set of kinetic steps. The spectral changes associated with these kinetic transitions, together with their dependence on NO concentration, have been determined and lead us to conclude that NO binding to cyt. c/CL takes place via an overall scheme comparable to that described for cytochrome c′ and guanylate cyclase, the final product being one in which NO resides on the proximal side of the heme. In addition, novel features not observed before in other heme proteins forming pentacoordinate nitrosyl species, include a high yield of NO escape after dissociation, rapid (<1 ms) dissociation of proximal histidine upon NO binding and its very fast binding (60 ps) after NO dissociation, and the formation of a hexacoordinate intermediate. These features all point at a remarkable mobility of the proximal heme environment induced by cardiolipin.