Kinetic studies on (N-formyltryptophyl)cytochrome c.

The formylation of the ring nitrogen atom of the tryptophan residue in cytochrome c was carried out and consequent changes in the kinetic properties of the protein were investigated. The reduction of formylated cytochrome c by Cr2+ was studied by stopped-flow techniques. At pH 6.5 the reduction process shows the presence of two phases. One phase (k = 4 X 10(4) M-1-s-1) is dependent on Cr2+ concentration and one phase (k = 5.0 s-1) is not. A study of the temperature dependence of the two phases yields values for their activation energies of 38.6kJ-mol-1 and 42.4kJ-mol-1 respectively. The reaction of the reduced formylated cytochrome c with CO was followed by means of both stopped-flow techniques and flash photolysis. The combination with CO at pH 6.8 measured in stopped-flow experiments shows two phases, both dependent on the concentration of CO (k1 = 1.8 X 10(2) M-1-s-1). If CO was dissociated from the protein by photolysis and then allowed to recombine with it, it was found to do so in a simple manner, at a rate which depended on the concentration of CO (k = 1.9 X 10(2) M-1-s-1). A tentative model which can accommodate these findings is proposed. The reaction of the oxidized form of formylated cytochrome c with NO was followed by means of stopped-flow techniques. The reaction was found to be biphasic with one phase dependent on the concentration of NO (k = 2.8 X 10(3) M-1-s-1) and one phase (k = 0.2x-1) independent of the concentration of NO. This behaviour is compared with that of the native molecule. A comparison of these kinetic observations with those on other tryptophan-specific modifications leads to the conclusion that the main alteration in kinetic properties is due, not to the nature of the modifying group, but rather to the disruption of the normal environment of the haem.