Oxidation of bis(terpyridine)cobalt(II) chloride by cytochrome c peroxidase compounds I and II

The bis(terpyridine)cobalt(II), Co(terpy)2(2+), reduction of cytochrome c peroxidase compound I, CcP-I, has been investigated using stopped-flow techniques as a function of ionic strength in pH 7.5 buffers at 25 degrees C. Co(terpy)2(2+) initially reduces the Trp191 radical site in CcP-I with an apparent second-order rate constant, k2, equal to 6.0+/-0.4x10(6) M(-1)s(-1) at 0.01 M ionic strength. A pseudo-first-order rate constant of 480 s(-1) was observed for the reduction of CcP-I by 79 microM Co(terpy)2(2+) at 0.01 M ionic strength. The one-electron reduction of CcP-I produces a second enzyme intermediate, CcP compound II (CcP-II), which contains an oxyferryl, Fe(IV), heme. Reduction of the Fe(IV) heme in CcP-II by Co(terpy)2(2+) shows saturation kinetics with a maximum observed rate constant, k3max, of 24+/-2 s(-1) at 0.01 M ionic strength. At low reductant concentrations, the apparent second-order rate constant for Co(terpy)2(2+) reduction of CcP-II, k3, is 1.2+/-0.5x10(6) M(-1) s-1. All three rate constants decrease with increasing ionic strength. At 0.10 M ionic strength, values of k2, k3, and k3max decrease to 6.0+/-0.8x10(5) M(-1) s(-1), 1.2+/-0.5x10(5) M(-1) s(-1), and 11+/-3 s(-1), respectively. Both the product, Co(terpy)2(3+), and ferricytochrome c inhibit the rate of Co(terpy)2(2+) reduction of CcP-I and CcP-II. Gel-filtration studies show that a minimum of two Co(terpy)2(3+) molecules bind to the native enzyme in low ionic strength buffers.