Kinetics and mechanism of reduction of ferricytochrome c by glutathione and L-cysteine: a comparative study.

The kinetics and mechanism of the reduction of ferricytochrome c [Cyt c(III)] by substrates namely glutathione (GSH) and L-cysteine (L-cys) have been investigated spectrophotometrically employing [substrate]T >> [Cyt c(III)]T. The reaction exhibits first order dependence in [substrate]T and [Cyt c(III)]T. The pseudo-first order rate constant increases with an increase in pH, indicating that the conjugate base form of the HCyt c(III) is a better oxidant than the parent HCyt c(III). The electron transfer rate constants between the oxidants and GSH for both the k1 and k2 paths are found to be greater than that with L-cysteine. Hence, GSH is a better reductant of Cyt c(III) as compared to L-cysteine. A suitable mechanism has been proposed on the basis of experimental findings. The deprotonation constant for HCyt c(III) and the second order rate constants of k1 and k2 paths for the present reaction at 25 degrees C have been determined.