The inactivation and catalytic pathways of horseradish peroxidase with m-chloroperoxybenzoic acid - A spectrophotometric and transient kinetic study

The kinetics of the catalytic cycle and irreversible inactivation of horseradish peroxidase C (HRP-C) reacting with m-chloroperoxybenzoic acid (mCPBA) have been studied by conventional and stopped-flow spectrophotometry, mCPBA oxidized HRP-C to compound I with a second order-rate constant k(1) = 3.6 x 10(7) M(-1) s(-1) at pH 7.0, 25 degrees C. Excess mCPBA subsequently acted as a one-electron reducing substrate, converting compound I to compound II and compound II to resting, ferric enzyme. In both of these reactions, spectrally distinct, transient forms of the enzyme were observed (lambda(max) = 411 nm, epsilon = 45 mM(-1) cm(-1) for compound I with mCPB-4, and lambda(max) = 408 nm, epsilon = 77 mM(-1) cm(-1) for compound II with mCPBA), The compound I-mCPBA intermediate (shown by near infrared spectroscopy to be identical to P965) decayed either to compound II in a catalytic cycle (k(3) = 6.4 x 10(-3) s(-1)) or, in a competing inactivation reaction, to verdohemoprotein (k(i) = 3.3 x 10(-3) s(-1)). Thus, a partition ratio of r = 2 is obtained for the inactivation of ferric HRP-C by mCPBA, The intermediate formed from compound II with mCPBA is not part of the inactivation pathway and only decays via the catalytic cycle to give resting, ferric enzyme (k(5) = 1.0 x 10(-3) s(-1)). The data are compared with those from earlier steady-state kinetic studies and demonstrate the importance of single turnover experiments, The results are discussed in terms of the physiologically relevant reactions of plant peroxidases with hydrogen peroxide.