pH Dependence and Structural Interpretation of the Reactions of Coprinus cinereus Peroxidase with Hydrogen Peroxide, Ferulic Acid, and 2,2‘-Azinobis(3-ethylbenzthiazoline-6-sulfonic acid)

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Abstract
Steady-state and transient-state analysis of Coprinus cinereus peroxidase, CIP (identical to Arthromyces ramosus peroxidase), was used to characterize the kinetics of the three fundamental steps in heme peroxidase catalysis: compound I (cpd I) formation, cpd I reduction, and compound II (cpd II) reduction. The rate constant k1 for cpd I formation determined by transient-state analysis is (9.9 ± 0.6) × 106 M-1 s-1. The k1 determined by steady-state analysis is (8.8 ± 0.6) × 106 M-1 s-1 in the presence of ferulic acid and (6.7 ± 0.2) × 106 M-1 s-1 in the presence of ABTS. The value of k1 is constant from pH 6 to 11. However, at low pH the value of k1 decreases, corresponding to titration of an enzyme group with a pKa of 5.0. Titration of this group is also detected from cyanide-binding kinetics. Furthermore, titration of this group is linked with marked spectroscopic changes unique to CIP. We ascribe these changes to protonation of proximal His183. A very low pKa is proposed for distal His55 in the resting state of CIP. The rate constants, k2 for cpd I and k3 for cpd II reduction, are very large for both ferulic acid and 2,2‘-azinobis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS). For ferulic acid, transient-state kinetic analysis shows that the values of k2 and k3 are identical at pH 5−6, and the ratio k2/k3 increases to 10 at pH 10. The similar magnitude of k2 and k3 is unusual for a peroxidase. Both k2 and k3 decrease with increasing pH, and both are influenced by two ionizations: one with a pKa value near 7, assumed to reflect the protonation of His55; and the other with pKa of 9.0 ± 0.7 for k2 and 8.8 ± 0.4 for k3, perhaps reflecting the phenol-linked deprotonation of ferulic acid. Steady-state analysis at pH 7.0 gave k2k3/( k2 + k3) = (2.2 ± 0.1) × 107 M-1 s-1 for ferulic acid, and (2.0 ± 0.7) × 107 M-1 s-1 for ABTS and revealed a unimolecular step with ku = 1500 s-1, ascribed to slow ABTS radical product release. From transient-state results at pH 7, the values of k2 and k3 were found to be identical also for ABTS. A mechanism for cpd I and II reduction involving distal histidine and arginine is proposed.

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