KINETICS AND EQUILIBRIA OF CYANIDE BINDING TO COPRINUS-CINEREUS PEROXIDASE

Equilibria and kinetics of cyanide binding to Coprinus cinereus peroxidase have been studied. Spectral results show the presence of one heme binding site per molecule of enzyme. Isosbestic points between the native enzyme and the enzyme-cyanide complex are 349, 413.5, 492, 521 and 640 nm. The dissociation constant is (0.52 +/- 0.03) mu-M at 25-degrees-C in phosphate buffer of 0.1 M ionic strength at pH 6.98. The pH dependence of the apparent second-order rate constant indicates the presence of an acidic heme-linked group in the enzyme with a pK(a) of 5.1 +/- 0.2. The protonated form of cyanide binds to the basic form of the enzyme with a second-order association rate constant of (7.7 +/- 0.2) x 10(5) M-1 s-1.