Catalase-peroxidases in cyanobacteria - Similarities and differences to ascorbate peroxidases

Cyanobacteria (blue-green algae) are oxygenic phototrophic bacteria carryin g out plant-type photosynthesis. The only hydrogen peroxide scavenging enzy mes in at least two unicellular species have been demonstrated to be bifunc tional cytosolic catalase-peroxidases (CatPXs) having considerable homology at the active site with plant ascorbate peroxidases (APXs). In this paper we examined optical and kinetic properties of CatPXs from the cyanobacteria Anacystis nidulans and Synechocystis PCC 6803 and discuss similarities and differences to plant APXs. Both CatPXs and APX showed similar spectra of t he ferric enzyme, the redox intermediate Compound I and the cyanide complex , whereas the spectrum of CatPX Compound II had characteristics reminescent of the spectrum of the native enzyme. Both steady-state and multi-mixing t ransient-state kinetic studies were performed in order to characterize the kinetic behaviour of CatPXs. Bimolecular rate constants of both formation a nd reduction of a CatPX Compound I are presented. Because of its intrinsic high catalase activity (which cannot be found in APXs), the rate constants for Compound I formation were measured with peroxoacetic acid and are shown to be 5.9 x 10(4) M-1 s(-1) for CatPX from A. nidulans and 8.7 x 10(3) M-1 s(-1) for the Synechocystis enzyme. Compared with o-dianisidine (2.7-6.7 x 10(6) M(-1)s(-1)) and pyrogallol (8.6 x 10(4)-1.6 x 10(5) M-1 s(-1)), the rate constant for Compound I reduction by ascorbate was extremely low (5.4 x 10(3) M-1 s(-1) at pH 7.0 and 15 degrees C), in marked contrast to the be haviour of APXs.