Catalase-peroxidase active site restructuring by a distant and "inactive" domain

Catalase-peroxidases are composed of two peroxidase-like domains. The N-terminal domain contains the heme-dependent, bifunctional active site. The C-terminal domain does not bind heme, has no catalytic activity, and is separated from the active site by >30 A. Nevertheless, without the C-terminal domain, the N-terminal domain exhibits neither catalase nor peroxidase activity due to the apparent coordination of the distal histidine to the heme iron. Here we report the ability of the separately expressed and isolated C-terminal domain (KatG(C)) to restructure the N-terminal domain (KatG(N)) to its bifunctional conformation. Addition of equimolar KatG(C) to KatG(N) decreased the hexacoordinate low-spin heme complex and increased the high-spin species (pentacoordinate and hexacoordinate). EPR spectra of the domain mixture showed a distribution between high-spin species nearly identical to that of wild-type KatG. The CD spectrum for the 1:1 physical mixture of the domains was identical to an arithmetic composite of individual spectra for KatG(N) and KatG(C). Both physical and arithmetic mixtures were nearly identical to the spectrum for wild-type KatG, suggesting that major shifts in secondary structure did not accompany active site reconfiguration. With the shift in heme environment, the parallel return of catalase and peroxidase activity was observed. Inclusion of bovine serum albumin instead of KatG(C) produced no activity, indicating that specific interdomain interactions were required to reestablish the bifunctional active site. Apparent constants for reactivation (k(react) approximately 4 x 10(-3) min(-1)) indicate that a slow process like movement of established structural elements may precede the restructuring of the heme environment and return of catalytic activity.