Enhancement of Laccase Activity through the Construction and Breakdown of a Hydrogen Bond at the Type I Copper Center in Escherichia coli CueO and the Deletion Mutant Δα5-7 CueO

CueO is a multicopper oxidase involved in a copper efflux system of Escherichia coli and has high cuprous oxidase activity but little or no oxidizing activity toward various organic substances. However, its activity toward oxidization of organic substrates was found to be considerably increased by the removal of the methionine-rich helical segment that covers the substrate-binding site (Delta alpha 5-7 CueO) [Kataoka, K., et al. (2007) J. Mol. Biol. 373, 141]. In the study presented here, mutations at Pro444 to construct a second NH-S hydrogen bond between the backbone amide and coordinating Cys500 thiolate of the type 1 copper are shown to result in positive shifts in the redox potential of this copper center and enhanced oxidase activity in CueO. Analogous enhancement of the activity of Delta alpha 5-7 CueO has been identified only in the Pro444Gly mutant because Pro444 mutants limit the incorporation of copper ions into the trinuclear copper center. The activities of both CueO and Delta alpha 5-7 CueO were also enhanced by mutations to break down the hydrogen bond between the imidazole group of His443 that is coordinated to the type I copper and the beta-carboxy group of Asp439 that is located in the outer sphere of the type I copper center. A synergetic effect of the positive shift in the redox potential of the type I copper center and the increase in enzyme activity has been achieved by the double mutation of Pro444 and Asp439 of CueO. Absorption, circular dichroism, and resonance Raman spectra indicate that the characteristics of the Cu(II)-S(Cys) bond were only minimally perturbed by mutations involving formation or disruption of a hydrogen bond from the coordinating groups to the type I copper. This study provides widely applicable strategies for tuning the activities of multicopper oxidases.