Insights Into How Heme Reduction Potentials Modulate Enzymatic Activities of a Myoglobin-based Functional Oxidase.

Heme-copper oxidase (HCO) is a class of respiratory enzymes that use a heme-copper center to catalyze O2 reduction to H2 O. While heme reduction potential (E°') of different HCO types has been found to vary >500 mV, its impact on HCO activity remains poorly understood. Here, we use a set of myoglobin-based functional HCO models to investigate the mechanism by which heme E°' modulates oxidase activity. Rapid stopped-flow kinetic measurements show that increasing heme E°' by ca. 210 mV results in increases in electron transfer (ET) rates by 30-fold, rate of O2 binding by 12-fold, O2 dissociation by 35-fold, while decreasing O2 affinity by 3-fold. Theoretical calculations reveal that E°' modulation has significant implications on electronic charge of both heme iron and O2 , resulting in increased O2 dissociation and reduced O2 affinity at high E°' values. Overall, this work suggests that fine-tuning E°' in HCOs and other heme enzymes can modulate their substrate affinity, ET rate and enzymatic activity.