Heme redox potentials hold the key to reactivity differences between nitric oxide reductase and heme-copper oxidase

Abstract

Significance Nitric oxide reductases (NORs) catalyze the reduction of NO as part of the denitrification cycle, while structurally related heme-copper oxidases (HCOs) catalyze oxygen reduction during respiration. Despite decades of investigation into the structure/function of HCOs and NORs, factors governing their reaction specificity remain unknown. By tuning E°′ of a biosynthetic model of NOR in myoglobin and using electrochemical, spectroscopic, and computational methods to understand the impact of such tuning, this work reveals heme E°′ as a key to determining the specificity of these two enzymes and explains their cross-reactivity. Beyond resolving such a long-standing issue, this work provides guidelines for the design and synthesis of artificial metalloenzymes that can catalyze reactions relevant to global nitrogen cycles and biological respiration that is important for oxygen reduction reaction in fuel cells.