Rewiring the ‘Push-Pull’ Catalytic Machinery of a Heme Enzyme using an Expanded Genetic Code

Nature employs a limited number of genetically encoded axial ligands to control diverse heme enzyme activities. Deciphering the functional significance of these ligands requires a quantitative understanding of how their electron donating capabilities modulate the structures and reactivities of the iconic ferryl intermediates compounds I and II. However, probing these relationships experimentally has proven challenging as ligand substitutions accessible via conventional mutagenesis do not allow fine tuning of electron donation and typically abolish catalytic function. Here we exploit engineered translation components to replace the histidine ligand of cytochrome c peroxidase (CcP) by a less electron donating Nδ-methyl histidine (Me-His) with little effect on enzyme structure. The rate of formation (k1) and the reactivity (k2) of compound I are unaffected by ligand substitution. In contrast, proton coupled electron transfer to compound II (k3) is 10-fold slower in CcP Me-His, providing a direct link between electron do-nation and compound II reactivity which can be explained by weaker electron donation from the Me-His ligand (‘the push’) affording an electron deficient ferryl-oxygen with reduced proton affinity (‘the pull’). The deleterious effects of the Me-His ligand can be fully compensated by introducing a W51F mutation, designed to increase ‘the pull’ by removing a hydrogen bond to the ferryl-oxygen. Analogous substitutions in ascorbate peroxidase (APX) lead to similar activity trends to those observed in CcP, suggesting a common mechanistic strategy is employed by enzymes using distinct electron transfer pathways. Our study highlights how non-canonical active site substitutions can be used to directly probe and deconstruct highly evolved bioinorganic mechanisms.