The Ph-Induced Selectivity Between Cysteine Or Histidine Coordinated Heme In An Artificial Alpha-Helical Metalloprotein

De Novo metalloprotein design assesses the relationship between metal active site architecture and catalytic reactivity. Herein, we use an alpha-helical scaffold to control the iron coordination geometry when a heme cofactor is allowed to bind to either histidine or cysteine ligands, within a single artificial protein. Consequently, we uncovered a reversible pH-induced switch of the heme axial ligation within this simplified scaffold. Characterization of the specific heme coordination modes was done by using UV/Vis and Electron Paramagnetic Resonance spectroscopies. The penta- or hexa-coordinate thiolate heme (9 <= pH <= 11) and the penta-coordinate imidazole heme (6 <= pH <= 8.5) reproduces well the heme ligation in chloroperoxidases or cyt P450 monooxygenases and peroxidases, respectively. The stability of heme coordination upon ferric/ferrous redox cycling is a crucial property of the construct. At basic pHs, the thiolate mini-heme protein can catalyze O-2 reduction when adsorbed onto a pyrolytic graphite electrode.