Heme, axial ligands, Lys22, Pro25, and CXXCH motif in Ht cyt c552. Hydrogen bonding interactions (heavy-atom distances of less than 3 Å) are shown with dashed lines. Shown in cyan are the two residues selected for mutation, Met13 and Lys22. The hydrogen bonding interaction between the axial His ligand and the backbone carbonyl of Pro is highly conserved in Class I cyts c.