Heme Axial Methionine Fluxion in Pseudomonas aeruginosa Asn64Gln Cytochrome c₅₅₁

Heme axial methionine ligands in ferricytochromes c₅₅₂ from Hydrogenobacter thermophilus (HT) and Nitrosomonas europaea, both members of the cyt c₈ family, display fluxional behavior. The ligand motion, proposed to be inversion at sulfur, results in an unusually small range of hyperfine shifts for heme substituents in these proteins. Herein, heme axial Met fluxion is induced in a structurally homologous cytochrome c₅₅₁ from Pseudomonas aeruginosa (PA) by substituting heme pocket residue Asn64 with Gln. The mutant, PA-N64Q, displays a highly compressed range of heme substituent hyperfine shifts, temperature-dependent heme methyl resonance line broadening, low rhombic magnetic anisotropy, and a magnetic axes orientation consistent with Met orientational averaging. Analysis of NMR properties of PA-N64Q demonstrates that the heme pocket of the mutant resembles that of HT. This result confirms the importance of peripheral interactions and, in particular, residue 64 in determining axial Met orientation and heme electronic structure in proteins in the cyt c₈ family.