Characterization of heme-coordinating histidyl residues of an engineered six-coordinated myoglobin mutant based on the reactivity with diethylpyrocarbonate, mass spectrometry, and electron paramagnetic resonance spectroscopy

A genetically engineered porcine myoglobin triple mutant (H64V/V68H/H93A) (VHA-Mb) contains 6 non-axial His residues (His24, His36, His48, His81, His82, and His119) besides two candidate axial His residues (His68 and His97). Although previous resonance Raman study on the ferric VHA-Mb were not conclusive for its coordination structure, present EPR parameters of the ferric VHA-Mb were consistent with bis-imidazole coordination of His68/His97. We further investigated the reactivity of these possible His ligands with diethylpyrocarbonate (DEPC) to clarify the coordination structure and their protonation states in ferric form. We found that the non-axial His residues were easily modified with a low concentration of DEPC based on UV spectral changes and MALDI-TOF-MS analyses. On the other hand, the two candidate axial His ligands were protected from the modification due to a limited steric exposure of their imidazoles to solvent, the Fe3+-Nɛ2 coordination bond, and the protonation of Nδ1 by forming a hydrogen bond with their immediate surroundings. However, once N-carbethoxylation occurred at Nɛ2 of His97, resulting in a disruption of the heme Fe3+-Nɛ2 coordination bond, it facilitated the second N-carbethoxylation to take place at Nδ1 of the same imidazole ring, leading to a bis-N-carbethoxylated derivative and further to a ring-opened derivative. These phenomena were consistent with the bis-His68/His97 coordination. Further, these were not observed at all for cytochrome b561, a transmembrane di-heme containing protein responsible for the ascorbate-specific transmembrane electron transfer, where only a specific Nδ1-carbethoxylation of axial His occurred at a low concentration of DEPC, leading to an inhibition of the electron acceptance from ascorbate without a release of the heme. These distinct results might be related to a specific physiological mechanism being operative at the cytosolic heme center of cytochrome b561.