Proton electron nuclear double resonance from nitrosyl horse heart myoglobin: The role of His-E7 and Val-E11

Electron nuclear double resonance (ENDOR) spectroscopy has been used to stu dy protons in nitrosyl horse heart myoglobin (MbNO). H-1 ENDOR spectra were recorded for different settings of the magnetic field. Detailed analysis o f the ENDOR powder spectra, using computer simulation, based on the "orient ation-selection" principle, leads to the identification of the available pr otons in the heme pocket. We observe hyperfine interactions of the N(HisF8) -Fe2+-N(NO) complex with five protons in axial and with eight protons in th e rhombic symmetry along different orientations, including those of the pri ncipal axes of the g-tensor. Protons from His-E7 and Val-E11 residues are i dentified in the two symmetries, rhombic and axial, exhibited by MbNO. Our results indicate that both residues are present inside the heme pocket and help to stabilize one particular conformation.