Trans-substitution of the proximal hydrogen bond in myoglobin: I. Structural consequences of hydrogen bond deletion

The structural role of a sidechain to side-chain protein hydrogen bond is e xamined using trans-substitution of the proximal histidine of myoglobin wit h methylimidazoles (Barrick, Biochemistry 1994;33:6546-6554), Modification of the chemical structure of exogenous ligands allows this hydrogen bond to be disrupted. Comparison of the crystal structures of H93G myoglobin compl exed 4-methylimidazole (4meimd; methylation at carbon 4) and 1-methylimidaz ole (1meimd; methylation at the adjacent nitrogen, preventing hydrogen bond ing between the imidazole ligand and the protein) shows that the polypeptid e, heme, and methylimidazole orientations are the same within error. For 4m eimd there appear to be major and minor conformations corresponding to diff erent tautomeric states of the ligand, Conformational heterogeneity is also seen in the hyperfine-shifted region of the NMR spectrum of 4meimd complex ed with high-spin H93G deoxyMb. The major conformation of the 4meimd ligand and the 1meimd ligand, as seen in the respective crystal structures, are q uite similar except that the proximal ligand NH-to-Ser92-OH hydrogen bond i s eliminated in the 1meimd complex, and instead the proximal ligand CH is a djacent to the Ser92-OH. Thus, this system provides a means to eliminate th e Mb proximal hydrogen bond in a chemically and structurally conservative w ay. (C) 2000 Wiley-Liss, Inc.