Trans-substitution of the proximal hydrogen bond in myoglobin: II. Energetics, functional consequences, and implications for hemoglobin allostery

The trans-substituted histidine to glycine mutant of sperm whale myoglobin (H93G Mb) is used to study energetics of proximal hydrogen bonding, proxima l ligand-heme interactions, and coupling to distal ligand binding, Comparis on of mono- and dimethylimidazole structural isomers shows that the hydroge n bond between the proximal ligand and the neighboring Ser92 hydroxyl (posi tion F7) is stabilizing. The range of hydrogen bond stabilities measured he re for different distal ligand complexes ranges from -0.7 kcal/mol (monomet hylimidazole isomers to MbCO) to -4.1 kcal/mol (dimethylimidazole isomers t o MbCN), This range of hydrogen bond stabilities, which is similar to that seen in protein mutagenesis unfolding studies, demonstrates the high sensit ivity of the hydrogen bond to modest structural perturbations, The degree t o which the a-methyl group destabilizes proximal ligand binding is found to depend inversely on the total electronic spin. For monomethylimidazole pro ximal ligands, distal ligand binding weakens the proximal hydrogen bond com pared to deoxyMb, Surprisingly, this trend is largely reversed for the dime thylimidazole proximal ligands. These results demonstrate strong coupling b etween the proximal protein matrix and distal ligand binding. These results provide an explanation for the strong avoidance of hydrogen bonding residu es at position F7 in hemoglobin sequences, (C) 2000 Wiley-Liss, Inc.