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Detailed comparisons of the carbon monoxide FTIR spectra and ligand-binding properties of a library of E7, E11, and B10 mutants indicate significant differences in the role of electrostatic interactions in the distal pockets of wild-type sperm whale myoglobin and soybean leghemoglobin. In myoglobin, strong hydrogen bonds from several closely related conformations of the distal histidine (HisE7) side chain preferentially stabilize bound oxygen. In leghemoglobin, the imidazole side chain of HisE7 is confined to a single conformation, which only weakly hydrogen bonds to bound ligands. The phenol side chain of TyrB10 appears to “fix” the position of HisE7, probably by donating a hydrogen bond to the Nä atom of the imidazole side chain. The proximal pocket of leghemoglobin is designed to favor strong coordination bonds between the heme iron and axial ligands. Thus, high oxygen affinity in leghemoglobin is established by a favorable staggered geometry of the proximal histidine. The interaction between HisE7 and TyrB10 prevents overstabilization of bound oxygen. If hydrogen bonding from HisE7 were as strong as it is in mammalian myoglobin, the resultant ultrahigh affinity of leghemoglobin would prevent oxygen transport in root nodules.