Dynamics and thermodynamics of acid-alkaline transitions in metmyoglobins lacking the usual distal histidine residue.

The kinetics of the acid-alkaline transition in the ferric myoglobins from the gastropodic mollusc Dolabella auricularia and the shark Mustelus japonicus, which possess the distal Val E7 and Gln E7, respectively, has been investigated using the paramagnetic 1H-NMR saturation transfer measurements in order to gain insight into functional properties of these non-His distal residues. Both myoglobins possess the penta-coordinated heme below the pK of the transition (7.8 and 10.0 for Dolabella and Mustelus myoglobins, respectively) and bind OH- above the pK. The pH dependence of the transition rates and the relatively high activation barrier (58 +/- 9 kJ/mol) for the dissociation of the Fe-bound OH- in Dolabella myoglobin indicate a strong interaction between the bound ligand and the guanidino NH proton of the Arg E10 in Dolabella myoglobin. Such a strong interaction between Fe-bound OH- and the Arg E10 side-chain in Dolabella myoglobin is also manifested in the EPR spectra. For Mustelus myoglobin, the pH and temperature dependence studies on the kinetics strongly suggest that the distal Gln E7 in this myoglobin does not contribute significantly to stabilize the Fe-bound ligand.