Engineering Ascaris hemoglobin oxygen affinity in sperm whale myoglobin: role of tyrosine B10.

The contribution to oxygen stabilization of a tyrosine residue in topological position (B10) has been studied in sperm whale myoglobin by simultaneous replacement of residues at positions (B10), (E7) and (E10) as suggested by analysis of the sequence of high oxygen affinity hemoglobins, such as that of the nematode Ascaris suum. Kinetic and equilibrium experiments with the gaseous ligands oxygen and carbon monoxide show that indeed the introduction of tyrosine (B10), together with replacement of the distal histidine (E7) with glutamine, is associated with a large decrease in the oxygen dissociation rate constant. Our results are consistent with the possible formation in the distal pocket of two hydrogen bonds with the iron-bound oxygen.