A cooperative oxygen finding hemoglobin from Mycobacterium tuberculosis - Stabilization of heme ligands by a distal tyrosine residue

The homodimeric hemoglobin (HbN) from Mycobacterium tuberculosis displays a n extremely high oxygen binding affinity and cooperativity, Sequence alignm ent with other hemoglobins suggests that the proximal F8 ligand is histidin e, the distal E7 residue is leucine, and the B10 position is occupied by ty rosine. To determine how these heme pocket residues regulate the ligand bin ding affinities and physiological functions of HbN, we have measured the re sonance Raman spectra of the O-2, CO, and OH- derivatives of the wild type protein and the B10 Tyr --> Leu and Phe mutants. Taken together these data demonstrate a unique distal environment in which the heme bound ligands str ongly interact with the B10 tyrosine residue. The implications of these dat a on the physiological functions of HbN and another heme-containing protein , cytochrome c oxidase, are considered.