Evidence for hydrogen bonding of bound dioxygen to the distal histidine of oxycobalt myoglobin and haemoglobin

The origin of the differences in oxygen binding energy in various haemoglobins and myoglobins has long been debated. PERtz¹ proposed that the haem-coordinated histidine (proximal histidine) strains the haem iron in low affinity globins but relaxes it in high affinity globins. The existence of such tension in T-structure deoxyhaemoglobin (deoxyHb) was recently confirmed by electron paramagnetic resonance (EPR)^2,3, resonance Raman ^4,5 and NMR⁶ spectroscopy. Although its contribution to the free energy of cooperativity is insignificant in the deoxy state, the tension at the haem is considered to be ∼1 kcal mol^-1 for the ligated form in which the haem iron moves into the porphyrin plane⁷. The remaining free energy is probably stored in other parts of the molecule. Therefore, a study of the stabilization mechanisms of the oxygenated form became increasingly important. A hydrogen bond between the bound oxygen and the distal histidine has been proposed by Pauling⁸; this would be expected to stabilize the oxy form of the protein and could contribute to the regulation of the oxygen affinity through the oxygen dissociation rate. A series of EPR and functional studies on various cobalt-substituted monomeric haemoglobins and myoglobins suggested the presence of such hydrogen bonding^8-12 and it has recently been established in crystals of oxy iron myoglobin (oxyFeMb) ¹³ and in oxyhaemoglobin¹⁴. Here we present resonance Raman spectra of the oxy forms of cobalt-porphyrin-substituted myoglobin and haemoglobin (CoMb and CoHb) recorded in buffered H₂O and D ₂O solutions at 406.7 nm excitation. Only the Raman lines corresponding to the O - O stretching mode of the bound oxygen¹⁵, appearing near 1,130 cm^-1, are shifted (2-5 cm^-1) on replacement of H₂O by D₂O; no other vibrations, including the Co - O₂ stretching mode, exhibit any frequency shifts. This indicates that the bound oxygen in oxyCoMb and in both subunits of oxyCoHb interacts with the adjacent exchangeable proton, and confirms the formation of a hydrogen bond between the bound oxygen and the distal histidine⁹.