부산대학교 도서관

Combined quantum chemical and molecular mechanics geometry optimisations have been performed on myoglobin without or with O2 or CO bound to the haem group. The results show that the distal histidine residue is protonated on the Nϵ2 atom and forms a hydrogen bond to the haem ligand both in the O2 and the CO complexes. We have also re-refined the crystal structure of CO–myoglobin by a combined quantum chemical and crystallographic refinement. Thereby, we probably obtain the most accurate available structure of the active site of this complex, showing a Fe–C–O angle of 171°, and Fe–C and C–O bond lengths of 170–171 and 116–117 pm. The resulting structures have been used to calculate the strength of the hydrogen bond between the distal histidine residue and O2 or CO in the protein. This amounts to 31–33 kJ/mol for O2 and 2–3 kJ/mol for CO. The difference in hydrogen-bond strength is 21–22 kJ/mol when corrected for entropy effects. This is slightly larger than the observed discrimination between O2 or CO by myoglobin, 17 kJ/mol. We have also estimated the strain of the active site inside the protein. It is 2–4 kJ/mol larger for the O2 complex than for the CO complex, independent of which crystal structure the calculations are based on. Together, these results clearly show that myoglobin discriminates between O2 and CO mainly by electrostatic interactions, rather than by steric strain. [Copyright &y& Elsevier]