THE DISTAL RESIDUE CO INTERACTION IN CARBONMONOXY MYOGLOBINS - A MOLECULAR-DYNAMICS STUDY OF 2 DISTAL HISTIDINE TAUTOMERS

Four independent 90 ps molecular dynamics simulations of sperm-whale w ild-type carbonmonoxy myoglobin (MbCO) have been calculated using a ne w AMBER force field for the haem prosthetic group. Two trajectories ha ve the distal 64N(delta) nitrogen protonated, and two have the 64N(eps ilon) nitrogen protonated; all water molecules within 16 Angstrom of t he carbonyl O are included. In three trajectories, the distal residue remains part of the haem pocket, with the protonated distal nitrogen p ointing into the active site. This is in contrast with the neutron dif fraction crystal structure, but is consistent with the solution phase CO stretching frequencies (nu(CO)) of MbCO and various of its mutants. There are significant differences in the ''closed'' pocket structures found for each tautomer: the 64N(epsilon)H trajectories both show sta ble distal-CO interactions, whereas the 64N(delta)H trajectory has a w eaker interaction resulting in a more mobile distal side chain. One tr ajectory (a 64N(delta)H tautomer) has the distal histidine moving out into the ''solvent,'' leaving the pocket in an ''open'' structure, wit h a large unhindered entrance to the active site. These trajectories s uggest that the three nu(CO) frequencies observed for wild-type MbCO i n solution, rather than representing significantly different Fe-C-O ge ometries as such, arise from three different haem pocket structures, e ach with different electric fields at the ligand. Each pocket structur e corresponds to a different distal histidine conformer: the A(3) band to the 64N(epsilon)H tautomer, the A(1,2) band to the 64N(delta)H tau tomer, and the A(0) band to the absence of any significant interaction with the distal side chain.