KINETIC-THEORY OF LIGAND RECOMBINATION OF MYOGLOBIN - A MODEL FOR A COMBINATION OF ENTROPIC AND ENTHALPIC EFFECTS

A kinetic theory of ligand recombination of myoglobin is obtained thro ugh a microscopic model. The macroscopic time dependent rate constant is obtained by the first passage time distribution random walk method. When the ligand is outside the haem pocket, it diffuses in a continuu m space. In this process, this rate corresponds to a Smoluchowski rate constant times the concentration of myoglobin. After penetrating thro ugh the hydration shell, the ligand waits in front of the gate or diff uses on the myoglobin surface for entering the gate. This waiting time refers to a large scale fluctuation of protein to open the gate. When the ligand is inside the pocket, the motion of the ligand ranges from a ballistic to a diffusive limit. To cover the whole range of frictio n, it is necessary to solve exactly a finite area random walk model wi th periodic gating in one-and two-dimensional finite lattices with sli ppery boundary conditions. Protein dynamics influence the ligand motio n indirectly through the collision between the ligand and the heme poc ket well. The first step corresponds to an adiabatic dissociation proc ess. A branching diagram method is used to show a detailed pathway ana lysis of the adiabaticity by introducing intermediate states in the qu intet states for the CO ligand binding. The rate theories of ligand re combination of myoglobin are a combination of entropic and enthalpic e ffects.