Hemoglobin-oxygen-carbon monoxide equilibria with the MWC model.

Fractional saturation equations for the Monod, Wyman and Changeux model are derived for the case of two distinct ligands bonding to a host molecule with four ligand sites and two conformational states. A variety of useful graphical studies can be derived from these equations when applied to normal human hemoglobin with O2 and CO as ligands. For example, the oxygen transport capability of hemoglobin can be assessed at different environmental CO levels and the concentrations of various liganded species can be displayed as a function of fractional saturation with oxygen. In addition, the CO pressure in the tissue, PCOtissue, can be calculated as a function of the tissue oxygen pressure, PO2tissue, at different environmental levels of CO. In an environment of a given CO concentration, PCOtissue decreases with PO2tissue until a minimum is reached. Further decrease in PO2tissue results in a fairly steep rise in PCOtissue.