Facilitated Diffusion and the Possible Role of Myoglobin as a Transport Mechanism

Abstract

This paper deals with the facilitated diffusion of oxygen in systems containing proteins, such as ferrohemoglobin, that combine reversibly with oxygen. The phenomenon is treated in terms of translational diffusion of free and bound oxygen molecules, and of the reaction rate of the oxygen molecules with the oxygen-binding protein. These considerations lead to a differential equation (see Equation 9) for the flux; analytical or numerical solutions for this equation are not yet available. However, the assumption that chemical equilibrium exists at every point in the diffusing system leads to a simple equation already derived by other workers. This in turn permits calculation of the partial pressure of oxygen as a function of distance, in facilitated diffusion across a flat membrane, for given values of the total oxygen flux. The data given by Wittenberg in the preceding paper are analyzed in these terms, with satisfactory agreement between experiment and calculation. The possible contribution of rotary diffusion to the facilitated flux of oxygen is analyzed in detail, and it is found to be negligible compared to that of translational diffusion. Calculations of facilitated diffusion in muscle resulting from the presence of myoglobin indicate that myoglobin may be responsible for a substantial part of the transport of oxygen in muscle, especially at low partial pressures of oxygen.