THE KINETICS OF FACILITATED DIFFUSION FOLLOWED BY ENZYMATIC CONVERSION OF THE SUBSTRATE

The uptake of a substrate that is transported into the cell by facilit ated diffusion and subsequently converted in a series of enzymatic rea ctions, measured as a function of the external concentration, does not usually show the rectangular hyperbolic function characteristic of Mi chaelis-Menten kinetics. Instead, a seemingly biphasic curve is observ ed, consisting of Michaelis-Menten kinetics at low concentrations and no further uptake at higher levels. By combining the equations for fac ilitated diffusion and an enzymatic reaction, we have derived an equat ion that describes the overall rate of uptake and metabolism of a subs trate that is transported across the plasma membrane by facilitated di ffusion. Modelling based on this equation simulated the kinetics found experimentally, as long as the kinetic parameters of the carrier were chosen to render it asymmetric. The overall rate was influenced by th e kinetics of both reactions over a wide range of concentrations, conf irming the principles of the 'Control Analysis' theory in an independe nt manner.