| Abstract: | A model transport system for cellular accumulation of sugar coupled to phosphorylation is described. Sugar permeates the cell membrane via a passive facilitated transport system. On the inside surface of the membrane the bound sugar is either phosphorylated to form impermeable hexose phosphate, which is released into the intracellular solution, or released directly into the cytosol. Sugar may be regenerated from hexose phosphate in the cytosol via a phosphatase reaction. The reduction of the proportion of sites on the inner membrane surface occupied by permeable sugar, caused by the kinase reaction, increases both net and unidirectional passive inflow and reduces both net and unidirectional exit of sugar, thereby permitting large stationary state gradients of free sugar to be maintained between the cytosol and bathing solution. In cells where there is a high passive membrane permeability to free sugar, steady-state accumulation of free sugar within the cytosol, linked to metabolism is inexplicable in terms of conventional transport kinetics based on equilibrium thermodynamic assumptions. This phenomenon is analysed in terms of non-equilibrium stationary state flows of ligands via a probability network. The effects of metabolism on exchange transport are also examined. The model provides a framework to explain how sugar transport is loosely coupled to phosphorylation in mammalian epithelial cells, adipocytes, yeasts and bacteria, so that a high rate of substrate accumulation is maintained without requiring a reduction in the intracellular concentration of permeable substrate below that in the external solution. |
