EXPERIMENTAL FIGURE 11-4 Cellular uptake of glucose mediated by GLUT proteins exhibits simple enzyme kinetics. The initial rate of glucose uptake, v (measured as micromoles per milliliter of cells per hour), in the first few seconds is plotted as a percentage of the maximum rate, V_max, against increasing glucose concentration in the extracellular medium. In this experiment, the initial concentration of glucose in the cells is always zero. Both GLUT1, expressed by erythrocytes, and GLUT2, expressed by liver cells, catalyze glucose uptake. Like enzyme-catalyzed reactions, GLUT-facilitated uptake of glucose exhibits a maximum rate (V_max). K_m is the concentration at which the rate of glucose uptake is half maximal. GLUT2, with a K_m of about 20 mM (not shown), has a much lower affinity for glucose than GLUT1, with a K_m of about 1.5 mM.