page 02 1. The diffusion of glucose across the plasma membrane of mammalian cells is facilitated by a family of related proteins called GLUT transporters. GLUT transporters have a specific binding site for glucose which alternately faces inside and outside the cell. The orientation of the binding site is brought about by a change in conformation as shown in Figure 1. Figure 1 plasma membrane extracellular intracellular glucose transporter glu glu glu = glucose Several studies have measured the changes in rates of glucose uptake by GLUT transporters as the concentration of glucose is increased. Results for four different GLUT transporters are shown in Figure 2. For each type of GLUT transporter, the rate of transport levels off to a maximum value that is termed Vmax. The glucose concentration at which the rate of transport is half Vmax is defined as the KM of the transporter. KM values for four types of GLUT are shown in Figure 2. Figure 2 0 3 6 9 12 15 18 21 0∙00 0∙01 0∙02 0∙03 0∙04 0∙05 Glucose concentration (mmol/l) Glucose uptake (mmol/min) GLUT3 (KM = 1∙4 mmol/l) GLUT4 (KM = 5·0 mmol/l) GLUT1 (KM = 3·0 mmol/l) GLUT2 (KM = 17·0 mmol/l) Key