S

Exterior GLUT1 • Glucose Glucose

Outward-facing Cytosol , ... 3

conformation

Inward-facing conformation

Inward-facing conformation

Outward-facing conformation

Outward-facing conformation

▲ FIGURE 7-4 Model of uniport transport by GLUT1. In one conformation, the glucose-binding site faces outward; in the other, the binding site faces inward. Binding of glucose to the outward-facing site (step |1|) triggers a conformational change in the transporter that results in the binding site's facing inward toward the cytosol (step |2|). Glucose then is released to the inside of the cell (step |3|). Finally, the transporter undergoes the reverse conformational change, regenerating the outward-facing binding site (step |4|). If the concentration of glucose is higher inside the cell than outside, the cycle will work in reverse (step |4|n step |1|), resulting in net movement of glucose from inside to out. The actual conformational changes are probably smaller than those depicted here.

in Chapter 3, we can derive the following expression for v, the initial transport rate for S into the cell catalyzed by GLUT1:

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