Most of the 14 GLUT transporters can move glucose, but only four are reported to

Question

Most of the 14 GLUT transporters can move glucose, but only four are reported to transport fructose, the other major dietary sugar. GLUT5 transporters are structurally similar to GLUT1, but a single amino acid difference (Trp -> Ala) in the sugar binding site changes the affinity almost entirely from glucose to fructose (Km = 10 mM for fructose). GLUT2 transporters can move both glucose (Km = 17 mM) and fructose (Km = 76 mM).

a. For which hexose, glucose or fructose, does GLUT2 have a higher affinity?

b. If both GLUT2 and GLUT5 transporters are found in equal numbers in the same membrane, which one is more likely to move fructose out of a cell if the cytosolic fructose concentration is 5 mM?

Explanation / Answer

a. Km has also been used as a measure of binding affinity. Hence, higher Km denotes lower binding affinity and lower Km denotes higher binding affinity. Thus, GLUT2 has a higher affinity for glucose.

b. GLUT5 is more likely to move fructose out of the cell as it will bind more efficiently as compared to GLUT2 (compare the Km values).

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