Suppose another patient, who is a child, enters the clinic and you discover that

Question

Suppose another patient, who is a child, enters the clinic and you discover that they have normal blood glucose concentrations of around 5.5mM, but they also show symptoms of wooziness, weakness, irritability and disorientation. After a series of tests, you eventually isolate a sample from the patient that allows you to test the transport of glucose by GLUT1 into whole red blood cells derived from the patient, which also express GLUT1 in addition to brain cells. A radiolabeled and non-metabolizable analog, 3-O-methyl-D-glucose, shown below, is used instead of glucose for these assays. 3-OMG is recognized by the glucose transporter readily with no change in Km compared to glucose but it is not a substrate of glycolytic machinery.

a. Which kinetic parameter is affected in this patient, Vmax or Km? What does this indicate about the patient’s GLUT1?

b. Why is it important to use 3-OMG, or another glucose analog, instead of glucose in the whole cells isolated from the patient? Explain this also in terms of the Km’s of hexokinase and GLUT1 for glucose and the analog.

Explanation / Answer

A. Vmax is changing. GluT1 in patient has no problem in binding of gluocse and transporter (GLUT1) but there is problem in intake of glucose to inside the RBC. Thats why patient Vmax is lesser compare to their parents. The velocity of moving glucose molecules from outside to inside is affected but the concentration of glucose binding to GLUT1 in outside (Km) is not affected.

B. We are using 3-OMG because it is non metabolize so we can able to calculate how mch molecule enter into the cells. If it is glucose and is metabolize means we not able to get the results due to glucose molecules are undergoes glycolysis process.

Km of hexokinase is decrease if we are using glucose as substrate. Affinity between glucose and hexokinase are strong so Km will decrease.

Km of hexokinase and the analog will increase. Affinity is less between glucose and hexokinase.

Km of glucose and GLUT1 and analog and GLUT1 are same. Both bind the GLUT1 has a same affinity so there is no change in the Km value.

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