Practice question

GLUT isoforms display divergent Michaelis constants reflecting tissue metabolic roles and sensing. GLUT2 product of SLC2A2 on 3q26 expressed strongly in hepatocyte sinusoidal membrane pancreatic beta cell plasma membrane intestinal basolateral side and kidney tubule exhibits unusually high Km fifteen to twenty millimolar for glucose also transporting fructose galactose low affinity corresponding to lowest affinity among transporters. This high Km enables effective glucose sensor rather than supply line where rate varies roughly linearly across physiological plasma glucose three to fifteen millimolar without early saturation allowing liver to take up proportionally after meals for glycogen synthesis when sugar high and release glucose during fasting via glucose-6-phosphatase. In beta cells low affinity coupled with high Km glucokinase ensures metabolism proportional to blood glucose linking insulin secretion to glycemia via ATP sensitive K+ channel closure. In intestine high capacity efflux of concentrated glucose via basolateral GLUT2 prevents buildup. By contrast GLUT1 and GLUT3 low Km near one and one point five millimolar provide constitutive high affinity uptake ensuring constant brain glucose despite fluctuations and GLUT4 intermediate Km five millimolar regulated by insulin for muscle storage.