Glucose transport and phosphorylation are the initial events in glucose utilization by all tissues and cell types. The necessary first step in glucose utilization, the movement of the molecule across the plasma membrane, is accomplished by a family of g lucos e tran s porters , design ated GLUT 1-5, that operate by facilitated diffusion to move glucose down a high to low concentration gradient ( 10, 78, 85). Glucose uptake occurs when the 0199-9885/93/07 1 5-0463$02.00 PRINTZ, MAGNUSON & GRANNER concentration of extracellular glucose exceeds the intracellular concentration, thus favoring glucose entry. This is generally the case, a notable exception being hepatocytes that export glucose when they are actively engaged in glycogenolysis or gluconeogenesis, or intestinal and renal cells that are involved in the trans-epithelial movement of glucose. This downhill concenÂ tration gradient is maintained by the conversion of glucose to its phosphate ester, glucose-6-phosphate (G-6-P) , which cannot exit the cell. In mammalian cells the phosphorylation of glucose to G-6-P is catalyzed by a family of closely related enzymes , the hexokinases. Four mammalian hexokinases have been characterized (38 , 55). These are designated hexokinase (HK) I, II, III, and IV , according to their relative
Annual Review of Nutrition – Annual Reviews
Published: Jul 1, 1993
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