The generalised form of this reaction is written as follows
There are three types of sugar transporters that carry sugars across the plasma membrane in to the cells. First are the energy independent facilitated diffusion transporters such as the glucose transporters family(GLUT) of hexose transporters seen in yeast and in mammalian cells. These proteins are encoded by SLC2A genes(solute carriers 2A). Second are the energy dependent transporters for example sodium dependent glucose transporters(SGLT) encoded by SLC5A genes in the intestine and in kidney epithelial cells. The third type of transporters couple ATP dependent phosphorylation with sugar import and are seen in bacteria.
This family of glucose transporters were first discovered in yeast where 18 genes have been identified. Humans have 14 GLUT homologs. All of the yeast glucose transporters are of the same size(40-55 Kilodaltons) and have similar structures containing 12 membrane spanning domains. These domains form a barrel with a small pore for the sugar to pass through. The only "sugar transport signatures" are a few widely scattered glycine and tryptophan residues and one PET tri-peptide sequence
The elevated levels of blood sugar and amino acids that occur following a meal signal pancreatic beta cells to release insulin into the bloodstream. Once in the vascular system, circulating insulin markedly enhances glucose transport into skeletal muscle and adipose tissue, the peripheral sites responsible for the majority of postprandial glucose disposal. In response to insulin, glucose enters muscle and fat cells through aqueous pores formed by the glucose transporter 4 (GLUT4) protein. GLUT4 is the fourth of 13 members of a family of facilitative sugar transporters and is the only iso-form that is widely accepted as being insulin-responsive. Like other GLUT family members, GLUT4 is a 12 trans-membrane protein; unlike most other isofoms, GLUT4 is predominantly localized to intracellular compartments in the basal state.
Activation of the insulin receptor triggers a large increase in the rate of GLUT4 vesicle exocytosis and a concomitant decrease in the rate of endocytosis. This insulin-dependent shift in GLUT4 vesicle trafficking results in a net increase of GLUT4 protein at the cell surface, thus allowing glucose to enter target cells. Once