Glucose transporters (GLUT) are a family of transmembrane proteins found in most mammalian cells. Their action allows glucose to be transferred across plasma membranes; we remember, in this regard, that this very important energetic substrate - being polar - is unable to spontaneously cross the phospholipid bilayer that characterizes them.
In humans, glucose transport can occur according to a gradient (facilitated diffusion) or against a gradient (active transport).
Active transport occurs in the intestine and renal tubules and requires the indirect utilization of biochemical energy (Na + / glucose symport). Facilitated diffusion does not require ATP and involves glucose transporters, the so-called GLUTs.
In man there are numerous isoforms of GLUT, indicated by increasing numbers (GLUT-1, GLUT-2, GLUT-3, GLUT-n), which - although they are quite similar - differ in some properties.
The best known and studied glucose transporter is GLUT-4, due to its direct sensitivity to insulin. Under normal conditions, this carrier is located in the cytoplasmic area and its translocation on the cell membrane is stimulated by the binding of insulin with the membrane receptor. This process promotes the movement of glucose from the interstitial fluid into the cell. When the blood glucose concentration normalizes and insulin is eliminated, the GLUT 4 molecules are slowly removed from the plasma membrane and sequestered by endocytosis in intracellular vesicles.
The glucose transporter GLUT-4 is mainly present in skeletal muscle, heart and white and brown adipose tissue, not surprisingly defined as insulin-dependent tissues. At the muscular level, the translocation of GLUT-4 transporters from intracellular sites (vesicles) to the plasma membrane is also favored by the contraction, by the increase in blood flow and by the low levels of glycogen that characterize long-lasting physical exercise. This explains why sport is a very useful medicine in the prevention of insulin resistance and in the treatment of diabetes mellitus.
A particular glucose transporter is GLUT-2, expressed mainly in the liver; this protein is in fact capable of working in both directions, favoring the passage of sugar from the cell to the interstitial fluid and vice versa. This function is very important because the liver is the primary seat of gluconeogenesis, that is, the ex-novo synthesis of glucose (which must then be released into the blood) starting from amino acids, glycerol and lactic acid. GLUT-2 can also transporter galactose, mannose and fructose, the latter capacity shared with the GLUT-5 transporter.
GLUT-2 is also expressed in pancreatic B cells, and is characterized by a low affinity associated with a "high capacity (it is never saturated by glucose).
Also in the liver we find the transport protein GLUT-7, which acts as an intracellular transporter of glucose, important for promoting the export of the amount produced during gluconeogenesis.
Not all tissues in the human body depend on insulin for their glucose supply. In addition to the liver, at the level of the brain and red blood cells we find, for example, a "high concentration of insulin-independent glucose transporters, such as GLUT-1 and GLUT-3.
