Facilitators glucose diffusion

The transport of must hexose (glucose and fructose) across the plasmic membrane activates a complex system of proteinic transporters not fully explained (Section 1.3.2). This mechanism facilitates the diffusion of must hexoses in the cytoplasm, where they are rapidly metabolized. Since solute moves in the direction of the concentration gradient, from the concentrated outer medium to the diluted inner medium, it is not an active transport system requiring energy. 

Glucose Transport in Erythrocytes Occurs by Facilitated Diffusion... 

Phloretin is the aglycon of phlorizin and inhibits the facilitated diffusion of glucose catalyzed by GLUT1 or GLUT4. It has been used to terminate the uptake of glucose in timed assays with isolated membranes or reconstituted transporters. 

Physiologically muscle-derived NO regulates skeletal muscle contractility and exercise-induced glucose uptake. nNOS is located at the plasma membrane of skeletal muscle and facilitates diffusion of NO to the vasculature to regulate muscle perfusion. 

Certain solutes, eg, glucose, enter cells by facilitated diffusion, along a downhill gradient from high to low concentration. Specific carrier molecules, or transporters, are involved in such processes. 

The entry rate of glucose into red blood cells is far greater than would be calculated for simple diffusion. Rather, it is an example of facilitated diffiision (Chapter 41). The specific protein involved in this process is called the glucose transporter or glucose permease. Some of its properties are summarized in Table 52-3-The process of entry of glucose into red blood cells is of major importance because it is the major fuel supply for these cells. About seven different but related glucose transporters have been isolated from various tissues unlike the red cell transporter, some of these are insidin-dependent (eg, in muscle and adipose tissue). There is considerable interest in the latter types of transporter because defects in their recruitment from intracellular sites to the surface of skeletal muscle cells may help explain the insulin resistance displayed by patients with type 2 diabetes mellitus. 

The history of observations of efflux associated with PTS carriers is nearly as old as PTS itself. Gachelin [82] reported that A -ethylmaleimide inactivation of a-methyl-glucoside transport and phosphorylation in E. coli was accompanied by the appearance of a facilitated diffusion movement of both a-methylglucoside and glucose in both directions, uptake and efflux. His results could not discriminate, however, between one carrier operating in two different modes, active transport for the native carrier and facilitated diffusion for the alkylated carrier, or two distinct carriers. Haguenauer and Kepes [83] went on to show that alkylation of the carrier was not even necessary to achieve efflux NaF treatment which inhibits P-enolpyruvate synthesis was sufficient but this study did not address the question of one carrier or two. 

By far the most complete study of the kinetics of mammalian passive glucose transporters has been done on the GLUT-1 isoform in the human erythrocyte. The transport of glucose in this cell type is a classic example of facilitated diffusion, the... 

Under certain conditions, the transfer of various molecules across the membrane is relatively easy. The membrane must contain a suitable transport mediator , and the process is then termed facilitated membrane transport . Transport mediators permit the transported hydrophilic substance to overcome the hydrophobic regions in the membrane. For example, the transport of glucose into the red blood cells has an activation energy of only 16 kJ mol-1—close to simple diffusion. 

Glucose and galactose enter the absorptive cells by way of secondary active transport. Cotransport carrier molecules associated with the disaccharidases in the brush border transport the monosaccharide and a Na+ ion from the lumen of the small intestine into the absorptive cell. This process is referred to as "secondary" because the cotransport carriers operate passively and do not require energy. However, they do require a concentration gradient for the transport of Na+ ions into the cell. This gradient is established by the active transport of Na+ ions out of the absorptive cell at the basolateral surface. Fructose enters the absorptive cells by way of facilitated diffusion. All monosaccharide molecules exit the absorptive cells by way of facilitated diffusion and enter the blood capillaries. 

Facilitated diffusion of glucose across the blood-brain barrier and into brain cells is catalyzed by GLUT1,- 2 and -3, products of the SLC2 gene superfamily 90... 

Glucose transporters in muscle and fat tissue operate by facilitated diffusion. 

Figure 4-6. Mechanism of facilitated diffusion mediated by a glucose transporter. This is an example of uniport. The reversible interconversion between conformations of the transporter in which the glucosebinding site is alternately exposed to the exterior and interior of the cell is called a ping-pong mechanism.

Facilitated diffusion is a mechanism of transmembrane transfer that is carrier mediated but not energy-dependent. The carrier molecule is usually a transmembrane protein, which binds molecules and releases them on the other side of the membrane. It is an important mechanism for endogenous substances, such as glucose. 

Facilitated diffusion is a carrier-mediated mechanism of transplacental transfer. It is important for endogenous substances, such as glucose. It is also the mechanism for some drugs, e.g. the antibiotic cefalexin. Active transport is a carrier-mediated process that requires energy against an electrochemical or concentration gradient. Amino acids and calcium are transported by this mechanism. Only a few drugs, such as a-methyidopa and 5-fluorouracil, are transferred by active transport. 

---