Facilitated diffusion transport

GLUT Sodium-independent facilitated diffusion transporter... 

There are, however, various types of active transport systems, involving protein carriers and known as uniports, symports, and antiports as indicated in Figure 3.7. Thus, symports and antiports involve the transport of two different molecules in either the same or a different direction. Uniports are carrier proteins, which actively or passively (see section "Facilitated Diffusion") transport one molecule through the membrane. Active transport requires a source of energy, usually ATP, which is hydrolyzed by the carrier protein, or the cotransport of ions such as Na+ or H+ down their electrochemical gradients. The transport proteins usually seem to traverse the lipid bilayer and appear to function like membrane-bound enzymes. Thus, the protein carrier has a specific binding site for the solute or solutes to be transferred. For example, with the Na+/K+ ATPase antiport, the solute (Na+) binds to the carrier on one side of... 

Glucose cannot diffuse directly into cells, but enters by one of two transport mechanisms a Na+-independent, facilitated diffusion transport system or a Na+-monosaccharide co-transporter system. 

Most known saturable transporters at the BBB are facilitated diffusion systems (Davson and Segal, 1996c). For example, GLUT-1, the transporter for glucose, is a facilitated diffusion transporter. If the level of glucose is artificially raised above that of serum (or if radioactive glucose is introduced into the CNS, but not the serum), efflux of glucose can be shown. 

Calcium is absorbed from the intestine by facilitated diffusion and active transport. In the former, Ca " moves from the mucosal to the serosal compartments along a concentration gradient. The active transport system requires a cation pump. In both processes, a calcium-binding protein (CaBP) is thought to be required for the transport. Synthesis of CaBP is activated by 1,25-DHCC. In the active transport, release of Ca " from the mucosal cell into... 

Glucose Transport in Erythrocytes Occurs by Facilitated Diffusion... 

All of the transport systems examined thus far are relatively large proteins. Several small molecule toxins produced by microorganisms facilitate ion transport across membranes. Due to their relative simplicity, these molecules, the lonophore antibiotics, represent paradigms of the mobile carrier and pore or charmel models for membrane transport. Mobile carriers are molecules that form complexes with particular ions and diffuse freely across a lipid membrane (Figure 10.38). Pores or channels, on the other hand, adopt a fixed orientation in a membrane, creating a hole that permits the transmembrane movement of ions. These pores or channels may be formed from monomeric or (more often) multimeric structures in the membrane. 

Does this transport operate by passive diffusion or 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. 

Molecules can passively traverse the bilayer down electrochemical gradients by simple diffusion ot by facilitated diffusion. This spontaneous movement toward equilibrium contrasts with active transport, which requires energy because it constitutes movement against an electrochemical gradient. Figure 41-8 provides a schematic representation of these mechanisms. 

PLASMA MEMBRANES ARE INVOLVED IN FACILITATED DIFFUSION, ACTIVE TRANSPORT, OTHER PROCESSES... 

Facilitated diffusion and active transport share many features. Both appear to involve carrier proteins, and both show specificity for ions, sugars, and amino acids. 

Mutations in bacteria and mammalian cells (including some that result in human disease) have supported these conclusions. Facilitated diffusion and active transport resemble a substrate-enzyme reaction except that no covalent interaction occurs. These points of resemblance are as follows (1) There is a specific binding site for the solute. (2) The carrier is saturable, so it has a maximum rate of transport (V Figure 41-11). (3) There is a binding constant (Al) ) for the solute, and... 

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. 

Unphosphorylated functioning according to Fig. 5 catalyzes facilitated diffusion of mannitol across the membrane. The same process has been reported for purified II reconstituted in proteoliposomes [70]. The relevance of this activity in terms of transport of mannitol into the bacterial cell is probably low, but it may have important implications for the mechanism by which E-IIs catalyze vectorial phosphorylation. It would indicate that the transmembrane C domain of Il is a mannitol translocating unit which is somehow coupled to the kinase activity of the cytoplasmic domains. We propose that the inwardly oriented binding site which is in contact with the internal water phase (Ecyt Mtl, see Fig. 5) is the site from where mannitol is phosphorylated when transport is coupled to phosphorylation. Meehan-... 

S. typhimurium normally transport trehalose via the galactose permease and are able to grow on this substrate in the complete absence of PTS phosphorylating activity. However, in S. typhimurium which lack a functional galactose permease, IfMan appears to be able to transport trehalose [78]. There is no evidence that trehalose is phosphorylated in this process, again pointing to Il "-dependent transport in the facilitated diffusion mode. 

Postma and Stock [81] showed that HPr or E-I mutants were unable to grow on PTS carbohydrates suggesting that transport without phosphorylation did not take place in apparent contradiction with the studies presented above. The explanation may be that facilitated diffusion via PTS carriers is observed only in abnormal situations, carbohydrate being transported by the incorrect PTS carrier (galactose via the mannose carrier) or transport via a mutated carrier. Efflux, which also reflects facilitated diffusion, is more common for PTS carriers. 

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. 

The phosphorylation of cytoplasmic sugar and the facilitated diffusion from the cytoplasm to the periplasm are catalyzed by the E-IIs under conditions where they are also active in the vectorial phosphorylation reaction. Therefore, the former two activities should be integral parts of any kinetic scheme representing the mechanism of E-IIs. Such a scheme should explain how vectorial phosphorylation, transport coupled to phosphorylation, is still achieved while the uncoupled pathways are integral parts of the scheme. 

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... 

In addition to enhancing surface reactions, water can also facilitate surface transport processes. First-principles ab initio molecular dynamics simulations of the aqueous/ metal interface for Rh(l 11) [Vassilev et al., 2002] and PtRu(OOOl) alloy [Desai et al., 2003b] surfaces showed that the aqueous interface enhanced the apparent transport or diffusion of OH intermediates across the metal surface. Adsorbed OH and H2O molecules engage in fast proton transfer, such that OH appears to diffuse across the surface. The oxygen atoms, however, remained fixed at the same positions, and it is only the proton that transfers. Transport occurs via the symmetric reaction... 

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. 

Mathematical approaches used to describe micelle-facilitated dissolution include film equilibrium and reaction plane models. The film equilibrium model assumes simultaneous diffusive transport of the drug and micelle in equilibrium within a common stagnant film at the surface of the solid as shown in Figure 7. The reaction plane approach has also been applied to micelle-facilitated dissolution and has the advantage of including a convective component in the transport analysis. While both models adequately predict micelle-facilitated dissolution, the scientific community perceives the film equilibrium model to be more mathematically tractable, so this model has found greater use. 

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