Uniport

Note These are examples of important transporters involved in substrate and ADP uptake into the matrix compartment as indicated, and most are reversible. These transporters are proteins and several have been isolated and sequenced. Other specific carriers occur in mitochondria from other tissues. The inner membrane does not allow rapid exchange of NAD or CoA but there are mechanisms for the slow uniport of cofactors synthesized extramitochondrially. 

Transport systems can be described in a functional sense according to the number of molecules moved and the direction of movement (Figure 41-10) or according to whether movement is toward or away from equilibrium. A uniport system moves one type of molecule bidirectionally. In cotransport systems, the transfer of one solute depends upon the stoichiometric simultaneous or sequential transfer of another solute. A symport moves these solutes in the same direction. Examples are the proton-sugar transporter in bacteria and the Na+ -sugar transporters (for glucose and certain other sugars) and Na -amino acid transporters in mammalian cells. Antiport systems move two molecules in opposite directions (eg, Na in and Ca out). 

Some specific solutes diffuse down electrochemical gradients across membranes more rapidly than might be expected from their size, charge, or partition coefficients. This facilitated diffusion exhibits properties distinct from those of simple diffusion. The rate of facilitated diffusion, a uniport system, can be saturated ie, the number of sites involved in diffusion of the specific solutes appears finite. Many facihtated diffusion systems are stereospecific but, fike simple diffusion, require no metabolic energy. 

The transceUular movement of sugars involves one additional component a uniport that allows the glucose accumulated within the cell to move across a different surface toward a new equihbrium this occurs in intestinal and renal cells, for example. 

Figure 41-14. The transcellular movement of glucose in an intestinal cell. Glucose follows Na+ across the luminal epithelial membrane. The Na+ gradient that drives this symport is established by Na+ -K+ exchange, which occurs at the basal membrane facing the extra-ceiiuiarfiuid compartment. Glucose at high concentration within the ceii moves "downhill" into the extracel-iuiarfiuid by fadiitated diffusion (a uniport mechanism).

NMDA/AMPA/KA receptor activation evokes a rise in cytosolic Ca2+, activating a high capacity ruthenium red sensitive Ca2+ uniporter that transports Ca2+ into the mitochondria, which then activates mtNOS [338-340,442]. Activation ofnNOS/mtNOS,... 

Secondary active uniport systems facilitating the permeation of a single solute, dependent on the electrochemical potentials of the solute molecules, are rare in bacteria. Only a glucose uptake system of Zymomonas mobilis has been studied in more detail [101]. 

Marger, M. D. and Saier, M. H., Jr (1993). A major super family of transmembrane facilitators that catalyze uniport, symport and antiport, Trends Biochem. ScL, 18, 13-20. 

Figure 9.7 (a) The epithelial brush border cells of the small intestine concentrate glucose from the intestinal lumen in symport with Na+ this is driven by the (Na+-K+)-ATPase located on the capillary side of the cell. The glucose is then exported by a passive uniport system. (From Voet and Voet, 2004. Reproduced with permission from John Wiley Sons., Inc.) (b) Two Na+-binding sites in the LeuT Na+-dependent pump. (From Gouax and MacKinnon, 2005. Copyright (2005) American Association for the Advancement of Science.)... 

The transport systems of the inner mitochondrial membrane use various mechanisms. Metabolites or ions can be transported alone (uniport, U), together with a second substance (symport, S), or in exchange for another molecule (antiport. A). Active transport—i. e., transport coupled to ATP hydrolysis—does not play an important role in mitochondria. The driving force is usually the proton gradient across the inner membrane (blue star) or the general membrane potential (red star see p. 126). 

Some cells couple the pure transport forms discussed on p. 218—i.e., passive transport (1) and active transport (2)—and use this mechanism to take up metabolites. In secondary active transport (3), which is used for example by epithelial cells in the small intestine and kidney to take up glucose and amino acids, there is a symport (S) located on the luminal side of the membrane, which takes up the metabolite M together with an Na" ion. An ATP-dependent Na transporter (Na /lC ATPase see p. 350) on the other side keeps the intracellular Na+ concentration low and thus indirectly drives the uptake of M. Finally, a uniport (U) releases M into the blood. 

A membrane protein or protein complex that is responsible for the simultaneous transport of two molecular entities or ions across the membrane, each entity traveling in opposite directions. See Uniport Symport Membrane Transport... 

STEREOCHEMICAL TERMINOLOGY, lUPAC RECOMMENDATIONS ANTI PORT UNIPORT SYMPORT... 

In isosmotlc solutions, movement of neutral amino acids such as prollne across the inner membrane into the matrix results in swelling of mitochondria (20). As shown in Figure 3C, the movement of proline is thought to occur via a uniport. The increased concentration of proline in the matrix produces an osmotic-induced swelling. Kinetics of the swelling response is shown in the 0 (control) trace. Again, quercetin inhibited this response in a concentration-dependent manner. 

Uniport is when one substance is transported in a single direction, eg, the GLUTl glucose transporter of the RBC. 

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.

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... 

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