Passive Mediated Transport Systems

Page 204
but when closed they appear to be more nearly parallel to a perpendicular to the membrane, suggesting that subunits slide over each other. The detailed mechanism of opening and closing, however, is unknown.
Like gap junctions nuclear pores cover two membranes, creating aqueous channels in the nuclear envelope. Pores are about 90 Å in diameter and permit the movement of large macromolecules. They are presumably lined with protein. The plasma membranes of Gram­negative bacteria also contain protein pores, termed porins. Over 40 different porins have been isolated and they range in size from 28 to 48 kDa. In contrast to most mammalian channels, these transmembrane segments are b ­sheets not a ­helices and exist in the membrane as trimers. Porins are water­filled transmembrane channels and range in diameter from 6 to 23 Å with some degree of selectivity for inorganic ions; some, however, permit the uptake of sugars.
Figure 5.37 Inhibitors of passive mediated transport of D­glucose in erythrocytes.
5.7— Passive Mediated Transport Systems
Passive mediated transport, also referred to as facilitated diffusion, leads to translocation of solutes through cell membranes without expenditure of metabolic energy (see Table 5.5, p. 199). As with nonmediated diffusion the direction of flow is always from a higher to a lower concentration. The distinguishing differences between measurements of simple diffusion and passive mediated transport are the demonstration of saturation kinetics, a structural specificity for the class of molecule moving across the membrane, and specific inhibition of solute movement.
Glucose Transport Is Facilitated
A family of passive mediated transporters for D­glucose, frequently referred to as glucose permeases, has been identified in the plasma membrane of mammalian cells. Six members have been described and are termed GLUT­1, GLUT­2, and so on. All have 12 hydrophobic segments considered to be the transmembrane regions. The physiological direction of movement is into the cell because the extracellular level of glucose is about 5 mM and most cells metabolize glucose rapidly, thus maintaining low intracellular concentrations. The transporter catalyzes a uniport mechanism and is most active with D­glucose. D­Galactose, D­mannose, D­arabinose, and several other D­sugars as well as glycerol are translocated by the same transporter. L­Isomers are not transported. It has been proposed that the b ­D­
glucopyranose is transported with carrier interaction at the hydrogen atoms on at least C­1, C­3, and C­6 of the sugar. The affinity of erythrocyte translocase for D­
glucose is highest with a Km of ~6.2 mM, whereas for other sugars Km values are much higher. The transporter has a very low affinity for D­fructose, precluding a role in cellular uptake of fructose; a separate carrier for fructose has been proposed. With isolated erythrocytes, glucose will move either into or out of the erythrocyte, depending on the direction of the experimentally established concentration gradient, demonstrating the reversibility of the system. Several sugar analogs as well as phoretin and 2,4,6­trihydroxyacetophenone (Figure 5.37) are competitive inhibitors. Some physiological aspects of the glucose translocase are presented on p. 881.
Figure 5.38 Passive anion antiport mechanism for movement of Cl– and HCO3– across the erythrocyte plasma membrane.
Cl– and HCO3– Are Transported by an Antiport Mechanism
An anion transporter in erythrocytes involves the antiport movement of Cl– and HCO3– (Figure 5.38). The transporter is referred to as the Cl––HCO3– exchanger, anion exchange protein, or band 3, the latter because of its position in SDS polyacrylamide gel electrophoresis of erythrocyte membrane proteins. The direction of ion flow is reversible and depends on the concentra­
Page 205
–
tion gradients of the ions across the membrane. The transporter has an important role in adjusting the erythrocyte HCO3 concentration in arterial and venous blood (see p. 1035).
Figure 5.39 Representative anion transport systems in liver mitochondria. Note that each is an antiport mechanism. Several other transport systems are known and are discussed in Chapter 6.
Mitochondria Contain a Number of Transport Systems
The inner mitochondrial membrane contains several antiport systems for the exchange of anions between the cytosol and mitochondrial matrix. These include (1) a transporter for exchange of ADP and ATP, (2) a transporter for exchange of phosphate and OH–, (3) a dicarboxylate carrier that catalyzes an exchange of malate for phosphate, and (4) a translocator for exchange of aspartate and glutamate (Figure 5.39). The relationship of these translocases and energy coupling are discussed on page 243. In the absence of an input of energy these transporters will catalyze a passive exchange of metabolites down their concentration gradient to achieve a thermodynamic equilibrium of all intermediates. As an antiport mechanism, a concentration gradient of one compound can drive the movement of the other solute. In several cases, the transporter catalyzes the antiport movement of an equal number of charges on the substrate; in such movement the mitochondrial membrane potential influences the equilibrium and the anions can be moved against their concentration gradients. ADP–ATP and the phosphate transporters, as well as an uncoupling protein that translocates H+, have significant amino acid homology and are presumably derived from a common ancestor. It has been suggested that each subunit has six transmembrane ­helices. The uncoupling protein, found in mitochondria of brown adipose tissue, has been proposed to be involved in generation of heat.
The ATP–ADP translocase is very specific for ATP and ADP and deoxyribose derivatives, dATP and dADP, but does not transport AMP or other nucleotides. It is a dimer containing two subunits of 33 kDa each and represents about 12% of the total protein in heart mitochondria. It is very hydrophobic and can exist in two conformations. Atractyloside and bongkrekic acid (Figure 5.40) are
Figure 5.40 Structure of two inhibitors of the ATP–ADP transport system of liver mitochondria.