H. Lodish - Molecular Cell Biology (5ed, Freeman, 2003), страница 104

These proteinsoften are called cotransporters, referring to their ability totransport two different solutes simultaneously.Like ATP pumps, cotransporters mediate coupled reactions in which an energetically unfavorable reaction(i.e., uphill movement of molecules) is coupled to an energetically favorable reaction. Note, however, that the natureof the energy-supplying reaction driving active transportby these two classes of proteins differs.

ATP pumps use energy from hydrolysis of ATP, whereas cotransporters usethe energy stored in an electrochemical gradient. This latter process sometimes is referred to as secondary activetransport.Table 7-1 summarizes the four mechanisms by whichsmall molecules and ions are transported across cellularmembranes. In this chapter, we focus on the properties andoperation of the membrane proteins that mediate the threeprotein-dependent transport mechanisms. Conformationalchanges are essential to the function of all transport proteins.248CHAPTER 7 • Transport of Ions and Small Molecules Across Cell MembranesTABLE 7-1Mechanisms for Transporting Ions and Small Molecules Across Cell MembranesTransport MechanismPropertyPassive DiffusionFacilitated DiffusionActive TransportCotransport*Requires specificproteinSolute transportedagainst its gradientCoupled to ATPhydrolysisDriven by movement ofa cotransported iondown its gradientExamples of moleculestransportedO2, CO2, steroidhormones, many drugsGlucose and aminoacids (uniporters); ionsand water (channels)Ions, small hydrophilicmolecules, lipids (ATPpowered pumps)Glucose and aminoacids (symporters);various ions andsucrose (antiporters)*Also called secondary active transport.ATP-powered pumps and transporters undergo a cycle ofconformational change exposing a binding site (or sites) toone side of the membrane in one conformation and to theother side in a second conformation.

Because each such cycleresults in movement of only one (or a few) substrate molecules, these proteins are characterized by relatively slow ratesof transport ranging from 100 to 104 ions or molecules persecond (see Figure 7-2). Ion channels shuttle between aclosed state and an open state, but many ions can passthrough an open channel without any further conformational change. For this reason, channels are characterized byvery fast rates of transport, up to 108 ions per second.Several Features Distinguish UniportTransport from Passive DiffusionThe protein-mediated movement of glucose and other smallhydrophilic molecules across a membrane, known as uniporttransport, exhibits the following distinguishing properties:1.

The rate of facilitated diffusion by uniporters is far higherthan passive diffusion through a pure phospholipid bilayer.2. Because the transported molecules never enter the hydrophobic core of the phospholipid bilayer, the partitioncoefficient K is irrelevant.3. Transport occurs via a limited number of uniportermolecules, rather than throughout the phospholipid bilayer.Consequently, there is a maximum transport rate Vmax thatis achieved when the concentration gradient across themembrane is very large and each uniporter is working atits maximal rate.4.

Transport is specific. Each uniporter transports onlya single species of molecule or a single group of closelyrelated molecules. A measure of the affinity of a transporterfor its substrate is Km, which is the concentration ofsubstrate at which transport is half-maximal.These properties also apply to transport mediated by theother classes of proteins depicted in Figure 7-2.One of the best-understood uniporters is the glucosetransporter GLUT1 found in the plasma membrane oferythrocytes.

The properties of GLUT1 and many othertransport proteins from mature erythrocytes have been extensively studied. These cells, which have no nucleus or otherinternal organelles, are essentially “bags” of hemoglobincontaining relatively few other intracellular proteins and asingle membrane, the plasma membrane (see Figure 5-3a).Because the erythrocyte plasma membrane can be isolatedin high purity, isolating and purifying a transport proteinfrom mature erythrocytes is a straightforward procedure.Figure 7-3 shows that glucose uptake by erythrocytes andliver cells exhibits kinetics characteristic of a simple enzymecatalyzed reaction involving a single substrate.

The kineticsof transport reactions mediated by other types of proteins aremore complicated than for uniporters. Nonetheless, allprotein-assisted transport reactions occur faster than allowedby passive diffusion, are substrate-specific as reflected inlower Km values for some substrates than others, and exhibita maximal rate (Vmax).7.1 • Overview of Membrane TransportVmaxInitial rate of glucose uptake, V0500GLUT1 (erythrocytes)1/2V250maxGLUT2 (liver cells)Passive diffusion01234 5 6 7 8 9 10 11 12 13 14External concentration of glucose (mM)KmGLUT1 Uniporter Transports Glucoseinto Most Mammalian CellsMost mammalian cells use blood glucose as the major sourceof cellular energy and express GLUT1. Since the glucose concentration usually is higher in the extracellular medium(blood in the case of erythrocytes) than in the cell, GLUT1generally catalyzes the net import of glucose from the extracellular medium into the cell. Under this condition, Vmax isachieved at high external glucose concentrations.Like other uniporters, GLUT1 alternates between twoconformational states: in one, a glucose-binding site faces theoutside of the membrane; in the other, a glucose-binding sitefaces the inside.

Figure 7-4 depicts the sequence of events occurring during the unidirectional transport of glucose fromthe cell exterior inward to the cytosol. GLUT1 also can catalyze the net export of glucose from the cytosol to the extra-ExteriorGLUT1249 EXPERIMENTAL FIGURE 7-3 Cellular uptake of glucosemediated by GLUT proteins exhibits simple enzyme kineticsand greatly exceeds the calculated rate of glucose entrysolely by passive diffusion.

The initial rate of glucose uptake(measured as micromoles per milliliter of cells per hour) in thefirst few seconds is plotted against increasing glucoseconcentration in the extracellular medium. In this experiment, theinitial concentration of glucose in the cells is always zero. BothGLUT1, expressed by erythrocytes, and GLUT2, expressed byliver cells, greatly increase the rate of glucose uptake (red andorange curves) compared with that associated with passivediffusion (blue curve) at all external concentrations.

Like enzymecatalyzed reactions, GLUT-facilitated uptake of glucose exhibits amaximum rate (Vmax). The Km is the concentration at which therate of glucose uptake is half maximal. GLUT2, with a Km ofabout 20 mM, has a much lower affinity for glucose than GLUT1,with a Km of about 1.5 mM.cellular medium exterior when the glucose concentration ishigher inside the cell than outside.The kinetics of the unidirectional transport of glucosefrom the outside of a cell inward via GLUT1 can be described by the same type of equation used to describe asimple enzyme-catalyzed chemical reaction. For simplicity,let’s assume that the substrate glucose, S, is present initially only on the outside of the membrane.

In this case, wecan write:Sout GLUT1KmSout GLUT1VmaxSin GLUT1where Sout GLUT1 represents GLUT1 in the outwardfacing conformation with a bound glucose. By a similarderivation used to arrive at the Michaelis-Menten equationGlucoseGlucose1Cytosol2Outward-facingconformation▲ FIGURE 7-4 Model of uniport transport by GLUT1. Inone conformation, the glucose-binding site faces outward; in theother, the binding site faces inward. Binding of glucose to theoutward-facing site (step 1 ) triggers a conformational changein the transporter that results in the binding site’s facing inwardtoward the cytosol (step 2 ). Glucose then is released to theinside of the cell (step 3 ). Finally, the transporter undergoes the3Inward-facingconformation4Outward-facingconformationreverse conformational change, regenerating the outward-facingbinding site (step 4 ).

If the concentration of glucose is higherinside the cell than outside, the cycle will work in reverse(step 4 n step 1 ), resulting in net movement of glucose frominside to out. The actual conformational changes are probablysmaller than those depicted here.250CHAPTER 7 • Transport of Ions and Small Molecules Across Cell<b>Текст обрезан, так как является слишком большим</b>.