3 Биологические мембраны. Обмен веществом (1160072), страница 16
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This group of related enzymes, the F-type ATPases (Table 105), will be discussed when we describe ATP formation in mitochondriaand chloroplasts (Chapter 18). (The F in their name originated in theiridentification as energy-coupling/actors.) They catalyze the reversibletransmembrane passage of protons, driven by ATP hydrolysis. Flow ofprotons across the membrane down their concentration gradient is accompanied by ATP synthesis from ADP and Pi, the reversal of ATPhydrolysis.
In this role, the F-type ATPase is more appropriatelynamed ATP synthase. In some cases, the formation of the proton gradient in energy-conserving processes is driven by an energy sourceother than ATP, such as substrate oxidation or sunlight, as we willdiscuss in Chapter 18.Chapter 10 Biological Membranes and TransportSolute AIon Gradients Provide the Energyfor Secondary Active TransportThe ion gradients formed by primary transport of Na + or H + driven bylight, oxidation, or ATP hydrolysis can themselves provide the drivingforce for the cotransport of other solutes (Fig. 10-24). Many cells contain transport systems that couple the spontaneous, downhill flow ofH + or Na + to the simultaneous uphill pumping of another ion, sugar,or amino acid (Table 10-6).
The galactoside permease of E. coli allowsthe accumulation of the disaccharide lactose to levels 100 times that inthe surrounding growth medium (Fig. 10-25). E. coli normally has aproton gradient across its plasma membrane, produced by energyyielding metabolism; protons tend spontaneously to flow back into thecell, down this gradient. The lipid bilayer is impermeable to protons,but the galactoside permease provides a route for proton reentry, andlactose is simultaneously carried into the cell on the symporter protein(permease). The endergonic accumulation is thereby coupled to theexergonic flow of protons; the total free-energy change for the coupledprocess is negative.Table 10-6 Cotransport systems driven by gradients of Na + or H +Transportedsolute (symportor antiport)Organism or tissueIntestine, kidneyVertebrate cells (many types)Higher plantsFungi (Neurospora)Solute BOutsideInsideC2 « CxC2>CiAGA is largeAGB is positiveand negativeFigure 10—24 In secondary active transport, a single cotransporter couples the flow of one solute (solute 1; e.g., H+ or Na+) down its concentration gradient to the pumping of a second solute (solute 2;e.g., lactose or glucose) against its concentrationgradient.
When AGX + AG2 < 0, cotransport occursspontaneously. A specific example of this generalprocess is shown in Fig. 10-25.CotransportedsoluteLactose (symport)Proline (symport)Dicarboxylic acids(symport)Glucose (symport)Amino acids (symport)Ca2+ (antiport)K+ (antiport)K+ (antiport)E. coli291H+H+H+Na +Na +Na+H+H+ProtonpumpGalactosidepermeaseFigure 10-25 Lactose uptake in E.
coli. (a) Theprimary transport of H+ out of the cell, driven bythe oxidation of a variety of fuels, establishes a proton gradient. Secondary active transport of lactoseinto the cell involves symport of H+ and lactose bythe galactoside permease. The uptake of lactoseagainst its concentration gradient is entirely dependent on this inflow of H + . (b) When the energyyielding oxidation reactions are blocked by cyanide(CN~), there is an efflux of lactose from the cell,and no further accumulation occurs. The brokenline represents the concentration of lactose in thesurrounding medium.
When active transport isblocked by cyanide, the galactoside permease allowsequilibration of lactose inside and outside the cell(passive transport).Lactose(outside)CN inhibition offuel oxidationEfflux[Lactose] m e d i u mLactose(inside)H+(a)292Part II Structure and CatalysisIn intestinal epithelial cells, glucose and certain amino acids areaccumulated by symport with Na+, using the Na+ gradient establishedby the Na+K+ ATPase. Most cells of vertebrate animals also have anantiport system that simultaneously pumps one Ca2+ ion out of a celland allows three Na+ ions in, thereby maintaining the very low intracellular Ca2+ concentration necessary for normal function.
The role ofNa+ in symport and antiport systems such as these requires the continued outward pumping of Na+ to maintain the transmembrane Na+gradient. Clearly, the Na+K+ ATPase is a central element in manycotransport processes.Because of the essential role of ion gradients in active transportand energy conservation, natural products and drugs that collapse theion gradients across cellular membranes are poisons, and may serve asantibiotics. Valinomycin is a small, cyclic peptide that folds around K+and neutralizes its positive charge (Fig. 10-26). The peptide then actsas a shuttle, carrying K+ across membranes down its concentrationgradient and deflating that gradient. Compounds that shuttle ionsacross membranes in this way are called ionophores, literally "ionbearers." Both valinomycin and monensin (a Na+-carrying ionophore)are antibiotics; they kill microbial cells by disrupting secondary transport processes and energy-conserving reactions.Figure 10—26 Valinomycin, a peptide ionophorethat binds K+.
The oxygen (red) atoms that bindK+ (the green atom at the center) are part of a central hydrophilic cavity. Hydrophobic amino acidside chains (yellow) coat the outside of the molecule. Because the exterior of the K+-valinomycincomplex is hydrophobic, it readily diffuses throughmembranes, carrying K+ down its concentrationgradient. The resulting dissipation of the transmembrane ion gradient kills cells, making valinomycin a potent antibiotic.Ion-Selective Channels Act in Signal TransductionsCH3PX0—CH2—CH2—N—CH3CH 3AcetylcholineA third transmembrane path for ions, distinct from both transportersand ionophores, is provided by ion channels, found in the plasmamembranes of neurons, muscle cells, and many other cells, both prokaryotic and eukaryotic. Various stimuli cause rapid changes in theelectrical potential across the plasma membranes of neurons and muscle cells, the result of the rapid opening and closing of ion channels.One of the best-studied ion channels is the acetylcholine receptor ofthe vertebrate synapse (the point of connection between two neurons;Fig.
10-27), which plays an essential role in the passage of a signalfrom one neuron to the next. When the electrical signal carried by thepresynaptic neuron reaches the synaptic end of the cell, the neurotransmitter acetylcholine is released into the synaptic cleft. Acetylcholine rapidly diffuses across the cleft to the postsynaptic neuron, whereChapter 10 Biological Membranes and TransportPresynaptic jneuronAction potential arrives atsynapse, causes release ofacetylcholineAcetylcholinereceptorNa +Acetylcholine bindsto receptor; ionchannel opensNa+ influx through receptorchannel depolarizesmembrane; nerve impulsehas been transmittedacross synapsePostsynapticneuronit binds to high-affinity sites on the acetylcholine receptor.
This binding induces a change in receptor structure, opening a transmembranechannel in the receptor protein. Cations in the extracellular fluid, present at higher concentration than in the cytosol of the postsynaptic neuron,flowthrough the opened channel into the cell, down their concentration gradient, thereby depolarizing (decreasing the transmembraneelectrochemical gradient of) the postsynaptic cell. The acetylcholinereceptor allows Na+ and K+ to pass with equal ease, but other cationsand all anions are unable to pass through it.The rate of Na+ movement through the acetylcholine receptor ionchannel is linear with respect to extracellular Na+ concentration; theprocess is not saturable in the way that transporter-catalyzed translocation is saturable with substrate (see Fig.
10-19). This kinetic property distinguishes ion channels from passive transporters. Na+ movement through the channel is very fast—almost at the rate expected forunhindered diffusion of the ion. Under physiological conditions of ionconcentrations and membrane potential, about 2 x 107 Na+ ions canpass through a single channel in 1 s. A comparison of this figure withthe turnover numbers for typical enzymes, which are in the range of 10to 105 s"1, shows the efficiency of transmembrane diffusion through anion channel. In short, the ion channel of the acetylcholine receptorbehaves as though it provided a hydrophilic pore through the lipidbilayer through which an ion of the right size, charge, and geometrycan diffuse very rapidly down its electrochemical gradient.
This receptor/channel is typical of many ion channels in cells that produce orrespond to electrical signals: it has a "gate" that opens in response tostimulation by acetylcholine, and an intrinsic timing mechanism thatcloses the gate after a split second. Thus the acetylcholine signal istransient—an essential feature of electrical signal conduction.293Figure 10-27 Communication occurs across thesynaptic cleft between adjoining neurons as acetylcholine released by the presynaptic cell diffuses tospecific receptors on the postsynaptic cell. Thebinding of acetylcholine changes the conformationof the receptor, and results in the opening of a receptor-associated ion channel.
Na+ flows in, downits concentration gradient, carrying positive chargeand reducing the membrane electrical potential (depolarizing the cell). Depolarization initiates an electrical signal (action potential) that sweeps throughthe postsynaptic neuron at very high speed and isconducted to the next synapse, where these eventsmay be repeated.294Part II Structure and CatalysisSummaryBiological membranes are central to life. They define cellular boundaries, divide cells into discretecompartments, organize complex reaction sequences, and act in signal reception and energytransformations.
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