B. Alberts, A. Johnson, J. Lewis и др. - Molecular Biology of The Cell (6th edition) (1120996), страница 62
Текст из файла (страница 62)
(Coordinatesdetermined by P. Nissen et al., Science270:1464–1472, 1995. PDB code: 1B23.)162Chapter 3: Proteinsreversible, and the protein can only wander randomly back and forth along thethread. We can look at this situation in another way. Since the directional movement of a protein does work, the laws of thermodynamics (discussed in Chapter 2)demand that such movement use free energy from some other source (otherwisethe protein could be used to make a perpetual motion machine). Therefore, without an input of energy, the protein molecule can only wander aimlessly.How can the cell make such a series of conformational changes unidirectional?To force the entire cycle to proceed in one direction, it is enough to make any oneof the changes in shape irreversible.
Most proteins that are able to walk in onedirection for long distances achieve this motion by coupling one of the conformational changes to the hydrolysis of an ATP molecule that is tightly bound to theprotein. The mechanism is similar to the one just discussed that drives allostericprotein shape changes by GTP hydrolysis. Because ATP (or GTP) hydrolysisreleases a great deal of free energy, it is very unlikely that the nucleotide-bindingprotein will undergo the reverse shape change needed for moving backward—since this would require that it also reverse the ATP hydrolysis by adding a phosphate molecule to ADP to form ATP.In the model shown in Figure 3–75A, ATP binding shifts a motor protein fromconformation 1 to conformation 2. The bound ATP is then hydrolyzed to produceADP and inorganic phosphate (Pi), causing a change from conformation 2 to conformation 3.
Finally, the release of the bound ADP and Pi drives the protein backto conformation 1. Because the energy provided by ATP hydrolysis drives the transition 2 → 3, this series of conformational changes is effectively irreversible. Thus,the entire cycle goes in only one direction, causing the protein molecule to walkcontinuously to the right in this example.Many motor proteins generate directional movement through the use ofa similar unidirectional ratchet, including the muscle motor protein myosin,123Figure 3–74 An allosteric “walking”protein.
Although its three differentconformations allow it to wander randomlyback and forthwhilebound to a thread or aMBoC6m3.76/3.68filament, the protein cannot move uniformlyin a single direction.myosin Vactin1ATPBINDINGAPPP2HYDROLYSISPAP(B)P3RELEASEA P PADPP1(A)direction ofmovement50 nmFigure 3–75 How a protein can walkin one direction. (A) An allostericmotor protein driven by ATP hydrolysis.The transition between three differentconformations includes a step driven bythe hydrolysis of a bound ATP molecule,creating a “unidirectional ratchet” thatmakes the entire cycle essentiallyirreversible.
By repeated cycles, the proteintherefore moves continuously to the rightalong the thread. (B) Direct visualizationof a walking myosin motor protein byhigh-speed atomic force microscopy; theelapsed time between steps was less than0.5 sec (see Movie 16.3). (B, modified fromN. Kodera et al., Nature 468:72–76, 2010.With permission from MacmillanPublishers Ltd.)PROTEIN FUNCTION163which walks along actin filaments (Figure 3–75B), and the kinesin proteins thatwalk along microtubules (both discussed in Chapter 16). These movements canbe rapid: some of the motor proteins involved in DNA replication (the DNA helicases) propel themselves along a DNA strand at rates as high as 1000 nucleotidesper second.Membrane-Bound Transporters Harness Energy to PumpMolecules Through MembranesWe have thus far seen how proteins that undergo allosteric shape changes can actas microprocessors (Src family kinases), as assembly factors (EF-Tu), and as generators of mechanical force and motion (motor proteins).
Allosteric proteins canalso harness energy derived from ATP hydrolysis, ion gradients, or electron-transport processes to pump specific ions or small molecules across a membrane. Weconsider one example here that will be discussed in more detail in Chapter 11.The ABC transporters (ATP-binding cassette transporters) constitute animportant class of membrane-bound pump proteins. In humans, at least 48 different genes encode them. These transporters mostly function to export hydrophobic molecules from the cytoplasm, serving to remove toxic molecules at themucosal surface of the intestinal tract, for example, or at the blood–brain barrier.The study of ABC transporters is of intense interest in clinical medicine, becausethe overproduction of proteins in this class contributes to the resistance of tumorcells to chemotherapeutic drugs.
In bacteria, the same types of proteins primarilyfunction to import essential nutrients into the cell.A typical ABC transporter contains a pair of membrane-spanning subunitslinked to a pair of ATP-binding subunits located just below the plasma membrane. As in other examples we have discussed, the hydrolysis of the bound ATPmolecules drives conformational changes in the protein, transmitting forces thatcause the membrane-spanning subunits to move their bound molecules acrossthe lipid bilayer (Figure 3–76).Humans have invented many different types of mechanical pumps, and itshould not be surprising that cells also contain membrane-bound pumps that(A)A BACTERIAL ABC TRANSPORTER(C)small moleculeCYTOSOLATPATPasedomains(B)ATP2 ADP + Pi2 ATPA EUKARYOTIC ABC TRANSPORTERCYTOSOLATPasedomainssmallmoleculeATP2 ATPATP2 ADP + PiFigure 3–76 The ABC (ATP-bindingcassette) transporter, a protein machinethat pumps molecules through amembrane.
(A) How this large family oftransporters pumps molecules into thecell in bacteria. As indicated, the bindingof two molecules of ATP causes the twoATP-binding domains to clamp togethertightly, opening a channel to the cell exterior.The binding of a substrate molecule to theextracellular face of the protein complex thentriggers ATP hydrolysis followed by ADPrelease, which opens the cytoplasmic gate;the pump is then reset for another cycle.(B) As discussed in Chapter 11, ineukaryotes an opposite process occurs,causing selected substrate molecules to bepumped out of the cell.
(C) The structureof a bacterial ABC transporter (see Movie11.5). (C, from R.J. Dawson and K.P. Locher,Nature 443:180–185, 2006. With permissionfrom Macmillan Publishers Ltd;PDB code: 2HYD).164Chapter 3: Proteinsfunction in other ways. Among the most notable are the rotary pumps that couplethe hydrolysis of ATP to the transport of H+ ions (protons). These pumps resemble miniature turbines, and they are used to acidify the interior of lysosomes andother eukaryotic organelles. Like other ion pumps that create ion gradients, theycan function in reverse to catalyze the reaction ADP + Pi → ATP, if the gradientacross their membrane of the ion that they transport is steep enough.One such pump, the ATP synthase, harnesses a gradient of proton concentration produced by electron-transport processes to produce most of the ATP used inthe living world.
This ubiquitous pump has a central role in energy conversion, andwe shall discuss its three-dimensional structure and mechanism in Chapter 14.Proteins Often Form Large Complexes That Function as ProteinMachinesLarge proteins formed from many domains are able to perform more elaboratefunctions than small, single-domain proteins. But large protein assemblies formedfrom many protein molecules linked together by noncovalent bonds performthe most impressive tasks. Now that it is possible to reconstruct most biologicalprocesses in cell-free systems in the laboratory, it is clear that each of the centralprocesses in a cell—such as DNA replication, protein synthesis, vesicle budding,or transmembrane signaling—is catalyzed by a highly coordinated, linked set of10 or more proteins.
In most such protein machines, an energetically favorablereaction such as the hydrolysis of bound nucleoside triphosphates (ATP or GTP)drives an ordered series of conformational changes in one or more of the individual protein subunits, enabling the ensemble of proteins to move coordinately.In this way, each enzyme can be moved directly into position, as the machinecatalyzes successive reactions in a series (Figure 3–77).
This is what occurs, forexample, in protein synthesis on a ribosome (discussed in Chapter 6)—or in DNAreplication, where a large multiprotein complex moves rapidly along the DNA(discussed in Chapter 5).Cells have evolved protein machines for the same reason that humans haveinvented mechanical and electronic machines. For accomplishing almost anytask, manipulations that are spatially and temporally coordinated through linkedprocesses are much more efficient than the use of many separate tools.ATPATPATPATPScaffolds Concentrate Sets of Interacting ProteinsAs scientists have learned more of the details of cell biology, they have recognizedan increasing degree of sophistication in cell chemistry.
Thus, not only do we nowknow that protein machines play a predominant role, but it has also become clearthat they are very often localized to specific sites in the cell, being assembled andactivated only where and when they are needed. As one example, when extracellular signaling molecules bind to receptor proteins in the plasma membrane,the activated receptors often recruit a set of other proteins to the inside surface ofthe plasma membrane to form a large protein complex that passes the signal on(discussed in Chapter 15).The mechanisms frequently involve scaffold proteins.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
