B. Alberts, A. Johnson, J. Lewis и др. - Molecular Biology of The Cell (6th edition) (1120996), страница 59
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A protein kinase catalyzesthis reaction, and the reaction is essentially unidirectional because of the largeamount of free energy released when the phosphate–phosphate bond in ATP isbroken to produce ADP (discussed in Chapter 2). A protein phosphatase catalyzes the reverse reaction of phosphate removal, or dephosphorylation. Cells contain hundreds of different protein kinases, each responsible for phosphorylating adifferent protein or set of proteins.
There are also many different protein phosphatases; some are highly specific and remove phosphate groups from only one or afew proteins, whereas others act on a broad range of proteins and are targeted tospecific substrates by regulatory subunits. The state of phosphorylation of a protein at any moment, and thus its activity, depends on the relative activities of theprotein kinases and phosphatases that modify it.The protein kinases that phosphorylate proteins in eukaryotic cells belong to avery large family of enzymes that share a catalytic (kinase) sequence of about 290amino acids. The various family members contain different amino acid sequenceson either end of the kinase sequence (for example, see Figure 3–10), and oftenhave short amino acid sequences inserted into loops within it.
Some of theseadditional amino acid sequences enable each kinase to recognize the specific setof proteins it phosphorylates, or to bind to structures that localize it in specificregions of the cell. Other parts of the protein regulate the activity of each kinase,so it can be turned on and off in response to different specific signals, as describedbelow.By comparing the number of amino acid sequence differences between thevarious members of a protein family, we can construct an “evolutionary tree” thatis thought to reflect the pattern of gene duplication and divergence that gave riseto the family.
Figure 3–62 shows an evolutionary tree of protein kinases. Kinaseswith related functions are often located on nearby branches of the tree: the protein kinases involved in cell signaling that phosphorylate tyrosine side chains, forexample, are all clustered in the top left corner of the tree. The other kinases shownserineCH2side chainCPO_OCH2PROTEINKINASECPROTEINPHOSPHATASEphosphorylatedproteinPi(A)kinasePOFFONphosphatasekinaseONPOFFphosphatase(B)Figure 3–61 Protein phosphorylation. Manythousands of proteins in a typical eukaryoticcell are modified by the covalent addition ofa phosphate group. (A) The general reactiontransfers a phosphategroup from ATP to anMBoC6 e4.38/3.57amino acid side chain of the target protein bya protein kinase. Removal of the phosphategroup is catalyzed by a second enzyme, aprotein phosphatase.
In this example, thephosphate is added to a serine side chain; inother cases, the phosphate is instead linkedto the –OH group of a threonine or a tyrosinein the protein. (B) The phosphorylation of aprotein by a protein kinase can either increaseor decrease the protein’s activity, dependingon the site of phosphorylation and thestructure of the protein.PROTEIN FUNCTION155Cdc7MAP kinasesubfamilyWee1PDGFreceptortyrosinekinasesubfamilyKSS1ERK1cyclin-dependentkinase subfamily(cell-cycle control)Cdk2Cdc2EGFreceptorSrccyclic-AMPdependent kinaseLckcyclic-GMPdependent kinaseRafMosprotein kinase CTGFβreceptormyosin lightchain kinases2+Ca /calmodulindependent kinasereceptor serinekinase subfamilyphosphorylate either a serine or a threonine side chain, and many are organizedinto clusters that seem to reflect their function—in transmembrane signal transduction, intracellular signal amplification, cell-cycle control, and so on.As a result of the combinedactivitiesof protein kinases and protein phosMBoC6m3.66/3.58phatases, the phosphate groups on proteins are continually turning over—beingadded and then rapidly removed.
Such phosphorylation cycles may seem wasteful, but they are important in allowing the phosphorylated proteins to switch rapidly from one state to another: the more rapid the cycle, the faster a population ofprotein molecules can change its state of phosphorylation in response to a sudden change in its phosphorylation rate (see Figure 15–14). The energy required todrive this phosphorylation cycle is derived from the free energy of ATP hydrolysis,one molecule of which is consumed for each phosphorylation event.The Regulation of the Src Protein Kinase Reveals How a ProteinCan Function as a MicroprocessorThe hundreds of different protein kinases in a eukaryotic cell are organized intocomplex networks of signaling pathways that help to coordinate the cell’s activities, drive the cell cycle, and relay signals into the cell from the cell’s environment.
Many of the extracellular signals involved need to be both integrated andamplified by the cell. Individual protein kinases (and other signaling proteins)serve as input–output devices, or “microprocessors,” in the integration process.An important part of the input to these signal-processing proteins comes from thecontrol that is exerted by phosphates added and removed from them by proteinkinases and protein phosphatases, respectively.The Src family of protein kinases (see Figure 3–10) exhibits such behavior. TheSrc protein (pronounced “sarc” and named for the type of tumor, a sarcoma, thatits deregulation can cause) was the first tyrosine kinase to be discovered.
It is nowknown to be part of a subfamily of nine very similar protein kinases, which arefound only in multicellular animals. As indicated by the evolutionary tree in Figure 3–62, sequence comparisons suggest that tyrosine kinases as a group werea relatively late innovation that branched off from the serine/threonine kinases,with the Src subfamily being only one subgroup of the tyrosine kinases created inthis way.The Src protein and its relatives contain a short N-terminal region that becomescovalently linked to a strongly hydrophobic fatty acid, which anchors the kinase atthe cytoplasmic face of the plasma membrane.
Next along the linear sequence ofFigure 3–62 An evolutionary tree ofselected protein kinases. A highereukaryotic cell contains hundreds of suchenzymes, and the human genome codesfor more than 500. Note that only some ofthese, those discussed in this book, areshown.156Chapter 3: ProteinsCOOHNH2SH3fattyacidSH2kinase domains500 amino acidsFigure 3–63 The domain structure of theSrc family of protein kinases, mappedalong the amino acid sequence.
For thethree-dimensional structure of Src, seeFigure 3–13.amino acids come two peptide-binding domains, a Src homology 3 (SH3) domainand an SH2 domain, followed by the kinase catalytic domain (Figure 3–63). Thesekinases normally exist in an inactive conformation, in which a phosphorylatedMBoC6m3.68/3.59tyrosine near the C-terminusis boundto the SH2 domain, and the SH3 domainis bound to an internal peptide in a way that distorts the active site of the enzymeand helps to render it inactive.As shown in Figure 3–64, turning the kinase on involves at least two specificinputs: removal of the C-terminal phosphate and the binding of the SH3 domainby a specific activating protein. In this way, the activation of the Src kinase signals the completion of a particular set of separate upstream events (Figure 3–65).Thus, the Src family of protein kinases serves as specific signal integrators, contributing to the web of information-processing events that enable the cell to compute useful responses to a complex set of different conditions.Proteins That Bind and Hydrolyze GTP Are Ubiquitous CellRegulatorsWe have described how the addition or removal of phosphate groups on a proteincan be used by a cell to control the protein’s activity.
In the example just discussed,a kinase transfers a phosphate from an ATP molecule to an amino acid side chainof a target protein. Eukaryotic cells also have another way to control protein activity by phosphate addition and removal. In this case, the phosphate is not attacheddirectly to the protein; instead, it is a part of the guanine nucleotide GTP, whichbinds very tightly to a class of proteins known as GTP-binding proteins.
In general,proteins regulated in this way are in their active conformations with GTP bound.The loss of a phosphate group occurs when the bound GTP is hydrolyzed to GDPin a reaction catalyzed by the protein itself, and in its GDP-bound state the proteinis inactive. In this way, GTP-binding proteins act as on–off switches whose activityis determined by the presence or absence of an additional phosphate on a boundGDP molecule (Figure 3–66).GTP-binding proteins (also called GTPases because of the GTP hydrolysisthey catalyze) comprise a large family of proteins that all contain variations onthe same GTP-binding globular domain. When a tightly bound GTP is hydrolyzedby the GTP-binding protein to GDP, this domain undergoes a conformationalactivating ligandkinase domainPPtyrosineSH3activekinasePiSH2POFFPHOSPHATEREMOVALLOOSENSSTRUCTUREACTIVATINGLIGAND BINDSTO SH3 DOMAINKINASE CAN NOWPHOSPHORYLATETYROSINE TOSELF-ACTIVATEPONFigure 3–64 The activation of a Src-type protein kinase by two sequential events.
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