B. Alberts, A. Johnson, J. Lewis и др. - Molecular Biology of The Cell (6th edition) (1120996), страница 57
Текст из файла (страница 57)
331).Eukaryotic cells have yet another way of increasing the rate of metabolic reactions: using their intracellular membrane systems. These membranes can segregate particular substrates and the enzymes that act on them into the same membrane-enclosed compartment, such as the endoplasmic reticulum or the cellnucleus. If, for example, a compartment occupies a total of 10% of the volume ofPROTEIN FUNCTION149FATTY ACID SYNTHASEacyl carrierdomainCN21453terminationdomain (TE)enzyme domains(A)1TE20 nm(D)PYRUVATE DEHYDROGENASE COMPLEX13425232143(B)5 nm(C)etc.(E)Figure 3–54 How unstructured regions of polypeptide chain serving as tethers allow reaction intermediates to bepassed from one active site to another in large multienzyme complexes. (A–C) The fatty acid synthase in mammals.
(A) Thelocation of seven protein domains with different activities in this 270 kilodalton protein. The numbers refer to the order in whicheach enzyme domain must function to complete each two-carbon addition step. After multiple cycles of two-carbon addition,the termination domain releases the final product once the desired length of fatty acid has been synthesized.
(B) The structureof the dimeric enzyme, with the location of the five active sites in one monomer indicated. (C) How a flexible tether allows thesubstrate that remains linked to the acyl carrier domain (red) to be passed from one active site to another in each monomer,sequentially elongating and modifying the bound fatty acid intermediate (yellow).
The five steps are repeated until the final lengthof fatty acid chain has been synthesized.(Only steps 1 through 4 are illustrated here.)MBoC6 n3.150/3.50(D) Multiple tethered subunits in the giant pyruvate dehydrogenase complex (9500 kilodaltons, larger than a ribosome) thatcatalyzes the conversion of pyruvate to acetyl CoA. As in (C), a covalently bound substrate held on a flexible tether (red ballswith yellow substrate) is serially passed through active sites on subunits (here labeled 1 through 3) to produce the final products.Here, subunit 1 catalyzes the decarboxylation of pyruvate accompanied by the reductive acetylation of a lipoyl group linked toone of the red balls. Subunit 2 transfers this acetyl group to CoA, forming acetyl CoA, and subunit 3 reoxidizes the lipoyl groupto prepare it for the next cycle.
Only one-tenth of the subunits labeled 1 and 3, attached to the core formed by subunit 2, areillustrated here. This important reaction takes place in the mammalian mitochondrion, as part of the pathway that oxidizessugars to CO2 and H2O (see page 82). (A–C, adapted from T. Maier et al., Quart. Rev. Biophys. 43:373–422, 2010;D, from J.L.S. Milne et al., J. Biol. Chem. 281:4364–4370, 2006.)the cell, the concentration of reactants in that compartment may be increased by10 times compared with a cell with the same number of enzyme and substratemolecules, but no compartmentalization. Reactions limited by the speed of diffusion can thereby be speeded up by a factor of 10.The Cell Regulates the Catalytic Activities of Its EnzymesA living cell contains thousands of enzymes, many of which operate at the sametime and in the same small volume of the cytosol. By their catalytic action, theseenzymes generate a complex web of metabolic pathways, each composed ofchains of chemical reactions in which the product of one enzyme becomes thesubstrate of the next.
In this maze of pathways, there are many branch points(nodes) where different enzymes compete for the same substrate. The system is150Chapter 3: Proteinscomplex (see Figure 2–63), and elaborate controls are required to regulate whenand how rapidly each reaction occurs.Regulation occurs at many levels. At one level, the cell controls how manymolecules of each enzyme it makes by regulating the expression of the gene thatencodes that enzyme (discussed in Chapter 7). The cell also controls enzymaticactivities by confining sets of enzymes to particular subcellular compartments,whether by enclosing them in a distinct membrane-bounded compartment (discussed in Chapters 12 and 14) or by concentrating them on a protein scaffold (seeFigure 3–77).
As will be explained later in this chapter, enzymes are also covalently modified to control their activity. The rate of protein destruction by targetedproteolysis represents yet another important regulatory mechanism (see Figure6–86). But the most general process that adjusts reaction rates operates througha direct, reversible change in the activity of an enzyme in response to the specificsmall molecules that it binds.The most common type of control occurs when an enzyme binds a moleculethat is not a substrate to a special regulatory site outside the active site, therebyaltering the rate at which the enzyme converts its substrates to products. For example, in feedback inhibition, a product produced late in a reaction pathway inhibits an enzyme that acts earlier in the pathway.
Thus, whenever large quantities ofthe final product begin to accumulate, this product binds to the enzyme and slowsdown its catalytic action, thereby limiting the further entry of substrates into thatreaction pathway (Figure 3–55). Where pathways branch or intersect, there areusually multiple points of control by different final products, each of which worksto regulate its own synthesis (Figure 3–56). Feedback inhibition can work almostinstantaneously, and it is rapidly reversed when the level of the product falls.ABCXnegativeregulationYZFigure 3–55 Feedback inhibition of asingle biosynthetic pathway. The endproduct Z inhibits the first enzyme that isunique to its synthesis and thereby controlsits own level in the cell.
This is an exampleof negative regulation.MBoC6 m3.56/3.51aspartateaspartylphosphateaspartatesemialdehydehomoserinelysinethreoninemethionineisoleucineFigure 3–56 Multiple feedback inhibition.In this example, which shows thebiosynthetic pathways for four differentamino acids in bacteria, the red linesindicate positions at which products feedback to inhibit enzymes. Each amino acidcontrols the first enzyme specific to itsown synthesis, thereby controlling its ownlevels and avoiding a wasteful, or evendangerous, buildup of intermediates.
Theproducts can also separately inhibit theinitial set of reactions common to all thesyntheses; in this case, three differentenzymes catalyze the initial reaction, eachinhibited by a different product.PROTEIN FUNCTIONFeedback inhibition is negative regulation: it prevents an enzyme from acting.Enzymes can also be subject to positive regulation, in which a regulatory moleculestimulates the enzyme’s activity rather than shutting the enzyme down.
Positiveregulation occurs when a product in one branch of the metabolic network stimulates the activity of an enzyme in another pathway. As one example, the accumulation of ADP activates several enzymes involved in the oxidation of sugar molecules, thereby stimulating the cell to convert more ADP to ATP.Allosteric Enzymes Have Two or More Binding Sites That InteractA striking feature of both positive and negative feedback regulation is that the regulatory molecule often has a shape totally different from the shape of the substrateof the enzyme.
This is why the effect on a protein is termed allostery (from theGreek words allos, meaning “other,” and stereos, meaning “solid” or “three-dimensional”). As biologists learned more about feedback regulation, they recognizedthat the enzymes involved must have at least two different binding sites on theirsurface—an active site that recognizes the substrates, and a regulatory site thatrecognizes a regulatory molecule. These two sites must somehow communicateso that the catalytic events at the active site can be influenced by the binding of theregulatory molecule at its separate site on the protein’s surface.The interaction between separated sites on a protein molecule is now knownto depend on a conformational change in the protein: binding at one of the sitescauses a shift from one folded shape to a slightly different folded shape. Duringfeedback inhibition, for example, the binding of an inhibitor at one site on theprotein causes the protein to shift to a conformation that incapacitates its activesite located elsewhere in the protein.It is thought that most protein molecules are allosteric.
They can adopt two ormore slightly different conformations, and a shift from one to another caused bythe binding of a ligand can alter their activity. This is true not only for enzymesbut also for many other proteins, including receptors, structural proteins, andmotor proteins. In all instances of allosteric regulation, each conformation of theprotein has somewhat different surface contours, and the protein’s binding sitesfor ligands are altered when the protein changes shape. Moreover, as we discussnext, each ligand will stabilize the conformation that it binds to most strongly, andthus—at high enough concentrations—will tend to “switch” the protein towardthe conformation that the ligand prefers.Two Ligands Whose Binding Sites Are Coupled Must ReciprocallyAffect Each Other’s BindingThe effects of ligand binding on a protein follow from a fundamental chemicalprinciple known as linkage.
Suppose, for example, that a protein that binds glucose also binds another molecule, X, at a distant site on the protein’s surface. Ifthe binding site for X changes shape as part of the conformational change in theprotein induced by glucose binding, the binding sites for X and for glucose aresaid to be coupled. Whenever two ligands prefer to bind to the same conformationof an allosteric protein, it follows from basic thermodynamic principles that eachligand must increase the affinity of the protein for the other. For example, if theshift of a protein to a conformation that binds glucose best also causes the bindingsite for X to fit X better, then the protein will bind glucose more tightly when X ispresent than when X is absent. In other words, X will positively regulate the protein’s binding of glucose (Figure 3–57).Conversely, linkage operates in a negative way if two ligands prefer to bindto different conformations of the same protein.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
