Enzyme kinetic models help to predict how fast an enzyme operates, how well it handles the substrate and how fast a catalyzed reaction will proceed.Finishing'up' on' e nzyme 'kine ti c'mode l• KMis the concentration at which one half of the enzyme active sites are occupied and therefore the concentration at which the enzymatic reaction is proceeding at one half its maximal rate.• KMis often close to the physiological concentration of a natural substrate.103ACTIVATED CARRIERS AND BIOSYNTHESISThe energy released by energetically favorable reactions such as the oxi-dation of food molecules must be stored temporarily before it can be used by cells to fuel energetically unfavorable reactions, such as the syn-thesis of all the other molecules needed by the cell. In most cases, the energy is stored as chemical-bond energy in a set of activated carriers, small organic molecules that contain one or more energy-rich covalent bonds. These molecules diffuse rapidly and carry their bond energy from the sites of energy generation to the sites where energy is used for bio-synthesis or for other energy-requiring cell activities (Figure 3–26).Activated carriers store energy in an easily exchangeable form, either as a readily transferable chemical group or as readily transferable (“high energy”) electrons. They can serve a dual role as a source of both energy and chemical groups for biosynthetic reactions. The most important acti-vated carriers are ATP and two molecules that are closely related to each other, NADH and NADPH. Cells use activated carriers like money to pay for the energetically unfavorable reactions that otherwise would not take place.The Formation of an Activated Carrier Is Coupled to an Energetically Favorable Reaction When a fuel molecule such as glucose is oxidized in a cell, enzyme-cat-alyzed reactions ensure that a large part of the free energy released is captured in a chemically useful form, rather than being released waste-fully as heat. (Oxidizing sugar in a cell allows you to power metabolic reactions, whereas burning a chocolate bar in the street will get you nowhere, producing no metabolically useful energy.) In cells, energy capture is achieved by means of a coupled reaction, in which an ener-getically favorable reaction is used to drive an energetically unfavorable one that produces an activated carrier or some other useful molecule. Figure 3–25 Enzymes cannot change the equilibrium point for reactions. Enzymes, like all catalysts, speed up the forward and reverse rates of a reaction by the same amount. Therefore, for both the (A) uncatalyzed and (B) catalyzed reactions shown here, the number of molecules undergoing the transition X  Y is equal to the number of molecules undergoing the transition Y  X when the ratio of Y molecules to X molecules is 3.5 to 1, as illustrated. In other words, both the catalyzed and uncatalyzed reactions will eventually reach the same equilibrium point, although the catalyzed reaction will reach equilibrium faster. foodmoleculeoxidized foodmoleculeENERGYENERGYCATABOLISM ANABOLISMnew moleculeneeded by cellmoleculeavailable in cellactivated carrierinactive carrierENERGYenergeticallyfavorablereactionenergeticallyunfavorablereactionECB4 e3.29/3.29Figure 3–26 Activated carriers can store and transfer energy in a form that cells can use. By serving as intracellular energy shuttles, activated carriers perform their function as go-betweens that link the release of energy from the breakdown of food molecules (catabolism) to the energy-requiring biosynthesis of small and large organic molecules (anabolism).Activated Carriers and Biosynthesis X Y XYUNCATALYZED REACTIONAT EQUILIBRIUM(A) (B) ENZYME-CATALYZED REACTIONAT EQUILIBRIUMECB4 e3.25/3.25Storing( ener gyHow'to'view'ener gy'in'bi ologyUsi ng(energyTransferenergy108 CHAPTER 3 Energy, Catalysis, and BiosynthesisAs shown in Figure 3–31, ATP is synthesized in an energetically unfa-vorable phosphorylation reaction, in which a phosphate group is added to ADP (adenosine 5-diphosphate). When required, ATP gives up this energy packet in an energetically favorable hydrolysis to ADP and inor-ganic phosphate (Pi). The regenerated ADP is then available to be used for another round of the phosphorylation reaction that forms ATP, creat-ing an ATP cycle in the cell.The energetically favorable reaction of ATP hydrolysis is coupled to many otherwise unfavorable reactions through which other molecules are synthesized. We will encounter several of these reactions in this chapter, where we will see exactly how this is done. ATP hydrolysis is often coupled to the transfer of the terminal phosphate in ATP to another molecule, as illustrated in Figure 3–32. Any reaction that involves the transfer of a phosphate group to a molecule is termed a phosphorylation reaction. Phosphorylation reactions are examples of condensation reac-tions (see Figure 2–25), and they occur in many important cell processes: they activate substrates, mediate the exchange of chemical energy, and serve as key constituents of intracellular signaling pathways (discussed in Chapter 16).+energy availableto drive energeticallyunfavorablereactionsECB4 e3.31/3.31O PO_O P O CH2ADENINERIBOSEOOO_PO_O_O_OPO_O P O CH2ADENINERIBOSEOOO_energy fromsunlight or fromthe breakdownof foodPO_O_O_Oinorganicphosphate ( )phosphoanhydride bondsΔGº > 0 ATPADPPiΔGº < 0 Figure 3–31 The interconversion of ATP and ADP occurs in a cycle. The two outermost phosphate groups in ATP are held to the rest of the molecule by high-energy phosphoanhydride bonds and are readily transferred to other organic molecules. Water can be added to ATP to form ADP and inorganic phosphate (Pi). Inside a cell, this hydrolysis of the terminal phosphate of ATP yields between 11 and 13 kcal/mole of usable energy. Although the Gº of this reaction is –7.3 kcal/mole, the G is much more negative, because the ratio of ATP to the products ADP and Pi is so high inside the cell. The large negative Gº of the reaction arises from a number of factors. Release of the terminal phosphate group removes an unfavorable repulsion between adjacent negative charges; in addition, the inorganic phosphate ion (Pi) released is stabilized by favorable hydrogen-bond formation with water. The formation of ATP from ADP and Pi reverses the hydrolysis reaction; because this condensation