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Anaminoacyl adenylate is first formed from the amino acid and ATP, withthe elimination of PPi. The adenylate group is then displaced by atransfer RNA, which is thereb}' joined to the amino acid. In this case,too, the PPi formed in the first step is hydrolyzed by inorganic pyrophosphatase. An unusual use of the cleavage of ATP to AMP and PPioccurs in the firefly, which uses ATP as an energy source to producelight flashes (Box 13-3, p. 382).The AMP produced in adenylate transfers is returned to the ATPcycle by the action of adenylate kinase, which catalyzes the reversible reactionATP + AMPADP + ADPO"O-P-O-P-O-P=OIIIooOGTPfirst anhydridebond broken^GO' - 0The ADP so formed can be phosphorylated to ATP, using reactionsdescribed in detail in later chapters.Assembly of Informational Macromolecules Requires EnergyWhen simple precursors are assembled into high molecular weightpolymers with defined sequences (DNA, RNA, proteins), as describedin detail in Part IV, energy is required both for the condensation ofmonomeric units and for the creation of ordered sequences.
The precursors for DNA and RNA synthesis are nucleoside triphosphates, andpolymerization is accompanied by cleavage of the phosphoric acid anhydride linkage between the a- and /3-phosphates, with the release ofPPi (Fig. 13-11). The moieties transferred to the growing polymer inthese polymerization reactions are adenylate (AMP), guanylate (GMP),cytidylate (CMP), or uridylate (UMP) for RNA synthesis, and theirdeoxy analogs for DNA synthesis.
We have seen that the activation ofamino acids for protein synthesis involves the donation of adenylategroups from ATP, and we shall see later that the formation of peptidebonds on the ribosome is also accompanied by GTP hydrolysis (Chapter26). In all of these cases, the exergonic breakdown of a nucleoside triphosphate is coupled to the endergonic process of synthesizing a polymer of a specific sequence.RNA chainlengthenedby onenucleotideFigure 13-11 Nucleoside triphosphates are thesubstrates for RNA synthesis.
With each nucleosidemonophosphate added to the growing chain, onePPi is released and then hydrolyzed to two P*. Thehydrolysis of two phosphoric acid anhydride bondsfor each nucleotide added provides energy for forming the bonds in the RNA polymer and for assembling a specific sequence of nucleotides.382Part III Bioenergetics and MetabolismBOX 13-3Firefly Flashes: Glowing Reports of ATPFigure 1 The firefly, a beetle of the Lampyridaefamily.Many fungi, marine microorganisms, jellyfish, andcrustaceans as well as the firefly (Fig. 1) are capable of generating bioluminescence, which requiresconsiderable amounts of energy.
In the firefly, ATPis used in a set of reactions that converts chemicalenergy into light energy. From many thousands offirefly lanterns collected by children in and aroundBaltimore, William McElroy and his colleagues atThe Johns Hopkins University isolated the principal biochemical components involved, luciferin(Fig.
2), a complex carboxylic acid, and luciferase,an enzyme. The generation of a light flash requiresactivation of luciferin by an enzymatic reactionwith ATP in which a pyrophosphate cleavage ofATP occurs, to form luciferyl adenylate (Fig. 2).This compound is then acted upon by molecularoxygen and luciferase to bring about the oxidativedecarboxylation of the luciferin to yield oxyluciferin. This reaction, which has intermediate steps,is accompanied by emission of light (Fig. 2). Thecolor of the light flash differs with firefly speciesand appears to be determined by differences in thestructure of the luciferase.
Luciferin is then regenerated from oxyluciferin in a subsequent series ofreactions. Other bioluminescent organisms useother types of enzymatic reactions to generatelight.In the laboratory, pure firefly luciferin and luciferase are used to measure minute quantities ofATP by the intensity of the light flash produced. Aslittle as a few picomoles (10~12 mol) of ATP can bemeasured in this way.LuciferyladenylateHNHsHOluciferas-Firefly luciferin.NHONXssOHHI„ C — O — P — O ^ Rib H Adenine !H ||n•= •OA M PATPLuciferinOxyluciferin,0Luciferyl adenylateAMPregeneratingreactionsFigure 2 Important components in firefly bioluminescence, and the firefly bioluminescence cycle.Chapter 13 Principles of Bioenergetics383ATP Energizes Active Transport across MembranesATP can supply the energy for transporting an ion or a molecule acrossa membrane into another aqueous compartment where its concentration is higher.
Recall from Chapter 10 that the free-energy change(AGt) for the transport of a nonionic solute from one compartment toanother is given byAGt =RT In(13-4)where Ci is the molar concentration of the solute in the compartmentfrom which the ion or molecule moves and C2 is its molar concentrationin the compartment into which it moves. When a proton or othercharged species moves across a membrane without a counterion, theseparation of electrical charge requires extra electrical work beyondthe osmotic work against a concentration gradient.
The extra electricalwork is ZJAi//, where Z is the (unitless) electrical charge of the transported species, At/j is the transmembrane electrical potential (in volts),and 3 is the Faraday constant (96.48 kJ/V • mol). The total energy costof moving a charged species against an electrochemical gradient isActinfilamentMyosinheadMyosinthickfilamentADPATP(a)AGt ={CJC{) + ZJAif;(13-5)Transport processes are major consumers of energy; in tissues suchas human kidney and brain, as much as two-thirds of the energy consumed at rest is used to pump Na + and K+ across plasma membranesvia the Na + K + ATPase.
Na + and K+ transport is driven by cyclic phosphorylation and dephosphorylation of the transporter protein, withATP as the phosphate donor (see Fig. 10-23). Na + -dependent phosphorylation of the Na + K + ATPase forces a change in the protein's conformation, and K + -dependent dephosphorylation favors return to theoriginal conformation. Each cycle in the transport process results inthe conversion of ATP to ADP and Pi, and it is the free-energy change ofATP hydrolysis that drives the pumping of Na + and K + . In animalcells, the net hydrolysis of one ATP is accompanied by the outwardtransport of three Na + ions and the uptake of two K+ ions.Head returns tooriginal conformation,causing sliding motionvijLDissociationof actin-myosin^~— H2OCa2+ triggersassociation ofmyosin headwith actin(c)(b)Hydrolysisof ATP bymyosin headATP Is the Energy Source for Muscle ContractionIn the contractile system of skeletal muscle cells, myosin and actin arespecialized to transduce the chemical energy of ATP into motion.
ATPbinds tightly but noncovalently to the head portion of one conformationof myosin, holding the protein in that conformation. When myosin(which is also an ATPase) catalyzes the hydrolysis of its bound ATP,the ADP and Pi produced dissociate from the protein, allowing it torelax into a second conformation until another molecule of ATP binds(Fig. 13-12). The binding and subsequent hydrolysis of ATP thus provide the energy that forces cyclic changes in the conformation of themyosin head.
The change in conformation of many individual myosinmolecules results in the sliding of myosin fibrils along actin filaments(see Fig. 7-32), which translates into macroscopic contraction of themuscle fiber.This production of mechanical motion at the expense of ATP is oneof the few cases in which ATP hydrolysis per se, and not group transferfrom ATP, is the source of the chemical energy in a coupled process.The energy-dependent reactions catalyzed by helicases, RecA protein,and some topoisomerases (Chapter 24) and by certain GTP-bindingproteins (Chapter 22) also involve direct hydrolysis of phosphoric acidanhydride bonds.Figure 13-12 ATP hydrolysis drives the crossbridge cycle during the sliding motion of actinmyosin complexes in muscle.
This proposed mechanism begins with each myosin head bound to anactin filament. Binding of ATP to myosin (a) causesdissociation of the actin-myosin cross-bridge. ATPhydrolysis (b) leaves myosin with bound ADP andPi, which favors a different conformation of themyosin head. In this conformation, the myosin headbinds to an adjacent actin filament (c) when elevated cytosolic Ca2+ signals contraction. Thiscross-bridge formation induces the release of boundADP and P{ (d), which provides the free energy fora conforrfiational change in the myosin head; thehead tilts, forcing the thin (actin) filament to sliderelative to the thick (myosin) filament, producingcontraction. ATP then binds to the myosin head todissociate the cross-bridge and start another cycle.Each cycle occurs in about 1 msec.394Part III Bioenergetics and Metabolismfree flavin nucleotide.
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