Transcript

Energy Transfer from Adenosine Triphosphate

John Ross*Department of Chemistry, Stanford UniVersity, Stanford, California 94305

ReceiVed: October 5, 2005; In Final Form: February 14, 2006

We suggest a direct molecular mechanism of energy transfer from adenosine triphosphate (ATP) in hydrolysisand phosphorylation reactions, from chemical energy into mechanical energy. Upon hydrolysis of ATP, saybound to a protein, the electrostatic energy of Coulombic repulsion of the ions adenosine diphosphate andphosphate is available to assert a force on a neighboring molecular group in the protein and can do work onthat group, or as the ions recede from each without asserting such a force, they gain relative kinetic energy,which, in the absence of dissipative collisions that turn this kinetic energy into heat, can be converted intoany other form of energy and work by an impulse, a collision with a neighboring group, without restrictions.Either possibility can be used as a source of activation energy for reactions, as a source of energy to surmountenergy barriers in conformational changes, and as a source of work to be done, as in muscle. In some systemswhere the Gibbs free energy change is fully utilized, all of this energy is turned into mechanical energy, andwe suggest a similar mechanism. From the literature we cite some experimental evidence and several quotationsindicative of the possibility of our suggestion.

I. Introduction

Adenosine triphosphate, ATP, is an universal energy carrierin biological systems; it hydrolyzes and carries out manyphosphorylation reactions that proceed spontaneously. For thereaction

the standard Gibbs free energy change is∆G° ) -31 kJ/molat pH 7 in the presence of magnesium at pMg) 3, and underphysiological conditions the Gibbs free energy change,∆G, isabout-50 kJ/mol.1

The free energy liberated in the hydrolysis of ATP isharnessed to drive reactions that require an input of free energy,such as muscle contraction. In turn, ATP is formed from ADPand HPO4

-2 when fuel molecules are oxidized in chemotrophsor when light is trapped in phototrophs. This ATP-ADP cycleis the fundamental mode of energy exchange in biologicalsystems.1 Reactions of this type1 are ubiquitious: for metabo-lism, for pumping ions against chemical and electrical gradients,for generating conformational changes in proteins, etc.

The universality of the use of ATP is a marvel: what is itsbasis; how and why was ATP (and similarly GTP) selected forthis role; how is energy stored in ATP; how is energy transferredwhen ATP reacts?

Westheimer wrote an incisive article in 1978,2 addressingsome of these questions. We quote from his summary:

...the existence of a genetic material such as DNA requires acompound for a connecting link that is at least divalent. In orderthat the resulting material remain within a membrane, it shouldalways be charged, and therefore the linking unit should havea third ionizable group. The linkage is conveniently made byester bonds, but, in order that the ester be hydrolytically stable,that charge should be negative and should be physically close

to the ester groups. All of these conditions are met by phosphoricacid, and no other alternative is obvious. Furthermore, phos-phoric acid can form monoesters of organic compounds thatcan decompose by mechanisms other than nucleophilic attack,a mechanism that allows them sufficient reactivity in intermedi-ary metabolism... This remarkable combination of thermody-namic instability and kinetic stability was noted... by Lippmann.3

In this article we suggest a direct molecular mechanism ofenergy release and tranfer in hydration and phosphorylationreactions of ATP, from chemical energy into mechanical energy.The emphasis is first on the Coulombic repulsion between ADPand phosphate, and second, on the release of all the availableenergy in ATP. We know of no other prior suggestion on thisspecific topic.

II. Proposal for Molecular Energy Release from ATP inHydrolysis and Phosphorylation Reactions

The energy source in ATP has been described as due to a“high energy bond” and is so presented in standard texts. Thesource of the high energy has been assigned to Coulombicrepulsion of ADP and phosphate and to resonance effects.1

We gain some insight into the order of magnitude of theCoulombic repulsion in two very simple, separate ways. First,compare∆G° of reaction 2, rewritten in greater detail, but withMg omitted with that of the hydrolysis of phosphoglycerate. In

reaction 3 there is only hydrolysis of a phosphorus-oxygen* Author to whom correspondence should be addressed. E-mail:

[email protected].

ATP-4 + H2O f ADP-3 + HPO4-2 + H+ (1)

6987J. Phys. Chem. B2006,110,6987-6990

10.1021/jp0556862 CCC: $33.50 © 2006 American Chemical SocietyPublished on Web 03/16/2006

bond and no Coulombic repulsion of the post-hydrolysisproducts. In reaction 2 there is also hydrolysis of a phosphorus-oxygen bond followed by formation of relative kinetic energyin the reaction products due to reduction of Coulombic repulsion.As the two negative ions recede from each other to infiniteseparation we may estimate the (maximum) relative kineticenergy to be the difference in the standard Gibbs free energyof reaction 2,-31 kJ/mol, and that of reaction 3,-10 kJ/mol,which is about- 21 kJ/mol.1 A possible difference in resonanceenergy in these two reactions is ignored in this estimate.

A second estimate can be made with the most simpleelectrostatic calculation. The change in relative kinetic energy,∆K, of two ions A and B, of like charge with magnitudeszA

andzB, with initial separationri and final separationrj is

whereε0 is the permittivity of free space, 8.85× 10-12 F/m, εis the relative permittivity of the medium, ande is the electroniccharge, 1.6× 10-19 C. To calculate ranges of possible valuesof ∆K, we need to choose ranges of the final separation andthe permittivity of the medium in which the hydrolysis of ATPtakes place. We take the initial separation to be 0.3 nm, whichis approximately the P-P distance in ATP, and the finalseparation to be 0.4 nm. The relative permittivity of water is78, and we estimate that the relative permittivity of a hydro-phobic environment, as in a protein fold, is 20.4 It could beconsiderably lower, perhaps 2-4. In Table 1 we list severalpossibilities, all forε ) 20 and the distances indicated: Thefirst line combines the charge of the proton, in reaction 2, withthe charge of the phosphate; the second line neglects the chargeof the proton. The next two lines consider the presence ofdivalent Mg on ADP. Line 3 combines the charge of the protonwith that of the phosphate; line 4 neglects the charge of theproton. To obtain the values of∆K for the relative permittivityof 10 (80), multiply the values of∆K in the table by a factor of2 (divide by 4); to obtain the values for the case of the finalseparation distance at infinity, multiply the values in the tableby 4. For these ranges of conditions the entries in the table arewithin an order of magnitude of the first estimate of 21 kJ/mol.Thus the kinetic energy obtained from a reduction of theCoulombic repulsion is in the range of two-thirds of the standardGibbs free energy change of hydrolysis and in the range ofsomewhat less than half of the Gibbs free energy change underphysiological conditions.

The discussion presented here holds equally well for reactionsof ATP with pyrophosphate as one of the products.

An increase in ionic strength of the solution in which reaction2

occurs increases the shielding of the two negative ions and hencereduces the Coulombic repulsion. (This shielding produces theprimary salt effect in the kinetics of ions.)

As the HPO4-2 ion moves away from the ADP ion due to

Coulombic repulsion, the relative kinetic energy of the two ions,in the absence of other interactions, increases. If ADP is held

in place, for example, by binding to a segment of an enzyme,then the HPO4-2 ion acquires all of this energy. If both ionsare free to move, then HPO4

-2 acquires about 70% of therelative kinetic energy.

The essence of the present proposal is:Upon hydrolysis of ATP, the force due to the Coulombic

repulsion of the product ions can act on a neighboring moleculargroup in the protein and can do mechanical work on it bydisplacing it. If the ions recede from each other without assertingsuch a force, then they gain kinetic energy, which can, prior todissipation into heat, be transferred in an impulse, a collisionwith a neighboring group, into mechanical work. In either casethe mechanical energy generated can be wholly converted intoany other form of energy or work without any restrictions. IfATP is tightly bound to an enzyme, then upon hydrolysis theγ-phosphate may not be able to move away from ADP and theCoulomb repulsion is retained as potential energy until thebinding is reduced.

In some systems the Gibbs free energy change available fromthe reaction of ATP is fully utilized and all of this energy isturned into mechanical energy. This energy conversion occurslikely in the same way, due to the remaining electronic repulsion,as for that part due to the Coulombic repulsion.

For full conversion from chemical to mechanical energy nodissipation may occur. Dissipation may take place throughcollisions in which mechanical translational energy is changedto a statistical distribution, heat, or through various types ofinelastic collisions. Such events are precluded for very shorttime scales. For a relative kinetic energy of the phosphate of20 kJ/mol, for example, the phosphate ion moves about 0.1 nmin 0.3 ps. If no other collisions occur in that short time interval,as is likely, then the phosphate can transfer its kinetic energyto a neighboring molecular group and can thus achieve, say, aconformational change.

The mechanical energy being discussed, 20-50 kJ/mol, ison the order of 10-20 times thermal energy, 0.25 kJ/mol, andtherefore thermal fluctuations can be expected to have a minoreffect on the present argument.

More sophisticated calculations are of course possible, yetthe simplest calculations yield interesting concordant results.

III. Utilization of Mechanical Energy Obtained from ATP

The mechanical energy, generated from chemical energy inATP, in the products to be formed in the hydrolysis andphosphorylation reactions of ATP may serve different purposes.

First, the mechanical energy can produce work at themolecular level. Human inventions of the conversion of chemi-cal energy into work in a cyclic process require either a batteryconnected to a motor, or require the production of heat and itsusage in thermal engines, at reduced efficiency.

Second, the mechanical kinetic energy can be directly usedto surmount energy barriers on the reaction coordinate. Comparethis efficient conversion of electrostatic energy into mechanicalenergy, say, for surmounting a barrier or any other need forenergy, with providing a reaction complex, as in a unimolecularreaction, with thermal energy (heat). This thermal energy isgenerally distributed over all degrees of freedom statistically.A fluctuation has to occur to place sufficient energy into a givendegree of freedom, say, one bond, for reaction to occur. It takestime for such a fluctuation to take place, and hence thermalexcitation is less efficient (requires more energy and takeslonger) than activation by mechanical energy.

Third, the mechanical energy produced can be transferred tobring about a conformational change in the protein that requires

TABLE 1: Estimates of ∆K, the Relative Kinetic Energya

zA zB

∆K(kJ/mol)

1. -3 -1 182. -3 -2 363. -1 -1 64. -1 -2 12

a The symbols are defined in the text.

∆K )zAzBe2

4πε0ε(1ri

- 1rj) (4)

6988 J. Phys. Chem. B, Vol. 110, No. 13, 2006 Ross

energy input simply by the force exerted by the Coulombicrepulsion between ADP and phosphate on neighboring groupsin the enzyme.

Fourth, the repulsion between the negative ions, both inhydrolysis and in phosphorylation, enhances the rate of separa-tion of the products and thus leads to rapid completion of thereactions.

The suggestion made here for the conversion of chemicalinto mechanical energy can encompass several different sce-narios: (1) Suppose transfer of chemical energy occurs at agiven time, say, to effect a configurational change in the enzyme.There may be subsequent steps of configurational changesenergized by the first one. (2) The transfer of chemical energymay occur at different times. Suppose ATP is tightly bound toan enzyme as in actomycin; hydrolysis may take place, but theγ-phosphate cannot move away from ADP. In this case therecan be no reduction in the Coulombic repulsion and hence noconversion of that potential energy into mechanical work. Aftersome time interval the binding may be reduced, some reductionin the Coulomb repulsion may occur, and some mechanical workmay be done on neighboring groups. (3) Step 2 may occur morethan once, and thus energy from ATP may be distributed overseveral steps of the enzyme cycle.

All these advantages lead to the universal use of ATP as anenergy source in plants and animals.

IV. Possible Experimental Evidence for the PresentSuggestion

In this section we seek experimental indications for theutilization of the mechanical energy generated from the chemicalenergy available from ATP as suggested here. There is no proofof the suggestion, but the subsections list reasonable indicationsof its possibility.

A. Efficiency of Utilization of Energy from ATP. In manycases the energy available from ATP is not fully used; someenergy is dissipated. In other cases, however, the energyavailable from ATP is fully utilized.5,6 In the latter case it isclear that the mechanical energy generated from the repulsionof the product ions is also fully utilized, and the free energychange of the hydrolysis step is small. In refs 5 and 6 it wasshown that one ATP is utilized for each mechanical step inmyosin-V. The step size is 36 nm, and the maximum load is2.5 pN. The product of these numbers translates to 54 kJ/mol,well within experimental error of the full utilization of the Gibbsfree energy change of the hydrolysis of ATP under physiologicalconditions. Another example of full utilization, “near 100%efficiency” in ATP synthase, is given in ref 7.

The research described in refs 5 and 6 was done in vitro, inwater, which is likely a place for dissipation to occur due tothe collision of the phosphate ion with water molecules. Henceit is worthwhile noting that no measurable dissipation of themechanical energy takes place in these examples.

B. Crystal Structure of Monomeric Actin in the ATPState.8 The authors compare the crystal structures of the “actinmonomer (G-actin) in the ATP and the ADP states... Acomparison of the structures in the two states reveals how therelease of the nucleotideγ-phosphate triggers a sequence ofevents that propagate into a loop to helix transition in the DNaseI-binding loop in subdomain 2.”

C. The Role of MeH73 in Actin Polymerization and ATPHydrolysis.9 “...the imidazole of (Me)H73 does not make directcontact with the terminal phosphate, but it is in a position tomediate conformational changes in the hairpin loops associated

with nucleotide binding thereby governing hydrolysis andpolymerization.”

D. Swing of the Lever Arm of a Myosin Motor at theIsomerization and Phosphate Release Steps.10 The authorsuse FRET to observe the working stroke, the tilt of a lever arm,in the myosin motor. They show

“that the hydroxyl terminal fluorophore swings at the isomer-ization step of the ATP hydrolysis cycle, and then swings backat the subsequent step in which inorganic phosphate is released,thereby mimicking the swing of the lever arm. The swing atthe phosphate release step may correspond to the working stroke,and the swing at the isomerization to the recovery stroke... thelever arm domain can generate sliding of myosin along actinfilaments in the appropriate direction for muscle action...”

E. Analysis of the Conformational Change of Myosinduring ATP Hydrolysis Using Fluorescence ResonanceEnergy Transfer.11 “The phosphate release step shifts themyosin from a weak to a strong binding conformation in theinteraction with actin and is the step believed to be associatedwith the force generation during muscle contraction.”

F. The Structural Basis of the Myosin ATPase Activity.12

“Contrary to initial expectations, ATP hydrolysis in myosin isnot coincident with the force generating step... The energytransduction step occurs during product release... Kinetic studiesindicate that the phophate ion is released prior to ADP duringthe contractile cycle.”

G. Photon Excitation of Myoglobin. Miller and co-workershave published a series of articles13 on photon excitation ofmyoglobin that leads to bond breaking. Thereupon they haveobserved, by means of transient phase grating spectroscopy, thatthe effect of this event is passed through a series of molecularfragments. Collective modes of motion are excited within about0.3 ps, which brings about a conformational change. The barrierfor this change is about 12 kJ/mol. In analogy, the impositionof a force, or the impact (a collision) of a phosphate ion, withhigh kinetic energy, unto a molecular fragment of a protein maybring about a conformational change. As seen in Table 1 theenergy of 12 kJ/mol may be available from the conversion ofelectrostatic repulsion to relative kinetic energy with a relativemotion of the phosphate ion from ADP of about 0.1 nm, or 1Å, in a medium of relative permittivity of 10, in the presenceof Mg, line 3 in Table 1.

H. Examples Where the Suggestion Seems To Play NoRole. In an hypothesis for the use of ATP for pumping protonsacross mitochondrial membranes, and in the reverse process ofATP synthesis, Boyer14,15 proposed a series of conformationalchanges in the F1F0-type ATP synthase to achieve thesefunctions. There seems to be no need for our suggestion.However further studies are necessary to relate these confor-mational changes to the utilization of ATP energy, whichaccording to the experiments in ref 7, is near 100%. In thesefurther studies our suggestion may find an application.

V. Possible Ions for the Choice of a Source of Energy

What ions are available, and which are likely candidates, fora physical source of energy based on establishing repulsionbetween ions of like charge? The ions present in seawater mustsurely be considered; a list of some such ions and theirconcentrations are given in Table 2. What combinations of likecharges exist, either by themselves or in combination with othermolecular moieties, which have the property of an energy sourceas suggested here? Among the positive ions there appears tobe none. Among the negative ions there are the phosphates

Energy Transfer from ATP J. Phys. Chem. B, Vol. 110, No. 13, 20066989

PO4-3 , P2O7

-3, and P3O10-4 , which can react analogously to

eq 1

This reaction occurs spontaneously at pH 7, but the rate is slowin the absence of phosphatases.

There is another possibility, that of the sulfates

for which ∆G° ) -142. kJ/mol. However S2O7-2 is not stable

at neutral pH.The conclusion reached here, on the basis of arguments not

considered by Westheimer,2 is the same as that given in ref 2,see the quote cited in the Introduction: that phosphates appearto be the only choice for the universal role of energy storageand transfer into mechanical and electrical energy in livingsystems at normal temperatures.

Acknowledgment. Helpful discussions are gratefully ac-knowledged with Professors S. Block, W. Huestis, C. Khosla,

E. Kool, V. Pande, J. Spudich, R. N. Zare, and Dr. Marcel Vlad.This work was supported in part by the National ScienceFoundation.

References and Notes

(1) Stryer, L.Biochemistry, 4th ed.; W. H. Freeman and Co.: NewYork, 1995.

(2) Westheimer, F. H.Science1987, 235, 1173-1178.(3) Lippmann, L. InPhosphorous Metabolism; McElroy, W. D., Glass,

H. B., Eds.; Johns Hopkins Press: Baltimore, MD, 1961; Vol. 1, p 521.

(4) Schutz, C. N.; Warshel, A.Proteins: Struct., Funct., Genet.2001,44, 400-427.

(5) Mehta, A. D.; Rock, R. S.; Rief, M.; Spudich, J. A.; Mooseker, M.S.; Cheney, R. E.Nature1999, 400, 590-593.

(6) Rief, M.; Rock, R. S.; Mehta, A. D.; Mooseker, M. S.; Cheney, R.E.; Spudich, J. A.Proc. Natl. Acad. Sci. U.S.A.2002, 97, 9482-9486.

(7) Kinosita, K. Jr.; Yasuda, R.; Noji, H.Essays Biochem.2000, 35,3-18.

(8) Graceffa, P.; Dominguez, R.J. Biol. Chem2003, 278, 34172-34180.

(9) Nyman, T.; Schuler, H.; Korenbaum, E.; Schutt, C. E.; Karlsson,R.; Lindberg, U.J. Mol. Biol. 2002, 317, 577-589.

(10) Suzuki, Y.; Yasunaga, T.; Ohkura, R.; Wakabayashi, T.; Sutho, K.Nature1998, 396, 380-383.

(11) Mizukura, Y.; Maruta, S.J. Biochem.2002, 132, 471-482.

(12) Rayment, I.J. Biol. Chem.1996, 271, 15850-15853.

(13) Armstrong, M. R.; Ogilvie, J. P.; Cowan, Nagy, A. M.; Miller, R.J. D. Proc. Natl. Acad. Sci. U.S.A.2003, 100, 4990-4994.

(14) Boyer, P. D.Biochim. Biophys. Acta1993, 1140, 215-250.

(15) Capaldi, R. A.; Aggeler, R.Trends Biochem. Sci.2002, 27, 154-160.

TABLE 2: Principle Constituents of Seawater

chloride 0.54 mol/kg of seawatersodium 0.46 mol/kg of seawatermagnesium 0.053 mol/kg of seawatersulfate 0.028 mol/kg of seawatercalcium 0.010 mol/kg of seawaterphosphate 0.1-0.3µmol/kg of seawater

P3O10-4 + H2O f P2O7

-3 + HPO4-2 + H+ (5)

S2O7-2 + H2O f 2SO4

-2 + 2H+ (6)

6990 J. Phys. Chem. B, Vol. 110, No. 13, 2006 Ross


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