Energy Transfer from Adenosine Triphosphate: Quantitative Analysis and Mechanistic Insights Sarang S. Nath Science Group, The Mother’s International School, Sri Aurobindo Marg, Hauz Khas, New Delhi 110016, India Sunil Nath* Department of Biochemical Engineering and Biotechnology, Indian Institute of Technology, Delhi, Hauz Khas, New Delhi 110016, India ReceiVed: NoVember 2, 2008; ReVised Manuscript ReceiVed: December 9, 2008 The ATP-ADP thermodynamic cycle is the fundamental mode of energy exchange in oxidative phospho- rylation, photophosphorylation, muscle contraction, and intracellular transport by various molecular motors and is therefore of vital importance in biological energy transduction and storage. Following a recent suggestion in the pages of this journal (Ross, J. J. Phys. Chem. B 2006, 110, 6987-6990), we have carried out a simple quantitative analysis of a direct molecular mechanism of energy transfer from adenosine triphosphate (ATP). The simulation provides new insights into the mechanistic events following terminal phosphorus-oxygen bond cleavage during ATP hydrolysis. This approach also allows for the division of the energy-transfer process into elementary steps and for the prediction of the distribution of the standard-state Gibbs free energy of the overall ATP hydrolysis process among the various steps of substrate binding, bond cleavage, and product release in the enzymatic cycle, which had proved very difficult to specify previously. These predictions are consistent with available experimental data on ATP hydrolysis by protein biomolecular machines. The fundamental biological implications arising from our results are also discussed in detail. The aspects considered in this work enable us to look at the entire process of ATP synthesis/hydrolysis and energy transduction and storage in various biological molecular machines in a logically consistent and unified way. I. Introduction Understanding the process of oxidative phosphorylation and the fundamentals of the mechanism of the synthesis of adenosine triphosphate (ATP), the universal biological energy carrier, and its hydrolysis by various molecular machines of the mammalian cell has stimulated considerable research work over the past few decades. 1-10 The free energy liberated by the hydrolysis of ATP to ADP and inorganic phosphate is utilized to drive reactions requiring an input of free energy, such as muscle contraction. 10-15 Because green plants and microorganisms also capture and utilize energy by the same reaction, 16-18 the ATP-ADP thermodynamic cycle is the universal mode of energy exchange in biological systems. This is therefore a very important type of energy transduction in all of biology, and we are challenged to understand the molecular mechanism of this vital process. Mechanisms of energy transduction into/from ATP can be divided into two classes: direct and indirect. A detailed molecular mechanism of direct transduction of chemical energy into mechanical energy and vice versa that has been proposed is the torsional mechanism of energy transduction and ATP synthesis/hydrolysis. 9,10,15,19-27 In this molecular mechanism, the crucial role of electrostatic interactions between two charges (such as MgADP and P i ) in order to effect dissipation-free energy transduction was emphasized. 10,21,27 The ATP molecule itself was modeled as two negatively charged spheres attached to the ends of two hinged bars by an inextensible string forcing the spheres to remain close to each other in a high-energy conformation. ATP hydrolysis was considered analogous to cutting the string, thereby freeing the charged spheres and allowing them to move away as a result of mutual electrostatic repulsion. This movement of charges was conceived to carry out useful work such as rotation of the γ-subunit in F 1 -ATPase or to be transduced and stored as in the uncoiling of the S-2 coiled coil in myosin during muscle contraction. 10,14,15,27 Re- cently, a suggestion along these lines was also made in the literature by Ross. 28 In this interesting suggestion, Ross envis- ages that the entire standard-state free energy of ATP hydrolysis, ΔG 0 ′, is released not during the bond cleavage step but subsequently, after the binding of the charges to their sites is reduced. 28 This suggestion helped us to formulate an explanation of the mechanism of hydrolysis of ATP in which the release of the energy transduced at the cleavage step was distributed over other elementary steps of the enzymatic cycle, especially during the release of the inorganic phosphate into the medium. 27 Here, we develop this insight mechanistically as well as quantitatively and show the interesting results of a simulation that help to further elucidate the molecular mechanism of ATP hydrolysis and energy transfer. II. Current Views of Energy Transduction by ATP-Utilizing Biological Molecular Machines Various views of energy transduction have been put forth in the scientific literature. According to several researchers in bioenergetics, the actual chemical synthesis of ATP (and thus cleavage of ATP in the hydrolysis mode) does not require any external energy input. They propose that the release of bound * To whom correspondence should be addressed. E-mail: sunath@ dbeb.iitd.ernet.in. J. Phys. Chem. B 2009, 113, 1533–1537 1533 10.1021/jp809678n CCC: $40.75  2009 American Chemical Society Published on Web 01/14/2009