In almost all organisms, the nucleoside triphosphate adenosine triphosphate, or ATP (Figure 2-31), is the most important molecule for capturing, transiently storing, and subsequently transferring energy to perform work (e.g., biosynthesis, mechanical motion). Commonly referred to as a cell’s energy “currency,” ATP is a type of usable potential energy that cells can “spend” in order to power their activities. The storied history of ATP begins with its discovery in 1929, apparently simultaneously by Kurt Lohmann, who was working with the great biochemist Otto Meyerhof in Germany and who published first, and by Cyrus Fiske and Yellapragada SubbaRow in the United States. Muscle contractions were shown to depend on ATP in the 1930s. The proposal that ATP is the main intermediary for the transfer of energy in cells is credited to Fritz Lipmann around 1941. Many Nobel Prizes have been awarded for the study of ATP and its role in cellular energy metabolism, and its importance in understanding molecular cell biology cannot be overstated.

The useful energy in an ATP molecule is contained in phosphoanhydride bonds, which are covalent bonds formed from the condensation of two molecules of phosphate by the loss of water:

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As shown in Figure 2-31, an ATP molecule has two key phosphoanhydride (also called phosphodiester) bonds. Forming these bonds (represented here by the symbol ∼) in ATP requires an input of energy. When these bonds are hydrolyzed, or broken by the addition of water, that energy is released. Hydrolysis of a phosphoanhydride bond in each of the following reactions has a highly negative ΔG°′ of about –7.3 kcal/mol:

P_i stands for inorganic phosphate (PO₄^3–) and PP_i for inorganic pyrophosphate, two phosphate groups linked by a phosphoanhydride bond. As the top two reactions show, the removal of a phosphate group from ATP leaves adenosine diphosphate (ADP), and the removal of a pyrophosphate group from ATP leaves adenosine monophosphate (AMP).

A phosphoanhydride bond or other “high-energy bond” (commonly denoted by ∼) is not intrinsically different from other covalent bonds. High-energy bonds simply release substantial amounts of energy when hydrolyzed. For instance, the ΔG°′ for hydrolysis of a phosphoanhydride bond in ATP (–7.3 kcal/mol) is more than three times the ΔG°′ for hydrolysis of the phosphoester bond (red) in glycerol 3-phosphate (–2.2 kcal/mol):

A principal reason for this difference is that ATP and its hydrolysis products, ADP and P_i, are charged at neutral pH. During synthesis of ATP, a large amount of energy must be used to force the negative charges in ADP and P_i together. Conversely, this energy is released when ATP is hydrolyzed to ADP and P_i. In comparison, formation of the phosphoester bond between an uncharged hydroxyl in glycerol and P_i requires less energy, and less energy is released when this bond is hydrolyzed.

Cells have evolved protein-mediated mechanisms for transferring the free energy released by hydrolysis of phosphoanhydride bonds to other molecules, thereby driving reactions that would otherwise be energetically unfavorable. For example, if the ΔG for the reaction B + C → D is positive but less than the ΔG for hydrolysis of ATP, the reaction can be driven to the right by coupling it to hydrolysis of the terminal phosphoanhydride bond in ATP. In one common mechanism of such energy coupling, some of the energy stored in this phosphoanhydride bond is transferred to one of the reactants (here, B) by the breaking of the bond in ATP and the formation of a covalent bond between the released phosphate group and that reactant. The phosphorylated intermediate generated in this way can then react with reactant C to form product D + P_i in a reaction that has an overall negative ΔG:

B + Ap ~p~p → B~p + Ap~p

B~p + C → D + P_i

The overall reaction

B + C + ATP ⇌ D + ADP + P_i

is energetically favorable (ΔG < 0). Similarly, hydrolysis of GTP to GDP can provide energy to perform work, including the synthesis of ATP (see Chapter 12), but most often GTP hydrolysis is used to control cellular systems (e.g., protein synthesis, hormonal signaling) rather than as a source of energy.

An alternative mechanism of energy coupling is to use the energy released by ATP hydrolysis to change the conformation of a molecule to an “energy-rich” stressed state. In turn, the energy stored as conformational stress can be released as the molecule “relaxes” back into its unstressed conformation. If this relaxation process can be coupled to another reaction, the released energy can be harnessed to drive cellular processes.

As with many biosynthetic reactions, transport of molecules into or out of the cell often has a positive ΔG and thus requires an input of energy to proceed. Such simple transport reactions do not directly involve the making or breaking of covalent bonds; thus their ΔG°′ is 0. In the case of a substance moving into a cell, Equation 2-7 becomes

where [C_in] is the initial concentration of the substance inside the cell and [C_out] is its concentration outside the cell. We can see from Equation 2-10 that ΔG is positive for transport of a substance into a cell against its concentration gradient (when [C_in] > [C_out]); the energy to drive such “uphill” transport is often supplied by the hydrolysis of ATP. Conversely, when a substance moves down its concentration gradient ([C_out] > [C_in]), ΔG is negative. Such “downhill” transport releases energy that can be coupled to an energy-requiring reaction, such as the movement of another substance uphill across a membrane or the synthesis of ATP itself (see Chapters 11 and 12).