Phosphoryl Group Transfers and ATP:- ATP Provides Energy by Group Transfers, Not by Simple Hydrolysis
Throughout this book you will encounter reactions or processes for which ATP supplies energy, and the contribution of ATP to these reactions is commonly indicated as in Figure 13–8a, with a single arrow showing the conversion of ATP to ADP and Pi (or, in some cases, of ATP to AMP and pyrophosphate, PPi). When written this way, these reactions of ATP appear to be simple hydrolysis reactions in which water displaces Pi (or PPi), and one is tempted to say that an ATP-dependent re action is “driven by the hydrolysis of ATP.” This is not the case. ATP hydrolysis per se usually accomplishes nothing but the liberation of heat, which cannot drive a chemical process in an isothermal system. A single re action arrow such as that in Figure 13–8a almost in variably represents a two-step process (Fig. 13–8b) in which part of the ATP molecule, a phosphoryl or pyrophosphoryl group or the adenylate moiety (AMP), is first transferred to a substrate molecule or to an amino acid residue in an enzyme, becoming covalently at tached to the substrate or the enzyme and raising its free-energy content. Then, in a second step, the phosphate-containing moiety transferred in the first step is displaced, generating Pi, PPi, or AMP. Thus, ATP participates covalently in the enzyme-catalyzed reaction to which it contributes free energy. Some processes do involve direct hydrolysis of ATP (or GTP), however. For example, noncovalent binding of ATP (or of GTP), followed by its hydrolysis to ADP (or GDP) and Pi, can provide the energy to cycle some proteins between two conformations, producing me chanical motion. This occurs in muscle contraction and in the movement of enzymes along DNA or of ribosomes along messenger RNA. The energy-dependent reactions catalyzed by helicases, RecA protein, and some topo isomerases also involve direct hydrolysis of phosphoanhydride bonds. GTP-binding proteins that act in signaling pathways directly hydrolyze GTP to drive conformational changes that terminate signals triggered by hormones or by other extracellular factors.
FIGURE 13–8 ATP hydrolysis in two steps. (a) The contribution of ATP to a reaction is often shown as a single step, but is almost always a two-step process. (b) Shown here is the reaction catalyzed by ATP dependent glutamine synthetase. 1 A phosphoryl group is transferred from ATP to glutamate, then 2 the phosphoryl group is displaced by NH3 and released as Pi.
The phosphate compounds found in living organisms can be divided somewhat arbitrarily into two groups, based on their standard free energies of hydrolysis (Fig. 13–9). “High-energy” compounds have a G of hydrolysis more negative than 25 kJ/mol; “low-energy” compounds have a less negative ΔG0. Based on this criterion, ATP, with a ΔG0 of hydrolysis of 30.5 kJ/mol ( 7.3 kcal/mol), is a high-energy compound; glucose 6-phosphate, with a ΔG0 of hydrolysis of 13.8 kJ/mol (-3.3 kcal/mol), is a low-energy compound. The term “high-energy phosphate bond,” long used by biochemists to describe the P-O bond broken in hydrolysis reactions, is incorrect and misleading as it wrongly suggests that the bond itself contains the en ergy. In fact, the breaking of all chemical bonds requires an input of energy. The free energy released by hydrolysis of phosphate compounds does not come from the specific bond that is broken; it results from the products of the reaction having a lower free-energy content than the reactants. For simplicity, we will sometimes use the term “high-energy phosphate compound” when referring to ATP or other phosphate compounds with a large, negative, standard free energy of hydrolysis. As is evident from the additivity of free-energy changes of sequential reactions, any phosphorylated compound can be synthesized by coupling the synthesis to the breakdown of another phosphorylated com pound with a more negative free energy of hydrolysis. For example, because cleavage of Pi from phospho enolpyruvate (PEP) releases more energy than is needed to drive the condensation of Pi with ADP, the direct donation of a phosphoryl group from PEP to ADP is thermodynamically feasible:
Notice that while the overall reaction above is repre sented as the algebraic sum of the first two reactions, the overall reaction is actually a third, distinct reaction that does not involve Pi; PEP donates a phosphoryl group directly to ADP. We can describe phosphorylated compounds as having a high or low phosphoryl group transfer potential, on the basis of their standard free energies of hydrolysis (as listed in Table 13–6). The phosphoryl group transfer potential of phosphoenolpyruvate is very high, that of ATP is high, and that of glucose 6 phosphate is low (Fig. 13–9). Much of catabolism is directed toward the synthesis of high-energy phosphate compounds, but their formation is not an end in itself; they are the means of activating a very wide variety of compounds for further chemical transformation. The transfer of a phosphoryl group to a compound effectively puts free energy into that compound, so that it has more free energy to give up during subsequent metabolic transformations. We described above how the synthesis of glucose 6-phosphate is accomplished by phosphoryl group transfer from ATP. In the next chapter we see how this phosphorylation of glucose activates, or “primes,” the glucose for catabolic reactions that occur in nearly every living cell. Because of its intermediate position on the scale of group trans fer potential, ATP can carry energy from high-energy phosphate compounds produced by catabolism to com pounds such as glucose, converting them into more re active species. ATP thus serves as the universal energy currency in all living cells. One more chemical feature of ATP is crucial to its role in metabolism: although in aqueous solution ATP is thermodynamically unstable and is therefore a good phosphoryl group donor, it is kineticallystable. Because of the huge activation energies (200 to 400 kJ/mol) re quired for uncatalyzed cleavage of its phosphoanhydride bonds, ATP does not spontaneously donate phosphoryl groups to water or to the hundreds of other potential acceptors in the cell. Only when specific enzymes are present to lower the energy of activation does phosphoryl group transfer from ATP proceed. The cell is therefore able to regulate the disposition of the energy carried by ATP by regulating the various enzymes that act on it.
FIGURE 13–9 Ranking of biological phosphate compounds by standard free energies of hydrolysis. This shows the flow of phosphoryl groups, represented by P , from high-energy phosphoryl donors via ATP to acceptor molecules (such as glucose and glycerol) to form their low-energy phosphate derivatives. This flow of phosphoryl groups, catalyzed by enzymes called kinases, proceeds with an overall loss of free energy under intracellular conditions. Hydrolysis of low energy phosphate compounds releases Pi, which has an even lower phosphoryl group transfer potential (as defined in the text).
