The most important and versatile of the activated carriers in cells is ATP (adenosine triphosphate). Just as the energy stored in the raised bucket of water in Figure 2–32B can drive a wide variety of hydraulic machines, ATP is a convenient and versatile store, or currency, of energy used to drive a variety of chemical reactions in cells. ATP is synthesized in an energetically unfavorable phosphorylation reaction in which a phosphate group is added to ADP (adenosine diphosphate). When required, ATP gives up its energy packet through its energetically favorable hydrolysis to ADP and inorganic phosphate (Figure 2–33). The regenerated ADP is then available to be used for another round of the phosphorylation reaction that forms ATP.
The energetically favorable reaction of ATP hydrolysis is coupled to many otherwise unfavorable reactions through which other molecules are synthesized. Many of these coupled reactions involve the transfer of the terminal phosphate in ATP to another molecule, as illustrated by the phosphorylation reaction in Figure 2–34.
As the most abundant activated carrier in cells, ATP is the principle energy currency. To give just two examples, it supplies energy for many of the pumps that transport substances into and out of the cell (discussed in Chapter 11), and it powers the molecular motors that enable muscle cells to contract and nerve cells to transport materials from one end of their long axons to another (discussed in Chapter 16).
energy stored in aTp is often harnessed to Join Two molecules Together
We have previously discussed one way in which an energetically favorable reaction can be coupled to an energetically unfavorable reaction, X → Y, so as to enable it to occur. In that scheme, a second enzyme catalyzes the energetically favorable reaction Y → Z, pulling all of the X to Y in the process. But when the required product is Y and not Z, this mechanism is not useful.
Figure 2–33 The hydrolysis of aTP to aDP and inorganic phosphate. The two outermost phosphates in aTp are held to the rest of the molecule by high-energy phosphoanhydride bonds and are readily transferred. as indicated, water can be added to aTp to form aDp and inorganic phosphate (pi). hydrolysis of the terminal phosphate of aTp yields between 46 and 54 kJ/mole of usable energy, depending on the intracellular conditions. The large negative ΔG of this reaction arises from several factors: release of the terminal phosphate group removes an unfavorable repulsion between adjacent negative charges, and the inorganic phosphate ion (pi) released is stabilized by resonance and by favorable hydrogen-bond formation with water.
Figure 2–34 an example of a phosphate transfer reaction. Because an energy-rich phosphoanhydride bond in aTp is converted to a phosphoester bond, this reaction is energetically favorable, having a large negative ΔG. Reactions of this type are involved in the synthesis of phospholipids and in the initial steps of reactions that catabolize sugars.
A typical biosynthetic reaction is one in which two molecules, A and B, are joined together to produce A–B in the energetically unfavorable condensation reaction
There is an indirect pathway that allows A–H and B–OH to form A–B, in which a coupling to ATP hydrolysis makes the reaction go. Here, energy from ATP hydrolysis is first used to convert B–OH to a higher-energy intermediate compound, which then reacts directly with A–H to give A–B. The simplest possible mechanism involves the transfer of a phosphate from ATP to B–OH to make B–O–PO3, in which case the reaction pathway contains only two steps: 1.
2.
A–H + B–O–PO3 →A–B + Pi Net result: B–OH + ATP + A–H →A–B + ADP + Pi
The condensation reaction, which by itself is energetically unfavorable, is forced to occur by being directly coupled to ATP hydrolysis in an enzyme-catalyzed reaction pathway (Figure 2–35A).