Free Energy, ATP and Energy Coupling Chapter 6 Free Energy, ATP and Energy Coupling

Free-Energy Change (G), Stability, and Equilibrium A living system’s free energy (G) is energy that can do work when temperature and pressure are uniform, as in a living cell. The free-energy change (G) of a reaction tells us whether or not the reaction occurs spontaneously. © 2014 Pearson Education, Inc. 2

(a) Gravitational motion (b) Diffusion (c) Chemical reaction Figure 6.5b The change in free energy (∆G) during a chemical reaction is the difference between the free energy of the final state and the free energy of the initial state. ∆G = Gfinal state – Ginitial state Only processes with a negative ∆G are spontaneous. Spontaneous processes can be harnessed to perform work. (a) Gravitational motion (b) Diffusion (c) Chemical reaction 3

Exergonic and Endergonic Reactions in Metabolism An exergonic reaction proceeds with a net release of free energy and is spontaneous; ∆G is negative. © 2014 Pearson Education, Inc. 4

Amount of energy released (G  0) Figure 6.6a (a) Exergonic reaction: energy released, spontaneous Reactants Amount of energy released (G  0) Energy Free energy Products Figure 6.6a Free energy changes (ΔG) in exergonic and endergonic reactions (part 1: exergonic) Progress of the reaction 5

An endergonic reaction absorbs free energy from its surroundings and is nonspontaneous; ∆G is positive. © 2014 Pearson Education, Inc. 6

Amount of energy required (G  0) Figure 6.6b (b) Endergonic reaction: energy required, nonspontaneous Products Amount of energy required (G  0) Energy Free energy Reactants Figure 6.6b Free energy changes (ΔG) in exergonic and endergonic reactions (part 2: endergonic) Progress of the reaction 7

Equilibrium = Death G  0 G  0 Reactions in a closed system eventually reach equilibrium and then do no work. Cells are not in equilibrium; they are open systems experiencing a constant flow of materials. A defining feature of life is that metabolism is never at equilibrium. 8

Food, or some other energy source like the sun. Exergonic To do work, cells manage energy resources by energy coupling, the use of an exergonic process to drive an endergonic one. Most energy coupling in cells is mediated by ATP. Endergonic 9

(a) The structure of ATP Figure 6.8a Adenine Phosphate groups Ribose ATP (adenosine triphosphate) is composed of ribose (a sugar), adenine (a nitrogenous base), and three phosphate groups. (a) The structure of ATP 11

Adenosine triphosphate (ATP) Figure 6.8b Adenosine triphosphate (ATP) The bonds between the phosphate groups of ATP can be broken by hydrolysis. Energy is released from ATP when the terminal phosphate bond is broken. This release of energy comes from the chemical change to a state of lower free energy, not from the phosphate bonds themselves. Energy Inorganic phosphate Adenosine diphosphate (ADP) (b) The hydrolysis of ATP 12

How the Hydrolysis of ATP Performs Work The three types of cellular work (mechanical, transport, and chemical) are powered by the hydrolysis of ATP. In the cell, the energy from the exergonic reaction of ATP hydrolysis can be used to drive an endergonic reaction. Overall, the coupled reactions are exergonic. © 2014 Pearson Education, Inc. 13

Will this reaction happen spontaneously? Glutamic acid GGlu  3.4 kcal/mol Glutamine Ammonia Will this reaction happen spontaneously? No Is this reaction catabolic or anabolic? Anabolic Is this reaction exergonic or endergonic? Endergonic

Phosphorylated intermediate Phosphorylated intermediate Figure 6.9b Glutamic acid Phosphorylated intermediate ATP drives endergonic reactions by phosphorylation, transferring a phosphate group to some other molecule, such as a reactant. The recipient molecule is now called a phosphorylated intermediate. ATP hydrolysis leads to a change in a protein’s shape and often its ability to bind to another molecule. Phosphorylated intermediate Glutamine (b) Conversion reaction coupled with ATP hydrolysis 15

(c) Free-energy change for coupled reaction G  −3.9 kcal/mol Net Figure 6.9c GGlu  3.4 kcal/mol GGlu  3.4 kcal/mol GATP  −7.3 kcal/mol GATP  −7.3 kcal/mol  Figure 6.9c How ATP drives chemical work: energy coupling using ATP hydrolysis (part 3: coupled free energy) (c) Free-energy change for coupled reaction G  −3.9 kcal/mol Net 16

Protein and vesicle moved Figure 6.10 Transport protein Solute Solute transported (a) Transport work: ATP phosphorylates transport proteins. Vesicle Cytoskeletal track Figure 6.10 How ATP drives transport and mechanical work Motor protein Protein and vesicle moved (b) Mechanical work: ATP binds noncovalently to motor proteins and then is hydrolyzed. 17

Energy from catabolism (exergonic, energy- releasing processes) Figure 6.11 Energy from catabolism (exergonic, energy- releasing processes) Energy for cellular work (endergonic, energy-consuming processes) Figure 6.11 The ATP cycle 18