Directions and Rates of Biochemical Processes

1. Directions and Rates of Biochemical Processes

2. On the platform, a diver has more potential energy. Diving converts potential energy to kinetic energy. In the water, a diver has less potential energy. Climbing up converts kinetic energy of muscle movement to potential energy. Figure 8.2 Transformations between kinetic and potential energy

3. Heat co2 + Chemical energy H2O (b) (a) First law of thermodynamics: Energy can be transferred or transformed but neither created nor destroyed. For example, the chemical (potential) energy in food will be converted to the kinetic energy of the cheetah’s movement in (b). Second law of thermodynamics: Every energy transfer or transformation increases the disorder (entropy) of the universe. For example, disorder is added to the cheetah’s surroundings in the form of heat and the small molecules that are the by-products of metabolism. Figure 8.3 The two laws of thermodynamics

5. 50 µm Figure 8.4 Order as a characteristic of life

6. More free energy (higher G) • Less stable • Greater work capacity • In a spontaneously change • The free energy of the system decreases (∆G<0) • The system becomes more stable • The released free energy can • be harnessed to do work • Less free energy (lower G) • More stable • Less work capacity (c) Gravitational motion. Objects move spontaneously from a higher altitude to a lower one. Diffusion. Molecules in a drop of dye diffuse until they are randomly dispersed. Chemical reaction. In a cell, a sugar molecule is broken down into simpler molecules. (b) (a) Figure 8.5 The relationship of free energy to stability, work capacity, and spontaneous change G = H-TS .

7. ∆G = Gproducts – Greactants Exergonic∆G < 0 spontaneous rx. Endergonic ∆G > 0

9. (b) An open hydroelectric system ∆G < 0 A multistepopen hydroelectric system (c) Figure 8.7 Equilibrium and work in closed and open systems ∆G < 0 ∆G = 0 (a) A closed hydroelectric system Metabolism as a whole is never at equilibrium! ∆G < 0 ∆G < 0 ∆G < 0

10. Directions and Rates of Biochemical Processes • How Does Thermodynamics Predict the Direction of a Reaction? • The First Law of Thermodynamics • The total amount of energy in any process stays constant. Energy cannot be created or destroyed, only converted from one form to another. • So, energy may switch from potential to kinetic and back again, but it is neither created nor destroyed. • For example, the potential energy stored in the chemical bonds of ATP is converted into kinetic energy when it is split to allow a muscle contraction to occur.

11. Directions and Rates of Biochemical Processes • How Does Thermodynamics Predict the Direction of a Reaction? • The Second Law of Thermodynamics • In any process, the energy available to do work decreases. • For example, when ATP is split to allow a muscle contraction, only a fraction of the energy from ATP is converted into useful work.The rest of the energy becomesheat which is largely wasted energy.

20. Bonds break and new bonds form, releasing energy to the surroundings. The reactants AB and CD must absorb enough energy from the surroundings to reach the unstable transition state, where bonds can break. A B D C Transition state B A EA D C Free energy Reactants B A ∆G < O C D Products Progress of the reaction Figure 8.14 Energy profile of an exergonic reaction

21. Course of reaction without enzyme EA without enzyme EA with enzyme is lower Reactants Free energy ∆G is unaffected by enzyme Course of reaction with enzyme Products Progress of the reaction Figure 8.15 The effect of enzymes on reaction rate.

22. Substrate Active site Enzyme- substrate complex Enzyme (b) (a) Figure 8.16 Induced fit between an enzyme and its substrate

23. 1 Substrates enter active site; enzyme changes shape so its active site embraces the substrates (induced fit). 2 Substrates held in active site by weak interactions, such as hydrogen bonds and ionic bonds. 3 Active site (and R groups of its amino acids) can lower EA and speed up a reaction by • acting as a template for substrate orientation, • stressing the substrates and stabilizing the transition state, • providing a favorable microenvironment, • participating directly in the catalytic reaction. Substrates Enzyme-substrate complex 6 Active site is available for two new substrate molecules. Enzyme 5 Products are Released. 4 Substrates are Converted into Products. Products Figure 8.17 The active site and catalytic cycle of an enzyme