Directions and Rates of Biochemical Processes

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 8.2 Transformations between kinetic and potential energy On the platform, a diver has more potential energy. Diving converts potential energy to kinetic energy. Climbing up converts kinetic energy of muscle movement to potential energy. In the water, a diver has less potential energy.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 8.3 The two laws of thermodynamics (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. (b) Chemical energy Heat co 2 H2OH2O +

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 8.4 Order as a characteristic of life 50 µm

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 8.5 The relationship of free energy to stability, work capacity, and spontaneous change. Chemical reaction. In a cell, a sugar molecule is broken down into simpler molecules. Diffusion. Molecules in a drop of dye diffuse until they are randomly dispersed. Gravitational motion. Objects move spontaneously from a higher altitude to a lower one. More free energy (higher G) Less stable Greater work capacity Less free energy (lower G) More stable Less 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 (a) (b) (c)  G =  H-T  S

∆G = G products – G reactants Exergonic ∆G < 0 spontaneous rx. Endergonic ∆G > 0

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings (a) A closed hydroelectric system ∆G < 0∆G = 0 (b) An open hydroelectric system ∆G < 0 A multistep open hydroelectric system (c) ∆G < 0 Figure 8.7 Equilibrium and work in closed and open systems Metabolism as a whole is never at equilibrium!

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.

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 becomes heat which is largely wasted energy.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 8.14 Energy profile of an exergonic reaction A C D A A B B B C C D D Transition state Products Progress of the reaction ∆G < O Reactants Free energy EAEA The reactants AB and CD must absorb enough energy from the surroundings to reach the unstable transition state, where bonds can break. Bonds break and new bonds form, releasing energy to the surroundings.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 8.15 The effect of enzymes on reaction rate. Progress of the reaction Products Course of reaction without enzyme Reactants Course of reaction with enzyme EAEA without enzyme E A with enzyme is lower ∆G is unaffected by enzyme Free energy

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Substrate Active site Enzyme (a) (b) Enzyme- substrate complex Figure 8.16 Induced fit between an enzyme and its substrate

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 8.17 The active site and catalytic cycle of an enzyme 1 Substrates enter active site; enzyme changes shape so its active site embraces the substrates (induced fit). Substrates Products Enzyme Enzyme-substrate complex 5 Products are Released. 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 E A 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. 4 Substrates are Converted into Products. 6 Active site is available for two new substrate molecules.