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Lecture 6

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Biology 1002B
Tom Haffie

Enzymes 4445 What Enzymes Do And Do Not DoThe laws of thermodynamics tell us if a process is spontaneous or not They do not tell us the speed of the reaction o A reaction can be spontaneous yet very very slowSpeed up a spontaneous reactionCannot provide energy therefore can not induce an endergonic reaction Although enzymes lower the activation energy of a reaction they do not alter the G of the reaction The free energy of the reactants and products is the same the only difference is the path the reaction takes Enzymes DO speed up the rate of spontaneous reactions by lowering activation energy Enzymes DO NOT supply free energy to a reaction Therefore enzymes CANNOT make an endergonic reaction proceed spontaneously Exergonic ReactionG is negative EActivation Energy Ao Chemical reactions require bonds to break and new bonds to be formed For bonds to be broken they must first be strained made less stable so that bond breakage can occur To get reacting molecules into a more unstable state requires a small input of energy Even if the reaction is spontaneous the reaction will not actually start until a small boost of energy is added This initial energy is called the activation energy and also explains why spontaneous reactions are not rapid o Activation energy is basically the amount of energy needed to get to transition state o Activation energy represents a kinetic barrier kinetic means speed as it is a barrier that prevents the reaction from going fasto Where do reactions get activation energy from Molecules taking part in chemical reactions are in constant motion at temperatures above zero and periodically reacting molecules may gain enough energy to reach transition state If a significant number of reactant molecules reach the transition state then the free energy that is released can be enough to get the remaining reactants to the transition state This is rare under normal conditions o Chemists usually use heat to provide the energy needed for reactant molecules to get to the transition state and thus speed up the rate of reaction This is problematic in biology High temperatures destroy the structural components of cells particularly proteins and can result in cell
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