The rate of biochemical reactions catalyzed by enzymes increases at higher temperature. Higher temperature means that the kinetic energy of motion of the molecules that make up cells and their biochemical contents has increased. Thus, temperature is a measure of the average kinetic energy of the molecules.
Reactions occur faster at higher temperatures, because the energy needed to break bonds, the initial step in a chemical reaction, is provided by the energetic collisions of the reacting molecules. Higher temperatures mean more productive collisions to convert reactants into products.
Activation Energy Is the Thermal Energy Needed for Reactants to Transition
Humans do not, however alter body temperature to control the reactions of metabolism. Biochemical reactions to modify bonds within small molecules in cellular biochemical pathways are controlled by enzymes. Enzymes exploit thermal energy to enhance the rate of specific reactions.
An enzyme is a protein that provides a chemically active surface that selectively accelerates a particular reaction by lowering the amount of thermal energy needed for the colliding reacting molecules, reactants, to change their configurations and bonds to produce alternative molecules, products.
The initial, energetic collision of reactants on the enzyme surface leads to a transition state for the reactants and the amount of thermal energy required to reach the transition state is called the activation energy. From the transition state, either products or a return to reactants can result. There is no predetermined direction to products or reactants from the transition state. (Enzymes do not use ATP as a source of chemical energy to supply the activation energy.)
Enzyme Catalysis Results from Lower Activation Energy
Collisions among the numerous, different biochemical molecules in a cell can result in thousands of different reactions, but the presence of enzymes reduces the actual number of reactions that take place, because less kinetic energy is required to reach the reaction transition state on an enzyme surface. The enzyme catalyzed reaction has a lower activation energy than the same type of reaction in the absence of the enzyme and so cellular biochemicals are channeled through established reactions catalyzed by the available enzymes.
Equilibrium Means Products and Reactants Reach Transition Equally
If reactants and products are not removed by other reactions, the number of reactant molecules reaching the transition state on the enzyme molecules will be equal to the number or products colliding with enzyme and also reaching the transition state. When the ratio of reactant to product does not change over time, equilibrium has been reached.
Reactants or Products Will Predominate at Equilibrium Depending on Activation Energies
The equilibrium ratio reflects the difference in chemical energy levels of the reactants and products. Thus, if the product molecules have a lower chemical energy, then at equilibrium the product molecules will predominate. The amount of thermal energy needed to reach the transition state from reactant and from product may be different. If the product has a lower chemical energy, then the activation energy for products to reach the transition state will be greater.
Enzyme Increases Reaction Rate but not Equilibrium
Enzymes lower the activation energy by the same amount for both reactants and products reaching the transition state. Thus, an enzyme increases reaction rates and equilibrium is approached more quickly, but the same ratio of reactants to products occurs at equilibrium with or without the enzyme.
Enzyme Surface Lowers Activation Energy and Increases Reaction Rate
The unique surface provided by the folded amino acids of an enzyme surface facilitates the formation of the transition state for reactant or product molecules that collide with the enzyme. The enzyme lowers the activation energy of the reaction and increases the reaction rate to channel cellular biochemicals through enzyme-determined pathways.
References:
Alberts, B. et al. 2008. Molecular Biology of the Cell, 5th ed., Garland Science.
Campbell, N.E, et al. 2007. Biology, 8th ed., Benjamin Cummings.
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