Topic 7 METABOLISM THERMODYNAMICS CHEMICAL EQUILIBRIA ENERGY COUPLING and CATALYSIS lectures 9 10 OBJECTIVES 1 Understand the concepts of kinetic vs potential energy 2 Understand the concepts of free energy and entropy use these concepts and thermodynamic principles to show whether a particular reaction is going be spontaneous or not 3 Be able to define equilibrium constant and how this relates to degree of spontaneity of a given reaction 4 Understand the process by which an endergonic reaction is coupled to a highly exergonic reaction and the role of ATP in biological systems 5 Understand the principle of mass action 6 Draw a free energy diagram to explain the concept of activation energy Ea and then show the impact of enzymatic catalysis on Ea 7 Understand the concepts of enzyme velocity maximal velocity Vmax and affinity as well as the factors substrate concentration pH temperature etc which impact the rate of enzyme catalyzed reactions Energy physico chemical term for the capacity to do work work moving a force over a distance units are in calorie or more commonly in Joule note force mass x acceleration There are two forms of energy 1 kinetic energy that is actively engaged in doing work 2 potential energy that is not actively engaged in doing work but has the potential to do so Energy transformation the process by which energy is converted from one form to another chemical energy into mechanical energy as would take place during muscle contraction chemical energy into covalent bonds as would take place during the biosynthesis of macromolecules Bioenergetics the study of energy conversion in biological systems Metabolism the sum total of all the chemical reactions taking place in an organism consists of a network of chemical reactions often called pathways Two general types of pathways 1 catabolic breakdown complex molecules into simpler molecules 2 anabolic form complex molecules from simpler molecules biosynthesis requires energy input Thermodynamics the study of energy transformations as applied to all physicochemical systems including biological 1 Consider the following model chemical reaction A B we ask the simple question what determines whether this reaction takes place spontaneously or not The principles of thermodynamics help us to understand this question First of all we need to define yet another termfree energy as applied to molecular reactions it is the energy available to do work often denoted by the symbol G for Gibb s free energy first law of thermodynamics energy transformations do not create nor destroy energy but simply result in the interconversion from one form to the other second law of thermodynamics all energy transformations result in an increase in disorder entropy is a term which is a measure of the extent of disorder in a system Thus the second law can be restated by saying that all energy transformations result in an increase in entropy in the system Now lets apply the above two laws to defining whether a reaction is spontaneous or not A B C D Gi Gf Si Sf where Gi free energy at initial state Gf free energy at final state and G Gf Gi and Si entropy at initial state Sf entropy at final state and S Sf Si Thus when G negative value reaction is spontaneous it is said to be exergonic spontaneous reactions lead to a decrease in free energy S positive value reaction is spontaneous spontaneous reactions lead to an increase in entropy Exergonic reactions lead to a decrease in free energy and an increase in entropy Endergonic reactions are not spontaneous movement in this direction would lead to an increase in free energy and a decrease in entropy A good example is biosynthesis of large molecules We ll see in a few minutes how it is possible to drive endergonic reactions by coupling them with exergonic reactions Fig 6 5 relationship of free energy to stability work capacity and spontaneous change Fig 6 6 exergonic vs endergonic reactions Chemical equilibria 2 Suppose you mix A and B together they will react to form accumulate C D will begin to react to form A B Eventually C and D which will A B C D reaction rate C D A B reaction rate at this point we can say that the reaction has reached chemical equilibrium Each kind of reaction has its own unique chemical equilibria which can be defined by the equilibrium constant Keq Keq product of concentrations of products at equilibrium product of concentrations of reactants at equilibrium in our example above Keq C x D A x B Equilibria and spontaneity 1 2 reactions which have Keq 1 are highly exergonic reactions which have Keq 1 are highly endergonic However you can make an endergonic reaction go in a non spontaneous direction by coupling it with an exergonic reaction Energy coupling the use of an exergonic process to drive an endergonic process suppose A B G A B positive value X Z G X Z negative value Gnet G proceed A B G X Z if Gnet is negative then the A B reaction will Energy coupling is extremely common in biological systems By far the most common coupling reaction is the reaction which involves the hydrolysis of a compound known as ATP adenosine triphosphate ATP ADP inorganic phosphate Pi fig 6 8 ATP is very unstable and is spontaneously hydrolyzed by water this reaction however can be coupled to another reaction as shown in fig 6 9 glutamine formation ATP is often referred to as the energy currency of cells it is constantly being utilized to drive endergonic processes In addition ATP is unstable If you were to add ATP to a beaker of water it would spontaneously hydrolyze so that at chemical equilibrium 99 99 of the ATP would have been hydrolyzed to ADP and Pi In cells the concentration of ATP is 100 times greater than ADP Thus cells keep the ATP hydrolysis reaction far displaced from chemical equilibrium This is accomplished 3 by a process known as cellular energy metabolism energy metabolism catabolism of organic molecules yielding ATP and other useful forms of chemical energy Fig 6 10 the ATP hydrolysis regeneration cycle in cells Rates of reactions For a chemical reaction like A B the rate of the reaction is a function of the concentrations of reactants and products Thus the principle of chemical mass action tells us that we can increase the rate of A B by increasing A decreasing B or both However biological systems have evolved enzymes which function as catalysts to speed up chemical reactions enzyme catalytic protein catalyst an agent which accelerates a chemical reaction without being consumed Energy
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