Basic_thermodynamics

Basic_thermodynamics - MCDB 108B John Lew Basic Biochemical...

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MCDB 108B John Lew Basic Biochemical Thermodynamics Equilibrium Metabolism deals with the interconversion of macromolecules (intermediates) that are linked in a series, constituting a metabolic pathway. A ' B ' C ' D ' E …. . Because all intermediates (A, B, C, D, E, etc.) along the pathway are interconvertable with each other, it is possible for an equilibrium to be established. At equilibrium, the relative concentrations of all intermediates is always the same, regardless of whether their absolute concentrations change. Consider: A ' B A and B are said to be interconvertible, meaning that this reaction is reversible: A can be converted to B, and B can be converted to A. At equilibrium, A is converted to B at the same rate that B is converted to A. Thus, at equilibrium, there is no net change in the relative concentrations of A and B. Their ratio ([B]/[A]) is always constant, the value of which is called K eq . Simplistically, K eq is determined by the structures of the reactants and products. In the reaction A ' B, K eq is dimensionless; it has no units associated with it. The ratio of [B]/[A] is referred to as the mass action ratio (MAR). K eq is thus the MAR at equilibrium . Relationship Between Free Energy and K eq A reaction (such as A ' B) is said to be in its lowest [free] energy state at equilibrium. This is why no net change in the concentrations of A or B is observed at equilibrium. If a change in the concentration of A or B is apparent over time, by definition this means that energy is either being liberated (if the reaction is proceeding toward equilibrium - ie. the MAR is approaching K eq ) or that energy is being consumed by the reaction (if MAR is being displaced away from K eq ). The liberated or consumed energy corresponds to energy that is available, or “free”, to do chemical work and is termed the Gibbs free energy of reaction , Δ G. The units for Δ G are kcal/mol or kJ/mol. As a reaction approaches equilibrium from some starting initial MAR (MAR init ) the absolute value for Δ G continually decreases until equilibrium is reached, at which point Δ G = 0. Conventions Meaning of Δ G o A special situation is recognized for a reaction that is displaced away from equilibrium, such that the initial MAR is held at 1 M B/1 M A . This situation (in which the ratio of products/reactants equals 1 M/1 M, respectively) is referred to as standard 1
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MCDB 108B John Lew conditions . The free energy required to maintain standard conditions against the natural tendency to relax to equilibrium is called the change in standard free energy , and is given the designation Δ G o . In the hypothetical case in which K eq were to equal standard conditions (ie. K eq = 1), no energy would be required to “hold” the MAR at 1. In such a case, Δ G o = 0. Meaning of negative vs. positive
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Basic_thermodynamics - MCDB 108B John Lew Basic Biochemical...

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