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l5_bioche_deltag

l5_bioche_deltag - MIT Department of Biology 7.014...

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7.014 Handout Biochemistry III & IV MIT Department of Biology 7.014 Introductory Biology, Spring 2005
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G and K eq For the chemical equilibrium: products reactants [products] The equilibrium constant, K eq is defined as: K = at equilibrium. eq [reactants] Where [] symbolizes concentration, for example, [products] indicates concentration of products. The equation that relates the free energy of the reaction, Δ G, to the standard free energy, Δ G o ' and the concentrations of reactants and products is: Δ G = Δ G o ' + RT ln [products] Equation (1) [reactants] where: R= the universal gas constant T= the temperature in o K kcal kcal i f T = 25 ˚C RT = 0.59 if T = 37 ˚C RT = 0.61 mol mol Under Standard Conditions where: [products] = 1 M and [reactants] = 1 M (M = moles/liter) (Equation 1) becomes: Δ G = Δ G o ' therefore, the Δ G o ' of the reaction is the free energy change ( Δ G) under standard conditions. At equilibrium , Δ G = 0, and equation (1) becomes: Δ G o ' - RT Δ G o ' = − RT ln [products] = RT ln K eq ) or: K = e ( eq [reactants] Significance of K eq : K eq > 1 Δ G o ' has negative value, therefore reaction is possible. K eq < 1 Δ G o ' has positive value, therefore reaction is possible if [react] > [prod]. K eq >> 1 Δ G o ' has large negative value, therefore reaction is irreversible. K eq << 1 Δ G o ' has large positive value, therefore reaction cannot occur. 2
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Enzymes (This handout is designed to supplement the coverage of enzymes in Purves et al. ) A catalyst is a molecule that increases the rate of a reaction but is not the substrate or product of that reaction. Enzymes are usually proteins that catalyze chemical reactions in cells (the exception is catalytic RNA). A substrate is a molecule upon which an enzyme acts to yield a product. The part of the enzyme that binds substrate is called the active site . Consider the reaction: A (substrate) ------> B (product) A graph of the free energy against the reaction progress is shown below: E Energy of activation Δ G substrate (A) product (B) a Reaction Progress Δ G is negative overall for forward reaction. The free energy of this reaction is not changed
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