Gibbs free energy and equilibrium constant

Gibbs Free Energy, G Is the thermodynamic function that is most useful for biochemistry. G is a function of Enthalpy, H, a measure of the energy (heat content) of the system at constant pressure, and Enthalpy, H, a measure of the energy (heat content) of the system at constant pressure, and Entropy, S, a measure of the randomness (disorder) of the system. Entropy, S, a measure of the randomness (disorder) of the system.

The change in Gibbs free energy) ΔG) for a reaction quantitatively measure the energy available to do useful work. It is related to the change in enthalpy and the change in entropy: Δ G = Δ H - T Δ S Δ G = Δ H - T Δ S

The actual free energy change) ΔG) depends on 2 parameters: the standard free energy change for that reaction) ΔG°) (and thus to Keq, defining where equilibrium for this reaction lies), and the actual mass action ratio, reflecting the actual starting conditions, the actual concentrations of reactants and products actual ΔG = ΔG° + RTln {actual mass action ratio}

A + B C + D All reactions/processes proceed in direction required to go TOWARD EQUILIBRIUM. For this reaction, the mass action ratio is given by: [C] [D]/ [A] [B] For this reaction, the mass action ratio is given by: [C] [D]/ [A] [B] The mass action ratio at equilibrium is the equilibrium constant for the reaction Keq =[ C] [D] / [A] [B] Keq =[ C] [D] / [A] [B]

The standard free energy change for a reaction )ΔG°) is the change in free energy under STANDARD CONDITIONS. It is related to the equilibrium constant by the equation: ΔG°- = RT ln Keq Where R : is the natural gas constant equal joules or Cal T :is the absolute temperature ln : natural logarithm. T :is the absolute temperature ln : natural logarithm. Keq : equilibrium constant Keq : equilibrium constant

At equilibrium ΔG = 0 At equilibrium ΔG = 0 and K eq = [C] [D]/ [A] [B] and K eq = [C] [D]/ [A] [B] hence hence 0 = ΔG° + RT ln Keq 0 = ΔG° + RT ln Keq ΔG °= -RT ln Keq ΔG °= -RT ln Keq

Continuation of life requires continuous chemical reaction Reactions that reach equilibrium have stopped, can't get out of that state without external change. Consumption of food provides a continued supply of substrates for reactions yielding net negative ΔG. Net negative ΔG ensures that reactions proceed in the required direction for continuation of life Consumption of food provides a continued supply of substrates for reactions yielding net negative ΔG. Net negative ΔG ensures that reactions proceed in the required direction for continuation of life. Energy made available from breakdown of some compounds, e.g. sugars, fats, amino acids, can be used to drive the synthesis of other molecules, e.g. structural components of cells, or compounds such as polysaccharides that store energy for future needs. Energy made available from breakdown of some compounds, e.g. sugars, fats, amino acids, can be used to drive the synthesis of other molecules, e.g. structural components of cells, or compounds such as polysaccharides that store energy for future needs.

Free energy change is dependent on concentration Δ G Actual free energy change under specified conditions, including concentration of reactants and products. Δ G Actual free energy change under specified conditions, including concentration of reactants and products. Δ G ° Standard Free energy change, all reactants and products in their standard states, i.e. 1 mol / L concentration.Unfortunately, if H + concentration is 1 mol / L, pH = 0, which is not consistent with biochemical processes. Standard Free Energy change Δ G°´ for the biochemical standard state, all reactants and products at 1 mol / L except [H + ] = [OH - ] = mol / L, which allows pH = 7.

phosphorylation of glucose to produce glucose-6-phosphate Very important reaction in the cell. first reaction in metabolism of glucose that enters a cell from the blood. Reaction 1: condensation of glucose (alcohol) with inorganic phosphate ion (acid) to make glucose-6-phosphate (an ester) Reaction 1: condensation of glucose (alcohol) with inorganic phosphate ion (acid) to make glucose-6-phosphate (an ester) Glucose + Pi glucose-6-phosphate +H 2 O Glucose + Pi glucose-6-phosphate +H 2 O ΔG°= KJ/mole. Endergonic reaction ΔG°= KJ/mole. Endergonic reaction

Reaction 2: hydrolysis of ATP, a phosphoanhydride, to generate ADP and inorganic phosphate. ATP +H 2 O ADP + Pi ATP +H 2 O ADP + Pi ΔG°= Kj/mole. Exergonic reaction To couple the 2 reactions (which requires some chemical mechanism, of course), add reactants on left, add products on right, and add ΔG o ' values to get Δ G o ' for coupled reaction: Glucose + ATP glucose-6-phosphate + ADP Δ G o - = 16.7 = kJ/mol Glucose + ATP glucose-6-phosphate + ADP Δ G o - = 16.7 = kJ/mol

The coupled reaction is exergonic, it will go spontaneously (forward, left to right) in the cell, but will it proceed at a rate consistent with cellular needs؟ Most biological reactions would proceed at a very slow rate indeed if they're not catalyzed. The biological catalyst enabling the coupled reaction above to proceed on a biological timescale is an enzyme, hexokinase. Free energy coupling, with enzymes as catalysts, is the strategy used in metabolic pathways.