Structural Biochemistry, Enzyme, Equilibrium constant

Equilibrium Constant

The equilibrium constant, K, reflects the ratio of the activities of the products versus the reactants, and represents the extent to which a reaction goes to completion.

In general, a reaction with a

• LARGE K, K > 103, is said to favor the products and thus forms more products than reactants
• SMALL K, K < 10-3, favors reactants and thus very little product is formed. Intermediate values of K, form similar amounts of both products and reactants.

The equilibrium constant is a thermodynamic property and thus the size of K does not indicate anything about the rate at which products form, only the amount. The equilibrium constant for the reaction:

aA + bB <--> cC + dD

can be calculated by,

K = aCcaDd/(aAaaBb)

where aI represents the activity of species I. The activities of the reaction species are raised to their corresponding stoichiometric coefficient.

The equilibrium constant can be very closely approximated by using concentrations for aqueous solutions and pressures for gasses, in place of activities. Solids and pure liquids are not factored into K and are given a value of 1.

The equilibrium constant is related to the standard gibbs free energy, delta G, by

delta G0 = -RTlnK

Kd is defined as dissociation constant that accounts for amount of reactant that dissociates reversibly to form component products; the constant deals with half of binding site of enzyme that binds for concentration of ligands, or the concentration for ligands that bind enzyme to be equal to that that are not; the unit of the constant is in molarity or M; it is a measurement that is indirectly proportional to affinity constant, for example, between enzyme and ligand; or the smaller dissociation constant means higher affinity constant as the stronger the bond between the enzyme and ligand, which is more difficult to be separated to two components.

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