Enzymes and ligand-binding proteins

3 Enzymes and ligand-binding proteins a. Chemical equilibria 

Enzymes are proteins having a catalytic function. Catalysts in general speed up reactions but remain unchanged by the reaction. Enzymes do not change the overall thermodynamics of a reaction (i.e. the free energy difference between the initial and equilibrium conditions) but speed up the reactions, that is, enzymes increase the rate at which equilibrium is achieved. 

At this point it is useful to specify G more precisely in a cell biological context, that is, in conditions of essentially constant temperature, volume and pressure. For a reaction at constant pressure the energy change (A j is the difference between heat produced (q) and work done by the system (w) (e.g. pressure X volume change (TAT) work)  

The directionality of a cellular reaction at constant pressure and at a particular temperature (T) (i.e. whether AG is positive or negative) is determined by both the enthalpy change (AH) and the change in disorder of the system described by the change in entropy (AS)  

An enzyme (E) will have an active site that can bind the reagent or substrate (S) to form an enzyme—substrate complex (E—S). The active site is highly specific for S (and structurally 

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