HOW ENZYMES WORK?

• Active site : The enzyme- catalyzed reaction takes place within the confines of a pocket on the enzyme called the active site.
• Substrate:  The molecule that is bound in the active site and acted upon by the enzyme is called the substrate.
• The surface of the active site is lined with amino acid residues with substituent groups that bind the substrate and catalyzed its chemical transformation.
• The Enzyme-Substrate complex (ES), was first proposed by Charles- Adolphe Wurtz in 1880.
• A simple enzymatic reaction might be written : E + S = ES = EP = E + P

Catalysts do not affect reaction equilibrium. The function of a catalyst is to increase the rate of a reaction. The starting point for either the forward or the reverse reaction is called the ground state. 

• The equilibrium between S and P reflects the difference in the free energies of their ground states. The free energy of the ground state of P is lower than that of S, so change in G^10 (biochemical standard free- energy change, pH=7, conc. of solute is low, mM, nM) for the reaction is negative and the equilibrium favors P. The position and direction of equilibrium are not affected by any catalyst.
• A favorable equilibrium does not mean that the S = P conversion will occur at a detectable rate.
• There is an energy barrier between S and P, the energy required for alignment of reacting groups, formation of transient unstable charges, bond rearrangements and other transformations required for the reaction to proceed in either direction.
• To undergo reaction, the molecules must overcome the barrier and therefore must be raised to a higher energy level. At the top of the energy hill is a point at which decay to the S or P state is equally probable. This is called the transition state.  It is not a chemical species with any significant stability and should not be confused with a reaction intermediate (EP or ES). It is simply a fleeting molecular moment.
• Activation Energy, G++ = The difference between the energy levels of the ground state and the transition state. The rate of a reaction reflects this activation energy: higher A.E corresponds to slower reaction.
• Reaction rates can be increased by raising the temperature, pressure thereby increasing the no. of molecules with sufficient energy to overcome the energy barrier.
• The activation energy can be lowered by adding a catalyst. Catalysts enhance reaction rates by lowering A.E.
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• Any reaction may have several steps, involving the formation and decay of transient chemical species called reaction intermediates. A reaction intermediate is any species on the reaction pathway that has a finite chemical lifetime. The ES and EP complexes can be considered intermediates.
• When several steps occur in a reaction, the overall rate is determined by the step with a highest activation energy; this is called the rate limiting step which is the slowest step.
• In practice, the rate limiting step can vary with reaction conditions, and for many enzymes several steps may have similar A.E, which means they are all partially rate- limiting.