SPECIFICITY OF AN ENZYME:

The same binding energy that provides energy for catalysis also gives an enzyme its specificity. It is the ability to discriminate between a substrate and a competing molecule. Specificity is easy to distinguish from catalysis, but this distinction is much more difficult to make experimentally, because catalysis and specificity arise from the same phenomenon. If an enzyme active site has functional groups arranged optimally to form a variety of weak interaction with a particular S in the transition state, the enzyme will not be able to interact to the same degree with any other molecule. In general, specificity is derived from the formation of many weak interaction between the E and its specific S molecule.

Consider what needs to occur for a reaction to take place.

• Prominent physical and thermodynamic factors contributing to change in G++, the barrier to reaction, may include:
• the entropy (freedom of motion) of molecules in solution, which reduces the possibility that they will react together.
• the solvation shell of hydrogen- bonded water that surrounds and helps to stabilize most biomolecules in aqueous solution.
• the distortion of substrates that must occur in many reactions.
• the need for proper alignment of catalytic functional groups on the enzyme. 

Binding energy can be used to overcome all these barriers:

1. First, a large restriction in the relative motions of two substrates that are to react, or entropy reduction is one obvious benefit of binding them to an enzyme. B.E. holds the substrates in the proper orientation to react- a substantial contribution to catalysis, because productive collisions between molecules in solution can be exceedingly rare. S can be precisely aligned on the enzymes, with many weak interaction between each substrate and strategically located groups on the enzyme clamping the substrate molecule into the proper positions. Studies have shown that constraining the motion of two reactants can produce rate enhancements of many orders of magnitude. 
2. Second, formation of weak bonds between S and E results in desolvation of the S. E-S interaction replace most or all of the H-bonds between the S and water.
3. Third, binding energy involving weak interaction formed only in the reaction transition state helps to compensate thermodynamically for any distortion, primarily electron redistribution, that the S must undergo to react.
4. Finally, the enzyme itself undergoes a change in conformation when the S binds induced by multiple weak interaction with the S. This is referred to as induced fit , a mechanism postulated by Daniel Koshland in 1958. The motions can affect a small part of the enzyme near the active site, or can involve changes in the  positioning of entire domains. Typically, a network of coupled motions occurs throughout the enzyme that ultimately brings about the required changes in the active site. Induced fit serves to bring specific functional groups on the enzyme into the proper position to catalyze the reaction. The conformational change also permits formation of additional weak bonding interaction in the transition state. In either case, the new enzyme conformation has enhanced catalytic properties. Induced fit is also important in the interaction of almost every enzyme with its substrate.