Active site

The active site of an enzyme is the region which connects the substrate and converts it into product. It is commonly a relatively small part of the overall enzyme molecule and is a 3-dimensional entity formed through amino acid residues which can lie far apart in the linear polypeptide chain. The active  site is frequent  a cleft  or  crevice  on  the  surface  of  the  enzyme  which  forms  a  predominantly nonpolar   environment   that   enhances   the   binding   of  the   substrate.   Substrates  is bound  in the  active  site  through several  weak  forces  like van der Waals  bonds,  hydrogen  bonds, electrostatic interactions,    hydrophobic  interactions and in some cases through reversible covalent bonds. Through Having bound the substrate molecule and formed enzyme–substrate complex catalytically active residues within the active site of the enzyme act on the substrate molecule to transform it first into the transition state complex and then into product, that is released into solution. Enzyme is now free to bind another molecule of substrate and starts its catalytic cycle again.

Initially two models were proposed to elaborate how an enzyme binds its substrate.  In  the  lock, key  model  proposed  through  Emil  Fischer  in  year 1894,  the shape  of  the  substrate  and  the  active  site  of  the  enzyme  are  thought  to  fit together like a key into its lock in Figure. The two shapes are considered as rigid and fixed and rightly complement each other when brought together in the

             Figure:  Binding of a substrate to an enzyme. (a) Lock-and-key  model; (b) induced-fit model.

right alignment.   In  the  induced-fit   model  proposed   in the year of 1958  through  Daniel  E. Koshland,  Jr., the binding of substrate  induces a conformational  change in the active  site  of  the  enzyme which is described in figure.  Additionally, the enzyme may distort the substrate forcing it into a conformation similar to that of the transition state. For instance, the binding of glucose to hexokinase induces a conformational modify in the structure of the enzyme like that the active site supposes a shape which is complementary to the substrate (glucose) only after it has bound to the enzyme.  The reality is which different enzymes show features of both models with some conformational and some change complementarity.