The enzymes are highly specific in nature. During enzymatic reaction the enzyme is so built that it binds to the substrate in a specific manner. The stepwise mechanism of enzyme catalysed reaction is being described as follows:
The reactant molecule (usually called substrate) binds itself to the active site on the surface of enzyme. The active site in the given enzyme is so shaped that only a specific substrate can fit in it, just as a lick can be opened only with a specific key. The specific binding results in the formation of enzyme-substrate complex also referred to as activated complex.
In the complex, the substrate is placed in the proper orientation to facilitate the chemical reaction and enhancing its rate. The complex finally decomposes to give products and generates enzyme.