<P> A tighter fit between an active site and the substrate molecule is believed to increase the efficiency of a reaction. If the tightness between the active site of DNA polymerase and its substrate was increased, the fidelity, which means the correct rate of DNA replication is also increased . Most enzymes have deeply buried active sites, which can be accessed by a substrate via access channels . </P> <P> There are two proposed models of how enzymes fit their specific substrate: the lock and key model and the induced fit model . </P> <P> This concept was suggested by the 19th - century chemist Emil Fischer . He proposed that the active site and substrate are two stable structures that fit perfectly without any further modification . This is just like a key fits into a lock . If one substrate perfectly binds to its active site, the bonds formed between them will be strongest with highest catalytic efficiency . </P> <P> As time goes by its limitation started to appear . For example, an enzyme inhibitor, methylglucoside, can bind tightly to the active site and perfectly fits into it . However, there is no reaction between them and Lock and Key hypothesis cannot explain this . In addition, this theory cannot explain the mechanism of non-competitive inhibitors as they do not bind to the active site . </P>

When a reactant binds to the active site of an enzyme