Selectivity of enzyme

Major advantage of enzymes is their selectivity. Enzymes are characterized by their ability to catalyze biochemical reactions with high selectivity (essentially 100 %). Most enzymes are only active for single reaction i.e. stereo chemical specificity of enzymes is absolute. Three types of selectivity are exerted by enzymes :

1. Chemoselectivity
2. Regioselectivity
3. Stereo selectivity
1. Chemoselectivity

   Enzyme can catalyze transformation of one type of functional group in the presence of other sensitive groups in the substrate molecule. This reduces undesired reactions to great extent. The lipase of Burkholderia cepacia catalyzes acylation of 1-phenylethanol hundred times faster than of the corresponding amine using methyl butyrate as acyl donor.

   

   As shown in the figure, in a transacylation reaction Burkholderia cepacia lipase ( Bcl) prefer the alcohol over the amine as acyl acceptor by a factor of 100.

2. Regioselectivity

   Enzymatic catalytic center has a complicated 3D structure that can distinguish between two or more identical functional groups located in different sites of the substrate molecule. As a result only one group participates in the reaction resulting in selective products.

   Example :

   During deacylation of polyacylated sugar, high yield (80-90 %) of the product as shown in following reaction is obtained. The other minor derivatives are products of deacylation at other positions.

   

3. Stereo selectivity
   
   

   Stereoisomers are defined as isomers of a substrate whose chemical compositions are same but their structure differ such that their mirror images are non-superimposable. Each one of the two stereoisomers is known as optical isomer or enantiomer. Production of only one enantiomer with 100 % selectivity is important as activity depends on stereochemistry. For the same substrate, one stereoisomer may be active while the other stereoisomer may be inactive or active for an undesirable reaction. This enantioselectivity is the most important feature of enzymes. Enzymes are capable of recognizing any type of chirality of substrate and synthesizing one particular enatiomers. There are three approaches to synthesis of enantiomerically enriched compounds using enzymes.

1. Kinetic resolution
2. Desymmetrization
3. Deracemization