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  Theories of Active Site

Theories of Active Site:

In the year 1894, Fischer planned that the substrate fits into the active site of the enzyme like a key fits into the lock that is illustrated in the below diagram. Due to this model, the theory is termed as lock and key theory of enzyme action.

157_theory of fischer.jpg


  

                                                        Diagram: Lock and Key Theory of Fischer

As per to lock and key theory, there are exact functional groups and structural features in the enzyme in which substrate molecule have to be fit. The area of the enzyme that complexes with the substrate are known as active site/catalytic site.  The theory cannot be get applied for all the enzymatic reactions since in some reactions the substrate molecules and the active site are not structurally identical to fit in with each other. Furthermore, in specific cases the catalytic activity is seen although a fit is not possible.

After that, lock and key theory was changed by Koshland in the year 1963 in the form of ‘induced fit mechanism’. The necessary feature of this theory is the flexibility of the enzyme active site. In Fisher model, the active site is supposed to be a inflexible preshaped structure to fit the substrate, whereas in the induced fit model the substrate induces the conformational modification in the enzyme illustrated in the below diagram so that the substrate and active site come close to each other in such type of a way that the substrate fits the active site in a more suitable manner.

1461_koshland.jpg


                                                            Diagram: Induced Fit Model of Koshland

On the enzyme molecule the active site exerts a binding force on the substrate molecule through the hydrophilic and hydrophobic catalytic groups. Enzyme substrate complexes are created through multiple bonding that is,

1. Covalent, electrostatic and
2. Hydrogen bonding with the substrate.

At the active site the functional group are arranged in an exact spatial way so that the ES complex formation is favourable.
Several enzymes need non proteinous group termed as coenzymes for their maximal activity. The enzymes needing coenzymes for their activity as well possess sites for the attachment of coenzymes. The complexes created in such cases are termed as enzyme-substrate-coenzyme complexes.

Specific enzymes need a metal ion, additionally to coenzyme for their full activity. These metallic ions are known as positive modifiers of enzyme activity. Instances of such types of enzymes involve alcohol peroxidase, dehydrogenase, catalase and xanthine oxidase etc. that consist of sites for binding metal ions. The elimination of metal from these enzymes frequently results in partial or total loss of enzymatic activity. These enzymes are also termed as metallo enzymes. The common metallic ions needed for enzymatic activity are K+, Cu+, Mg++, Ca++ etc.

 

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