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The active site of an enzyme is the region that binds the substrates (and the cofactor, if any). It also contains the residues that directly participate in the making and breaking of bonds. These residues are called the catalytic groups. Although enzymes differ widely in structure, specificity, and mode of catalysis, a number of generalizations concerning their active sites can be stated:

Characteristics of active site

1) The active site takes the form of a cleft or pocket which is formed by groups that come from different parts of the amino acid sequences. The residues far apart in the sequence may interact more strongly than adjacent residues in the amino acid sequence. Amino acids near to one another in the primary structure are often sterically constrained from adopting the structural relations necessary to form the active site.

2) The active sites of multimeric enzymes are located at the interface between subunits and recruit residues from more than one monomer.

3) It takes up a relatively small part of the total volume of an enzyme. Most of the amino acid residues in an enzyme are not in contact with the substrate. Nearly all enzymes are made up of more than 100 amino acid residues, the “extra” amino acids serve as a scaffold to create the three-dimensional active site from amino acids that are far apart in the primary structure. In many proteins, the remaining amino acids also constitute regulatory sites, sites of interaction with other proteins, or channels to bring the substrates to the active sites.

4) Substrates are bound to enzymes by multiple weak attractions. The noncovalent interactions in  Enzyme Substrate (ES) complex are much weaker than covalent bonds.The electrostatic interactions, hydrogen bonds, van der Waals forces, and hydrophobic interactions mediate reversible interactions of biomolecules.

5) The specificity of binding depends on the precisely defined arrangement of atoms in an active site. Because the enzyme and the substrate interact by means of short-range forces that require close contact, a substrate must have a matching shape to fit into the site.

Chemistry of active site for Enzyme substrate binding

Two models have been proposed to explain how an enzyme binds its substrate: the lock-and –key model and the induced-fit model.

1) Lock-and-Key Model of Enzyme-Substrate Binding–  In this model, the active site of the unbound enzyme is complementary in shape to the substrate.

 Lock and key model

Figure-1 showing the lock and key model of Enzyme substrate model

“lock and key model” accounted for the exquisite specificity of enzyme-substrate interactions, the implied rigidity of the enzyme’s active site failed to account for the dynamic changes that accompany catalysis.

2) Induced-Fit Model of Enzyme-Substrate Binding  In this model, the enzyme changes shape on substrate binding. The active site forms a shape complementary to the substrate only after the substrate has been bound.When  a substrate approaches and binds to an enzyme they induce a conformational change, a change analogous to placing a hand (substrate) into a glove (enzyme).

 Induced fit model

Figure-2- Showing induced fit model of enzyme substrate binding

The induced fit model has been amply confirmed by biophysical studies of enzyme motion during substrate binding. Besides explaining the specificity, it also explains the regulation of enzyme activity and the dynamic changes occurring at the active site.

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