Mechanisms of Enzymes

Mechanisms of Enzymes:

There are two types of mechanisms involved to explain substrate-enzyme complex formation; lock and key theory (template model), and induced-fit theory.

(i) Lock and Key Theory:

Emil Fischer (1894) explained the specific action of an enzyme with a single substrate using a theory of Lock and Key analog . According to this theory, reaction of sub-state and enzyme is analogous to lock and key. Enzyme is analogous to key, where the geometrical configuration of socket is fixed. Similarly substrate has also got fixed geo­metrical configuration like that of key. A particular lock can be opened or closed by a particular key. According to the particular substrate can be found at active site of particular enzyme forming substrate-enzyme complex. Enzyme-substrate complex remains in tight fitting and active sites of enzymes are complementary to substrate molecules. Subsequently, enzyme-substrate complexes result in the transformation of substrate into the product formation due to activity of reaction sites.

Since product has lower free energy, it is released. Enzymes are fixed to receive another molecule of substrate and thus enzyme activity continues. In this analogy, the lock is the substrate and the key is the enzyme. Only the correctly sized key (substrate) fits into the key hole (active site) of the lock (enzyme).

Smaller keys, larger keys, or incorrectly positioned teeth on keys (incorrectly shaped or sized substrate molecules) do not fit into the lock (enzyme).

Lock and Key Model for Mechanism of Enzyme Action

(ii) Induced Fit Theory:

In 1958, Koshland modified the Fischer’s model for the formation of an enzyme-substrate complex to explain the enzyme property more efficiently. According the Fischer’s model the nature of the active site of enzyme is rigid, but it is able to be pre-shaped to fit the substrate.

Koshland explains that the enzyme molecule does not retain its original shape and structure, but the contact of the substrate induces some geometrical changes in the active site of the enzyme molecule. The enzyme molecule is made to fit completely the configuration and active centers of the substrate. At the same time, other amino acid residues may become buried in the interior of the molecule.

The hydrophobic and charged group both are involved in substrate binding. A phosphoserine (-P) and SH group of cysteine residue are involved in catalysis.

Residue of the other amino acid such as lysine (Lys) and methionine (Met) are not involved in either binding or catalysis. In the absence of substrate, the substrate binding group and catalytic group are far apart from each other.

But the contact of the substrate induces a conformational changes in the enzyme molecule and aligns both the groups for substrate binding and catalysis. Simultaneously, the spatial orientation of the other region also changed. This causes the lysine and methionine much closer.

Reference http://www.biologydiscussion.com/metabolism/microbial-metabolism/enzymes-definition-mechanisms-and-classification-microbiology/65516