Enzymes are proteins that act as biological catalysts and accelerate chemical reactions. They contain an active site that binds specifically to substrate molecules. Originally, it was thought that the active site and substrate were perfectly complementary in shape, as described by the lock-and-key model. However, the induced fit model proposes that the active site is flexible and changes conformation upon substrate binding to optimize the enzyme-substrate interaction. Specifically, when a substrate interacts with the active site, it induces a change in the active site's shape to better fit the substrate, forming a stable complex and lowering the energy of activation for the chemical reaction.

1. Mechanism of Enzyme Action
Dr.Ga.BAKAVATHIAPPAN,M.Sc.,M.Sc (Yoga)., M.Phil., B.Ed., Ph.D.,
Associate Professor, HOD of Zoology,
Saiva Bhanu Kshatriya College
Aruppukottai -626 101.
Virudhunagar District.
Tamilnadu

2. Enzymes are proteins that act as biological catalysts
(biocatalysts). Catalysts accelerate chemical reactions.
The study of enzymes is called Enzymology.
• Enzymes are complex macromolecules with high
molecular weight.
• They catalyse biochemical reactions in a cell.
They help in the breakdown of large molecules
into smaller molecules or bring together two
smaller molecules to form a larger molecule.
• Enzymes are specific in action.
• The molecules upon which enzymes may act are
called substrates, and the enzyme converts the
substrates into different molecules known as
products.

3. Active site
All enzymes molecules contain a special cleft or pocket in its
structure which is actively involved in catalysis. This cleft or pocket is
known as active site of enzyme.
Active site binds with substrate and the functional group present in
active site modify the substrate during catalysis.

7. • Enzymes are highly specific.
• They must bind to a specific substrate before they can catalyze a
chemical reaction.
• At present, there are two models, which attempt to explain enzyme
specificity:
• (1) lock-and-key model and
• (2) induced fit model.
• Like a key into a lock, only the correct size and shape of the substrate
(the key) would fit into the active site (the key hole) of the enzyme
(the lock).

8. Mechanism of Enzyme Action
1. Lock and Key model
• The lock and key model theory first postulated by Emil Fischer in
1894 shows the high specificity of enzymes.
According to this model, shape of active site of enzyme is
complementary to the shape of substrate molecules.
i.e. the substrate is like a key whose shape is complementary to the
enzyme which is supposed to be lock and they fit perfectly.
• Enzymes catalyze only those substrates which fit perfectly on the
active site of that enzyme.
– Once the product is formed, they no longer fit into the active site
and escape into surrounding medium.
•

9. • According to lock and key model, enzymes behave as rigid molecules.
However, most enzymes are globular and are flexible with varying shape.
• Most enzymes are far larger than the substrates molecules that act on and
the active site is usually a very small portion of the enzyme, between 3
and 12 amino acids. The remaining amino acids which make the bulk of
the enzyme, function to maintain the correct globular shape of the
enzyme.

10. In 1959, Koshland suggested a modification to the ‘Lock and Key’
hypothesis which is known as ‘Induced fit’ hypothesis.
• Working from evidence that suggested that some enzymes and
their active site are more flexible. To this, he proposed that the
active site can modify its shape as the substrate interact with the
enzyme.
• The amino acids which make up the active site are moulded into
precise shape which enable the enzyme to perform its catalytic
function most efficiently.
• For instance, a suitable analogy to describe Induced fit model would
be that of a hand changing the shape of the glove as the individual
put on the glove. Therefore in this case, glove is the active site of
enzyme and the hand is substrate.
• However, in some cases, the substrate molecules changes slightly as
it enters the active site before binding.
2. Induced fit model.

12. • Part of an enzyme where substrate molecules get attached is
known as an active site. On attachment to the active site, the
substrate is changed into the product.
• Bonds like hydrogen bonds, hydrophobic interactions, or
temporary non-covalent interactions are formed which leads
to the formation of an enzyme-substrate complex.
• The induced-fit model is generally considered the more
correct version. This theory maintains that the active site and
the substrate are, initially, not perfect matches for each other.
• Rather, the substrate induces a change of shape in the
enzyme. This is similar to placing your hand in a glove.
Getting the first finger in may be difficult, but, once you
complete this initial step, the glove slides on easily (read,
'with much less energy') because it is now properly aligned
for your hand.
• Induced-fit model stated that when the substrate comes in
contact with the active site, the active site modulates
according to the shape of the substrate to form an enzyme-
substrate complex.