Enzymes catalyze chemical reactions by binding substrates to their active site. The "lock and key" model from 1894 describes the active site as having a rigid and complementary shape to the substrate. The "induced fit" model from 1958 proposes the active site changes shape upon substrate binding. Coenzymes are small organic molecules that bind loosely to enzymes and help carry out catalysis, acting as cofactors. Examples include B vitamins and vitamin C, which produce coenzymes needed for reactions. The main difference between enzymes and coenzymes is that enzymes are proteins while coenzymes are small organic molecules that bind to enzymes.

1. Enzyme Action and
Coenzymes
By – Vidya Sukare

2. Enzyme Action
The mechanism by which the enzyme action begins is with the binding of
substrates to the active site of the enzyme. The active site is the specific
region of enzyme which combines with the substrate. An enzyme attracts
the substrates to it's an active site, catalyses the chemical reaction by
which products are formed and then allows the products to dissociate.
Ex-

3. Lock and Key Model-
 The lock and key theory explains
the high specificity to enzyme
activity.
 The enzyme active site is
complementary in conformation to
the substrate,so that enzyme and
substrate recognise one another.
-active site has a rigid shape
-enzyme only binds substrates that
exactly fit the active site
-enzyme is analogous to a lock
-substrate is the key that fits that
lock.
Postulated by Emil Fischer in 1894

4. Induced-fit Model
Proposed by Koshland in 1958

5. Co-enzymes are small, non-protein
molecules that bind loosely to an
enzyme. They are organic molecules that
are required by certain enzymes to carry
out catalysis. Co-enzyme is a type of co-
factor and can be removed freely from
enzyme whereas cofactors can be
removed only when denaturing an
enzyme. A catalyst is a chemical that
increases the rate of a reaction without
itself being changed.
If vitamin intake is too low, then an
organism will not have the coenzymes
needed to catalyze reactions. Water-
soluble vitamins, which include all B
complex vitamins and vitamin C, lead to
the production of coenzymes.
Coenzymes

6. Some examples of Coenzymes-

7. Enzyme Action can be explained
through two models i.e. Lock and Key
Model and Induced fit model.
An enzyme is a globular protein that
serves as a biological catalyst. But, a
coenzyme is a small, organic
molecule, which binds to the enzyme
to activate it. Enzymes do not
change their structure during the
reaction while coenzymes change
their structure by binding to the
functional groups. The main
difference between enzyme and
coenzyme is their structure and
function.
Conclusion