Enzymes

Enzymes are biological catalysts that accelerate the rate of biochemical reactions in living organisms without undergoing any permanent chemical change. They are essential for carrying out various metabolic processes under the mild conditions of temperature and pressure present in living systems.

Nature of Enzymes

Enzymes are generally globular proteins with complex three-dimensional structures. Their catalytic activity depends upon the specific arrangement of amino acid residues that form the active site.

• A small number of enzymes contain a non-protein component in addition to the protein part.
• Such enzymes require cofactors for their activity.
• The protein portion alone is called the apoenzyme, while the complete active enzyme along with its cofactor is known as the holoenzyme.

Characteristics of Enzymes

Enzymes possess several important characteristics that distinguish them from ordinary chemical catalysts.

• They are highly efficient catalysts and increase the rate of biochemical reactions considerably.
• They are highly specific and usually catalyze only a particular reaction or act on a specific substrate.
• They function under mild physiological conditions of temperature and pH.
• They are not consumed during the reaction and can be reused repeatedly.
• Small quantities of enzymes are sufficient to catalyze large amounts of reactants.

Mechanism of Enzyme Action

The catalytic activity of an enzyme is associated with a specific region known as the active site. The substrate molecule binds to the active site to form an enzyme-substrate complex.

• The active site possesses a specific shape that complements the structure of the substrate.
• After binding, the enzyme facilitates the conversion of the substrate into products, which are then released, leaving the enzyme unchanged and ready to participate in another reaction.
• The specificity of enzymes is often explained by the lock-and-key model, according to which the substrate fits precisely into the active site of the enzyme.

This process may be represented as:

Enzyme + Substrate ⇌ Enzyme – Substrate Complex → Enzyme + Product

Factors Affecting Enzyme Activity

The activity of enzymes depends on several factors.

1. Temperature: Each enzyme has an optimum temperature at which its activity is maximum. Excessively high temperatures may denature the enzyme and reduce its activity.

2. pH: Enzymes function best within a specific pH range. Changes in pH can alter the structure of the enzyme and affect its catalytic activity.

3. Substrate Concentration: Increasing substrate concentration generally increases the reaction rate until all active sites become occupied.

4. Presence of Inhibitors: Certain substances called inhibitors reduce or prevent enzyme activity by interfering with substrate binding or catalytic action.