Enzyme Action

It is a molecule that helps to catalyse the chemical reactions by lowering the activation energy required to initiate the reaction. It is not consumed in the reactions so, can be reused (Fig. 1).

Induced-fit hypothesis:
This is the most accepted hypothesis to explain the action of enzymes.
Enzymes bind to the substrate at the active site of the enzyme. When the substrate comes in contact with an enzyme, the shape of the enzyme changes. This results in a tight fit of the substrate in the active site. As the fit is induced, this is known as an induced-fit hypothesis (Fig. 2).
The enzymes bind to the specific substrate or a specific group of the substrates.
When the substrate binds with the enzyme, an enzyme-substrate complex is formed. At this stage, catalysis occurs, and the products are formed.

Active site

An active site is the part of an enzyme that directly binds to a substrate and  undergo a chemical reaction.

Enzyme substrate

An enzyme substrate is the material upon which an enzyme acts.

Factors affecting the activity of the enzyme:
1. Enzyme concentration: More availability of enzyme increases the rate of reaction as more substrates can be converted to the product in lesser time. However, when the concentration of the enzymes extends beyond the concentration of the substrate, the rate of the reaction is not increased further.
2. Substrate concentration: The increase in the concentration of the substrate increases the chances of binding of substrate to the enzymes. Thus, the rate of the reaction is increased. After a certain extent, when the availability of substrate extends beyond the availability of the enzymes, the rate of the reaction will not increase further. 
3. Temperature: At low temperature, the rate of the reaction is less as less heat energy is available for the enzymes to move. So, when the temperature is optimum, that is, 37 to 40-degree Celcius, the rate of the reaction is increased. When the temperature extends beyond this, the rate of the reaction decreases as the enzymes denature at higher temperature as they are proteins.
4. pH: Like temperature, enzymes have different optimum pH at which their activity is maximum. When the pH increases or decreases from the optimum level, the activity of the enzyme is also decreased. The optimum pH is 6-8 for most of the enzymes.

Cofactors:
These are non-proteinaceous molecules that help to increase the activity of the enzymes. These are inorganic molecules like ions. Magnesium is a cofactor for enzyme DNA polymerase.
Coenzymes:
These are organic molecules such as vitamins that help to increase the activity of the enzymes.
Enzyme inhibitors:
The activity of enzymes can be inhibited by certain molecules. Inhibition can be competitive or non-competitive.

1. Competitive inhibition
In this type, a molecule competes with the substrate to bind on the active site of the enzyme. If the inhibitor molecule binds to the active site, it is not available for the substrate thus, the enzyme activity of catalyzing the reaction is inhibited. This is seen when the inhibitor molecules are more in concentration than the substrate molecules.
This can be reversed by increasing the concentration of the substrate molecules and thus, known as reversible inhibition (Fig 3a).

2. Non-competitive inhibition
In this type, the inhibitor bind to a site other than the activity site. It changes the structure of the enzyme preventing binding of the substrate to it. Even when the concentration of substrate is increased, the enzymes remain inhibited. Thus, it is irreversible. The site where the inhibitor bind is known as the allosteric site and it is known as allosteric inhibition (Fig. 3b).

Feedback inhibition
In this type of inhibition, an increase in the product concentration suppresses the activity of the enzyme to produce less new products.

classes of enzymes and the type of reactions they catalyze

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