ENZYMES AND Mechanism of Enzyme Action

Enzymes are macromolecular biological catalysts that accelerate chemical reactions without being utilized themselves. The term enzyme was coined by W. Kuhne (1878).

Enzymes occur in a colloidal state and are often produced in an inactive form called proenzymes or zymogens and are converted to active enzymes in the presence of specific factors like pH, substrate, etc.

With few exceptions, all enzymes are proteins. Ribozyme, ribonuclease-P and peptidyl transferase are three non-protein enzymes.

Enzymes are of two types - simple enzymes and conjugated enzymes.

Simple enzymes are made of only proteins, e.g., amylase, trypsin, etc. Conjugated enzymes or holoenzymes contain a protein part (the apoenzyme) and a non-protein part (prosthetic group). The prosthetic group is firmly attached to the apoenzyme.
Another type of organic co-factors are co-enzymes that are loosely attached to the apoenzyme.

There are six classes of enzymes -
(i) Oxidoreductase: Catalyse oxidation and reduction reaction or take part in the transfer of electrons.

(ii) Transferase: They catalyze the transfer of a group between a pair of substrates.

(iii) Hydrolases: Catalyse hydrolysis of bonds like ester, ether, peptide, etc.

(iv) Lyases: Catalyse removal of groups from substrates by mechanisms other than hydrolysis.

(v) isomerases: Catalyse interconversion of optical, geometrical or positional isomers.

(vi) Ligases: Catalyse the linking together of two compounds.



Mechanism of Enzyme Action
There are two modes by which enzymes are supposed to bring about the chemical reaction: (i) Formation of enzyme-substrate (ES) complex and (ii) Lowering of activation energy.

Enzymes have a specific three-dimensional structure. In the active site of the enzyme, substrate fits to proceed with chemical reaction.

The point where the substrate is bound on the active site is known as the substrate-binding site.

This is the first step of an enzymatic reaction where the enzyme forms a temporary association with its substrate, hence the name enzyme-substrate complex.

Two models have been proposed to explain the formation of ES complex:

Lock and key hypothesis: It was proposed by Emil Fischer in 1894. According to this, both enzyme and substrate molecules have specific geometrical shapes. The region of the active site of the enzyme is such that it allows the particular substrate molecule to be held over it. This also explains the specificity of enzyme action.

Induced-fit hypothesis: itis the modification of lock and key hypothesis an ee and was proposed by Koshland in 1959. According to this theory, when the substrate binds to an enzyme, it may induce a conformational change in enzyme molecules in such a way that it is fit for the substrate-enzyme interaction.
The number of substrate molecules converted to products per minute by enzyme molecule is called turn over number.

Activation Energy: All the molecules of substrate possess energy, which is utilized to collide with other molecules to reach that transition state between the reactant and product. This energy is called activation energy. It is quite high for the reaction to proceed and is lowered by the enzyme.

Factors Affecting Enzyme Activity
The conditions which can alter the tertiary structure of a protein can affect the activity of an enzyme.
* Optimum temperature for enzyme activity is 25°C - 40°C. High temperature denatures enzymes due to degradation of
linkages in its polypeptide chains and low temperature inactivates them due to the reduction in speed of molecular movement.
Every enzyme has an optimum pH which is most effective. Most enzymes function near-neutral pH with the exception of several digestive enzymes.
With the increase in substrate concentration, the velocity of the enzymatic reaction fises at first. The reaction ultimately
reaches a maximum velocity (Vmax) which is not exceeded by any further rise in the concentration of the substrate. This is because
the enzyme molecules are fewer than the substrate molecules and after saturation of these molecules, there are no free enzyme molecules to bind with the additional substrate molecule.
Michaelis-Menten equation: To determine the effect of substrate concentration in enzymatic reaction Leonor Michaelis
and Maud Menten (1913) proposed a mathematical model and derived a relationship which is mathematically expressed as
VT: where, Kp, = Michaelis-Menten constant, ie., the substrate concentration to produce half maximum velocity,
V = Velocity of reaction, Vina, = Maximum velocity, [$] = Substrate concentration.

Inhibition of Enzyme Action

The reduction of enzyme activity due to the presence of certain adverse conditions or chemicals is called enzyme inhibition. It can
be classified as :

{i) Reversible inhibition is the type of inhibition that can be overcome by the withdrawal of the inhibitor because the effect of the latter is of temporary in nature. Reversible inhibitors can bind to enzymes through weak non-covalent interactions such as ionic bonds, etc.

(ii) Irreversible inhibition is of permanent type as the enzyme conformation is harmed, e.g., denaturation of the enzyme.

(iii) Competitive inhibition is caused by blockage of the active site of the enzyme by a chemical which is similar in structure to
the substrate. This type of inhibition is usually reversible as the conformational changes do not occur.

(iv) Non-competitive inhibition is caused by alteration of conformation of the enzymes by a chemical that binds to a site
other than the active site. It is usually irreversible because it cannot be overcome by increasing substrate concentration.

Post By : Preeti Rai 27 Dec, 2019 1152 views Biology