What are Enzymes and what do they do?

Enzymes: An Overview What are Enzymes and what do they do? Enzymes are biochemical catalysts, which catalyze almost all the biochemical reac...

Enzymes: An Overview

What are Enzymes and what do they do?

Enzymes are biochemical catalysts, which catalyze almost all the biochemical reactions occurring in various metabolic processes be it digestion of food or energy production by cellular respiration, etc. Enzymes are mostly proteins, except for ribozymes, which are catalytic RNAs (ribonucleic acid).

Similar to chemical catalysts, enzymes are required in very small amounts and increase the rate of reaction by reducing the activation energy. The difference between chemical catalysts and enzymes is that enzymes get denatured at higher temperatures (>40℃) and work efficiently only in the optimum temperature range. However thermophilic organisms have enzymes that retain their activity even at higher temperatures (80-90℃).

Enzymes are highly specific to the reactions they catalyze and for the substrate. The substrate fits into the active site, which is a crevice or pocket formed due to the protein chain folding into a tertiary structure. They are generally named after the compounds on which they work, e.g. maltase enzyme catalyzes the hydrolysis of maltose into glucose similarly the protease enzyme catalyzes the breakdown of proteins into smaller peptides and amino acids.

Mechanism of Enzyme Action

Enzymes have an active site to which the substrate (S) binds. A transient ES complex (Enzyme-substrate) is formed, which facilitates the transformation of the substrate to the product and it then dissociates from the enzyme. The unchanged enzyme remains after the product gets released from the short-lived EP complex (Enzyme-product).

E + S ⇌ ES → EP → E + P

The binding of enzymes to a substrate and their specificity is explained by Emil Fischer by the “Lock and key model”, but it failed to explain the transition state stabilization. Daniel Koshland later gave the “Induced fit model” for binding of substrate to enzyme, according to which binding of substrate induces slight alteration in the enzyme structure to fit more tightly around the substrate. The close proximity of substrates facilitates bond breaking and formation.

There are various factors, which influence enzyme activity such as temperature, pH, and concentration of substrate. At optimum pH and temperature, each enzyme shows maximum activity. The optimum pH and temperature differ for each enzyme, e.g. pepsin enzyme requires acidic pH in the stomach, whereas the pH of the intestine is maintained alkaline for enzymatic activities. Generally, at a very low temperature, enzymes remain inactive and at a higher temperature, they are destroyed due to the denaturation of proteins.

Certain chemicals may act as an inhibitor as they compete for binding sites with the substrate due to structural similarities, e.g. malonate inhibits the activity of succinic dehydrogenase due to structural similarities with succinate. This is known as competitive inhibition.

Enzyme activity is also controlled by allosteric regulation. Enzymes have an allosteric site, which is different from the binding site and is known as a regulatory site. An effector binds to the allosteric site and brings about conformation changes in the protein structure. An effector can be an allosteric activator or inhibitor.

Classification of Enzymes

Enzymes are mostly classified on the basis of reactions they catalyze. Based on the type of reactions they catalyze, there are primarily six classes of enzymes:

1. Oxidoreductases/dehydrogenases - catalyze oxidation-reduction reactions, e.g. catalase enzyme, glyceraldehyde-3-phosphate dehydrogenase, etc.

2. Transferases - catalyze the transfer of functional groups, e.g. aminotransferase

3. Hydrolases - catalyze the hydrolysis of various bonds, such as ester, peptide, glycosidic, ether, etc. E.g. Protease enzymes, nucleases, lipases, etc.

4. Lyases - removal of groups other than hydrolysis, e.g. decarboxylases, adenylyl cyclase, etc.

5. Isomerases - catalyze isomerization, e.g. epimerase, triose-phosphate isomerase, etc.

6. Ligases - catalyze joining or covalent bond formation such as C-O, C-S, C-N, etc. E.g. DNA ligase, etc.

Several enzymes have a non-protein part, which is essential for their activity. It is called a cofactor. The enzymatically inactive protein part of an enzyme, which requires a cofactor for its activity is known as an apoenzyme. An apoenzyme becomes enzymatically active when bound to its cofactors and is called a holoenzyme. Catalase enzyme, which catalyzes the breakdown of hydrogen peroxide to oxygen and water contains haem as the prosthetic group, which is tightly bound to apoenzyme and is a part of the active site.

Cofactors can also be coenzymes such as NAD, NADP, etc., or metal ions such as Zn, Mg, Fe, etc. which are essential for enzyme activity.

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