Cellular Biology

Enzymes and their Functions



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Enzymes are organic catalysts which are protein in nature. They are found in all living things and produced in all living cells. As catalysts, they speed up or reduce the rate of chemical reactions in the body without being used up themselves. There are two types of enzymes, the intracellular and the extracellular. The intracellular are secreted and used to catalyze chemical reactions within the cells which produced them while the extracellular are produced within the cells but are used outside the cells which produced them. Some of the biological activities catalyzed by enzymes include digestion and respiration.

All enzymes are known to be protein in nature although there others which contain a non-protein part known as the prosthetic group. The prosthetic group may be a metallic ion hence called a cofactor or it might be an organic molecule hence called a coenzyme. Without the prosthetic group, enzymes which contain them become inactive. The substances which the enzymes work on are called substrates.

Some of the properties of enzymes are;

·          They are protein in nature. Therefore, they are denatured by temperatures above 400C.

·         They are substrate specific and only act on one substrate. For example, maltase will only act on maltose.

·         They are not broken down during their catalytic activity. During the chemical reactions, the enzymes remain unchanged and can be used over and over again hence are efficient in small numbers.

·         Most of the enzyme catalyzed reactions are reversible.

During the chemical reaction, the enzyme combines with its specific substrate to form an enzyme-substrate complex. The substrate molecules react within their complex to form the products which remain bound to the enzyme as an enzyme-product complex. The complex dissociates to release the products and free the enzyme which can then combine with other substrate molecules.

Enzymes have a specific place on their surface known as the catalytic site to which the substrate molecules can attach themselves. The shape of the catalytic site and the presence of various chemical groups ensure that only those substrates with a complimentary structure can bind to it. When the substrate concentration is increased, the rate of the enzyme reaction is also increased to a certain level as all catalytic sites of an enzyme are occupied. However, further increase in substrate concentration does not increase the rate of chemical reaction because at low substrate concentration, only a few of the enzyme catalytic sites are occupied by substrates.

Inhibition can occur when certain chemicals, enzyme inhibitors, slow down the activity of the enzymes. There are two types of inhibitors; the competitive inhibitors and the non-competitive inhibitors. The competitive inhibitors have a structure similar to that of the normal substrate and compete with the normal substrate for the catalytic sites. They bind themselves temporarily to the enzyme’s catalytic site but are not converted into products hence it can remain in its catalytic site longer than the substrate.

Non-competitive inhibitors do not compete with the substrate for the catalytic site but instead, they bind themselves at a site different from the catalytic site altering the structure of the site. This prevents the substrate from interacting with the enzyme. Examples of the non-competitive inhibitors include poisons such as the cyanides and metals such as mercury, silver and lead. Competitive inhibitors can be eased by increasing the concentration of the substrate but non-competitive inhibitors can not be eased by increasing the substrate concentration.

Importance of enzymes

·         They speed up cellular reactions so that they proceed at a rate that is appropriate for sustaining life.

·         They control the chemical reactions hence preventing violent incidents in the cell.


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