Enzymes are biological catalysts. They may be defined as organic catalysts, which speed upon various biochemical reactions without being changed.

Structure of Enzymes:

All the enzymes are proteins. This protein nature of the enzyme was first discovered by Sumner (1920). However, all the proteins are not enzymes. Some enzymes like pepsin, amylase, urease, etc. exclusively made up of protein. Other enzymes have a protein and a non-protein component. The protein part is called apoenzyme whereas non-protein component is called cofactor or prosthetic group. The apoenzyme with prosthetic group forms holoenzyme.

The prosthetic group may be inorganic or organic. Inorganic prosthetic groups are found in ascorbic acid, oxidase, catalase, phosphates, etc. Organic prosthetic group which is found in dehydrogenase and it is also called coenzyme. The coenzyme constitute about one percent of the entire enzyme molecule and can be separated from the holoenzymes by dialysis.

Mode of Enzyme Action:

The mechanism of enzyme action can be studied under following two headings:

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(1) Formation of enzyme-substrate complex

(2) Lowering of activation energy.

(1) Formation of enzyme-substrate complex:

The first step in an enzymatic reaction is that the enzyme forms a temporary chemical compound with its substrate called enzyme-substrate complex. Subsequently, the complex breaks of releasing the product and regenerating the original enzyme molecules for reuse.

Substrate + Enzyme → Enzyme – substrate complex → Enzyme + Products

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It is amazing that the enzyme-substrate complex breaks up into chemical products different from these, which participated in its formation (i.e. substrates).

On the surface of each enzyme there are many specific sites for binding substrate molecules called active sites. Structurally, each active site is an indenation on enzyme surface. It is lined by approximately 20 amino acids. During the course of reaction, the substrate molecules occupy these sites. The active sites are located close to each other hence the substrate molecules also come close and react with one another (Fig).

It is thought that when enzyme substrates bind together, the shape of the enzyme molecule undergoes slight change. This produces strains in critical bonds in the substrate molecules and as a result, these bonds break and new bonds are formed. The new chemical thus formed has little affinity for the enzyme and moves away from it.

Following model have been proposed to explain the formation of enzyme-substrate complex.

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(i) Lock and Key model:

This model is proposed by a German chemist named Emil Fisher (1898). According to this model, substrate of specific shape fits into a specific active site on the enzyme surface (fig. above).

(ii) Induced fit model:

This model was proposed by D.Koshland (1988). According to this view, the active sites of an enzyme are not rigid when the substrate binds to enzyme. It may induce a change in shape of the enzyme molecule in such a way that it fits for the substrate-enzyme interaction (fig.). The change in shape of the molecules can put strain on the substrate. The trees may help bonds to break, thus promoting reaction.

(2) Lowering of Activation Energy:

All molecules require certain amount of energy for activation of the molecule and therefore, in the presence of enzymes the substrates are converted into products with lesser input of energy (fig). For example, the activation energy for the decomposition of H2O2 is 18 kcal mol-1 but in the presence of enzyme catalase it is only 6.4 kcal. The enzyme reduces the activation energy by approximately 65%.