Structure of Enzyme: Do you know that digestion of food starts at the mouth itself! But what helps the food to digest in the mouth? It’s the enzyme which is called salivary amylase that helps in the digestion of carbohydrates present in food. There are different enzymes present in our body that catalyses different reactions. But what enzymes are made of? What is its structure? How is its structure important to catalyse any reaction? Read this article to learn more about enzymes and their structure.

Enzymes Definition

Enzymes are catalysts that catalyse biochemical reactions at the body temperature. They are called biocatalysts as they can accelerate different reactions in the body.

History of Enzymes

1. Wilhelm Kühne, in 1878 gave the term “Enzyme” for these substances.
2. Edward Buchner, in 1897 accidentally discovered that a juice extracted from the yeast cells could bring out fermentation. He coined the term zymase to designate the active principle involved in fermentation.
3. James B. Sumner, in 1926 isolated the enzyme urease from jack beans in pure crystalline form.
4. Northrop in 1930 isolated many enzymes in pure crystalline form.

What are Enzymes?

1. Enzymes are mostly proteins except for ribozymes, made of RNA. Enzymes are produced by living cells to speed up the biochemical reactions in and outside the cells of the body. Enzymes are also called biocatalysts.
2. The reactants in the reaction catalysed by enzymes are called substrates, while the substances produced in the reaction are called products.
3. The study of enzymes’ function and composition is known as enzymology. Enzymologists are the people who study enzymes.
4. Some examples of enzymes are carbonic anhydrase, lysozyme, catalase, and peroxidase.

What is the Structure of Enzyme?

1. Most of the enzymes are proteins, except ribozymes (composed of RNA).
2. They are specific and have a three-dimensional tertiary structure.
3. Tertiary structure is formed by repeated folding or supercoiling of peptide chains to form crevices or pockets.
4. A part of the enzyme called the active site is the pockets or crevices where the substrate fits. The point where the substrate is bound on the active site is known as the substrate-binding site.
5. The active site in an enzyme consists of few amino acids which come together in a particular manner during the tertiary folding of a protein molecule and its attachment with the cofactor.
6. For example, The active site of pyruvate oxidase comprises amino acids such as aspartic acid-cysteine-alanine, and for aldolase, the amino acid is glycine-histidine alanine. The remaining amino acids help to maintain the shape of the enzyme molecule. An enzyme may have more than one active site.
7. The active site is specific to the substrate, i.e. a specific substrate can bind to a specific active site of the enzyme by forming various weak bond interactions.
8. The protein structure and function is determined by the amino acid sequence. So, a change in the sequence of amino acids changes the structure of the enzyme and its active site. Thus changing its function too.
9. Amino acids chains in the enzyme range from \(62\) amino acids to \(2,500\) amino acids.

Fig: Enzyme and its Active Site

10. Allosteric sites: Apart from active sites, enzymes have allosteric sites or inhibitor sites. Inhibitors may join an enzyme at an active site or allosteric site. The binding of inhibitors to allosteric sites modifies the structure of the active site, thus preventing the binding of substrate to the enzyme. This process is called allostery or allosteric inhibition. The enzymes with allosteric sites are called allosteric enzymes.

Formation of Enzyme-substrate Complex

1. Both enzymes and substrates have specific geometrical shapes. The surface configuration of the active site is such as to allow the particular substrate molecules to be held over it. This concept was given by biologist Emil Fisher in his lock and key structure model.
2. Daniel Koshland gave an induced fit model, which states that the active site does not have a rigid lock-and-key confirmation for the substrates. Rather it has a flexible structure. It changes its shapes as the substrates enter the active sites until a structure is obtained which fits completely or closely with the substrate.
3. This induced fit is due to the flexibility of the protein molecules. The tight fit conformation holds the substrates at the correct angle for the reaction to occur.
4. This tight binding enables the enzyme to catalyse the reaction.
5. After coming in contact with the active site of the enzyme, the substrate molecules or reactants form a complex called the enzyme-substrate complex.
6. The enzyme also pulls on the substrates’ bonds and “loosens” them. This brings the substrate into an intermediate state called the transition state between the substrate and the products.
7. This complex lowers the activation energy by bringing the reactant close to each other and orienting in such a way that allows easy interaction between the substrates.
8. Such interactions result in the formation of bonds between the reactants that result in the formation of products. While in some, a single reactant is broken down into different products.
9. The products having different structural configuration does not fit the last conformation of the active site and thus are released from the enzyme molecule or active site.
10. Unchanged enzymes are ready to catalyse other new reactions.

Fig: Active Site of Enzyme

Summary

Enzymes are the biocatalyst of the body, and without enzymes, metabolic reactions would take place at an extremely slow rate. Enzymes are chemically proteins in nature, except ribozymes that are chemically RNA molecules.

Enzymes have active sites to bind to the substrate (enzyme-substrate complex), forming products by allowing the formation of bonds between the substrates. While in some, a single reactant is broken down into different products. Products leave the active site of the enzyme, and again that enzyme is ready to bind another substrate.

FAQs

Q.1. What is the structure and function of an enzyme?
Ans: The enzymes are organic catalysts or biocatalysts which catalyse biochemical reactions at a specific biological temperature. They regulate the rate of biochemical reactions without being utilised in these reactions. Its function is to speed up metabolic reactions.

Q.2. Why is the structure of an enzyme important?
Ans: The structure and shape of an enzyme are important because this would determine which king of metabolic reactions can be carried out depending on the specificity of the substrate that is bound to the active site of an enzyme.

Q.3. What is an enzyme? Give an example?
Ans: Enzymes are the organic compounds necessary for normal metabolism in the cells. It acts as a biocatalyst, i.e., it can accelerate different reactions in the body: for example- salivary amylase.

Q.4. Is an enzyme a protein?
Ans: Yes, most of the enzymes are proteins, except ribozyme.

Q.5. Are enzymes catalysts?
Ans: Yes, enzymes are catalysts as they speed up metabolic reactions.

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