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What are Enzymes and How Do They Work?

How Do Enzymes Work
Enzymes play a very important role in the chemical reactions involved in both cellular and extracellular processes. This article deals with the working mechanism of enzymes.
Sonia Nair
Last Updated: Mar 7, 2018
We all know that cells are the fundamental building blocks of life. Every living organism is made of cells, which have specialized functions. While some living organisms (like bacteria) are unicellular, humans have thousands of cells in their body. It is said that an average adult human has around 100 trillion cells. Several chemical reactions take place inside each and every cell, and these reactions are crucial for the survival of the organism. This applies to unicellular organisms too, wherein numerous chemical reactions are carried out by the single cell. Now can you imagine the innumerable chemical reactions that are carried out inside a human body, which has thousands of cells? Most of these chemical reactions that occur inside the cells are facilitated by the action of enzymes. However, the actions of enzymes are not limited to intracellular activities. They are involved in extracellular reactions too.
What are Enzymes
Enzymes are special proteins that are found in the cells of living organisms. Like any other protein, enzymes are also made up of long chains of amino acids held together by peptide bonds. Enzymes play an important role in performing or controlling a host of chemical reactions that take place in the body. There are various types of enzymes and usually, a specific enzyme is responsible for a particular chemical reaction only. While most of them are associated with metabolic processes like digestion and breathing, there are some, which are related to blood clotting, healing of wounds, controlling production of hormones, and destroying pathogens and environmental toxins.
Enzymes can be broadly classified into metabolic, digestive, and food enzymes. While the first two types are produced by the body itself, the third type is derived from the food we eat. Metabolic ones are mainly responsible for reactions related to detoxification and energy production. Even though they are produced by almost all living cells, cells in the liver, gallbladder, and pancreas, are mainly responsible for the production of metabolic enzymes. Digestive enzymes are involved in chemical reactions that break down the food we eat and convert them into energy. They are secreted along the digestive tract. The enzymes of the digestive tract include pepsin, trypsin, and peptidases, which break down proteins into amino acids. Another digestive enzyme called amylase breaks down starch into simple sugars and lipase breaks down fat into glycerol and fatty acids. The body gets food enzymes from the food we eat or through consumption of enzyme supplements. The enzyme called cellulase, which is responsible for digesting fibers is not produced by the body. So it has to be derived from the raw food we eat. While some enzymes help to break down molecules, some others are meant for merging small molecules to form large ones. Certain enzymes can transform one molecule to another.
Working Mechanism of Enzymes
As mentioned above, most of the enzymes are produced in the cells of living organisms. The production of enzymes is carried out by the cell, based on the instructions from the genes of that cell. So defects in the genes may result in defective enzymes, which do not work properly. The structure and function of each enzyme is different. They have to act upon different targets, that vary from one enzyme to another. Usually, a particular enzyme can act upon a specific target only. The course of action of enzymes are different and complex and so, there are various theories regarding this subject.
An Overview: In general, the working mechanism of an enzyme can be described as follows. Each enzyme acts upon a specific target called substrate, which is transformed into usable products through the action of the enzyme. In other words, the enzyme reacts with the substrate forming an enzyme-substrate complex. Once the reaction is complete, the enzyme remains the same, but the substrate transforms to products. For example, the enzyme sucrase acts upon the substrate sucrose to form products - fructose and glucose.
Lock and Key Theory: This is one of the theories that explain the working mechanism of enzymes. As per this theory, each enzyme has a specific area (called active site) that is meant for a particular substrate to get attached. The active site of the enzyme is complementary to a specific part of the substrate, as far as the shapes are concerned. The substrate will fit into the active site perfectly, and the reaction between them takes place.
substrate of enzyme
The right substrate will fit into the active site of the enzyme and form an enzyme-substrate complex. It is at this active site that the substrate is transformed to usable products. Once the reaction is complete, and the products are released, the active site remains the same and is ready to react with new substrates. This theory was postulated by Emil Fischer in 1894. This theory provides a basic overview about the action of enzymes on the substrate. However, there are certain factors that remain unexplained. As per this theory, the amino acids (in unbound state) at the active site are responsible for its specific shape. There are certain enzymes that do not form any shape in the unbound form. The lock and key theory fails to explain the action of such enzymes.
Induced-fit Theory: This theory was formulated by Daniel E. Koshland, Jr. in 1958. This theory too supports the lock and key hypothesis that the active site and substrate fits perfectly and their shapes are complementary. According to the induced-fit theory, the shape of the active site is not rigid. It is flexible and changes, as the substrate comes into contact with the enzyme.
To be more precise, once the enzyme identifies the right substrate, the shape of its active site changes so as to fit the latter exactly. This results in formation of the enzyme-substrate complex and further reactions. As this theory explains the working mechanism of numerous enzymes, it is widely accepted than the lock and key hypothesis.
Factors that Affect Enzymes' Action: The activities of enzymes are affected by various factors, like the temperature, pH, and concentration. Usually, high temperatures boost the rate of reactions involving enzymes. The optimal temperature for such reactions are said to be around 37 ºC to 40 ºC. Once the temperature rises above this level, the enzymes get denatured and they are no longer fit for reaction with substrates. Variations in pH may also affect the working mechanism of enzymes. The optimum pH level may vary from one enzyme to another, as per the site of their action. Variations from that pH level may slow down the activity of enzymes and very high or low pH results in denatured enzymes that cannot hold the substrate properly. The rate of enzymatic activities may increase with the concentration of enzymes and substrates.
This is only a brief overview about the working mechanism of enzymes. Human body produces numerous enzymes that are responsible for a wide range of chemical reactions, which are necessary for our survival. Right from respiration and digestion, enzymes are involved in so many functions. Some of these enzymes are used for industrial purposes too. Enzymes in laundry detergents are responsible for removing the stains and make the clothes clean. Some are used in preparing foods and beverages.
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