Explanation Discovery of Enzymes The German chemist Edward Buchner discovered enzyme, accidentally, at the end of the nineteenth century. Buchner was trying to obtain a fluid extract from yeast for medicinal use. But the fluid extract kept getting spoilt. To prevent this spoilage, he added sugar to the fluid extract. He found to his surprise that the sugar had fermented to alcohol. Louis Pasteur had already demonstrated nearly twenty years ago, that yeast could ferment sugar to alcohol. But Buchner was able to demonstrate that living cells of yeast were not required for fermentation, but the liquid extract from yeast was responsible for the fermentation. The active ingredient in the fluid extract of the yeast that promoted fermentation was called 'enzyme'. The term 'enzyme' literally means 'in yeast'. Today the term 'enzyme' is used to designate the numerous compounds that have been extracted from cells that catalyze specific chemical reactions. Properties of enzymes 1. Enzymes are biological catalysts. They accelerate the rate of chemical reactions in the cell without undergoing any change itself. The presence of the enzyme does not alter the nature or properties of the end products of the reaction. 2. Enzymes are globular proteins. As they are proteins, enzymes are coded for by DNA. 3. Enzymes exhibit colloidal nature and provide larger surface area for biochemical reactions 4. Enzymes are thermo-labile, that is, heat sensitive. As they are proteins, they get denatured at high temperature. 5. Enzymes are substrate specific. An enzyme catalyses a particular biochemical reaction and acts only on a specific substrate. 6. Enzymes catalyze reactions by lowering the activation energy. The extra energy needed to destabilize existing chemical bonds and to initiate a chemical reaction is called activation energy. The rate of reaction is dependent on the activation energy required to initiate it. Enzymes lower or reduce the activation energy and so increase the rates of reaction. But enzymes do not alter the final proportion of reactants and products. 7. Enzymes are functional at very low concentration. 8. Enzymes have high turn over number. The number of substrate molecules that can be converted into products per minute is called the turn over number. 9. Enzymes can catalyse reversible reactions. 10. Enzyme activity is affected by temperature, pH, substrate concentration and enzyme concentration. How do enzymes work? Enzymes are globular proteins. Enzymes contain one or more clefts or depressions on their surface called active sites, where the substrate binds, thus forming the enzyme-substrate-complex. The chemical substance on which an enzyme works is called its substrate. The substance that is formed is called the product. The enzyme catalyses the reaction and the enzyme-substrate-complex breaks up into products and enzyme. The enzyme remains unchanged and is free to bind with other substrate molecules. This can be summarized by the equation: E+S = ES = E+P Where E=enzyme, S=substrate, ES=enzyme-substrate-complex and P=products. The two main hypotheses proposed to explain the mechanism of enzyme action are: 1. Lock and Key hypothesis and 2. Induced Fit hypothesis Lock and Key hypothesis The Lock and Key hypothesis of enzyme action was suggested by Fisher in 1890. According to the Lock and Key hypothesis, each enzyme possesses an active site, which closely resembles the shape of the substrate molecule. The active site is rigid. Just like a key fits into the lock, the substrate gets attached to the active site and forms the enzyme-substrate-complex. The enzyme does not change in composition while the substrate is broken into products. Induced fit Hypothesis Induced Fit hypothesis was suggested by Koshland in 1959. Induced fit hypothesis states that a substrate induces some structural change in the enzyme. As a result the active site of the enzyme becomes flexible. It changes its shape according to the requirement of the substrate molecule. The substrate gets attached to the active site and forms the enzyme-substrate -complex. The enzyme does not change in composition while the substrate is broken into products.