BIOLOGY CHAPTER 3 ENZYMES

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1 BIOLOGY CHAPTER 3 ENZYMES Catalyst In chemistry, a catalyst is a substance, which can alter or speed up a chemical reaction without itself being chemically changed at the end of the reaction. For example, when potassium chlorate is heated strongly in a test-tube, oxygen is given off. strong heat potassium chlorate > potassium chloride + oxygen This reaction occurs slowly and requires a high temperature. Gentle heating only melts the potassium chlorate and no oxygen is produced, but if little black Manganese dioxide or Manganese (IV) oxide is added to the melted substance the reaction is speeded up and there is a rapid evolution evolution of oxygen. Manganese dioxide or Manganese (IV) oxide is therefore a catalyst. At the end of the reaction, the same mass of Manganese dioxide remains, showing that it has not been changed in the reaction. Enzymes There are many reactions that can be speeded up by the use of catalyst. To break down the carbohydrates, fats or proteins in the laboratory without a catalyst requires the use of complex apparatus and high temperatures. The living body can bring about the same reactions quite rapidly without having to raise the body temperature. It does this, using catalysts called enzymes. Definition of enzymes: Enzymes are biological catalysts that can alter or speed up the rate of chemical reactions without themselves being chemically changed or used up at the end of the reaction. Example: Inside the alimentary canal, large molecules are broken down to smaller ones in the process of digestion. Enzymes speed up these reactions. A different enzyme is needed for each kind of food. 1

2 For example starch is digested to the sugar maltose by an enzyme called amylase. protein is digested into amino acids by protease. These enzymes are also found in plants for example in germinating seeds, where they digest the food stores for the growing seedling. Another enzyme, which speed up the breakdown of a substance, is catalase. Catalase, however, does not work in the alimentary canal. It works inside the cells of living organisms both animal and plants for example, in liver cells or potato cells. It breaks down hydrogen peroxide to water and oxygen. This is necessary because hydrogen peroxide is produced by many of the chemical reactions, which take place inside cells. Hydrogen peroxide is a very toxic chemical, and must be immediately broken down. 'BREAKING DOWN' REACTIONS (CATABOLIC) ENZYME USAGE EFFECT Protease Digestion Protease converts proteins into amino acids Amylase Digestion Amylase converts i) starch to sugar maltose and then ii) maltose to glucose. Cellulase Industrial use Cellulase breaks down cellulose found in cell wall of plant cells. This allows softening of vegetables and also removal of seed coats. Lipase Digestion Lipase converts fats to fatty acids and glycerol. Catalase Digestion (in both plant and animal cells) Catalase breaks down hydrogen peroxide (toxic) to produce water and oxygen. However, not all enzymes help to break things down. Many enzymes help to make large molecules from small ones (anabolic reaction). One example of this kind of enzyme is starch phosphorylase, which builds starch molecules from glucose molecules inside plant cells. Enzymes Change Substrates To Products A chemical reaction always involves one substance changing into another. The substance, which is present at the beginning of a reaction is called the substrate. The substance, which is made at the end of the reaction is the product. Some examples of substrates and products for reactions catalysed by particular enzyme include: 2

3 SUBSTRATE ENZYME PRODUCT Starch Amylase Maltose Glucose Starch phosphorylase Starch Hydrogen peroxide Catalase Water and oxygen Classification of Enzymes Enzymes are classified according to the chemical reactions they catalyse. Hydrolases Digestion is an example of a chemical reaction called hydrolysis. In hydrolysis, water molecules are needed to break down a complex molecule into simples molecules. Therefore enzymes that catalyse hydrolytic reactions are known as hydrolases. For example: Enzyme Carbohydrases that digest carbohydrates include: Amylases, e.g. salivary amylase/ptyalin and pancreatic amylase Proteases e.g. pepsin in the stomach Lipases in the pancreatic juice and intestinal juice. Process Digest or hydrolyse starch into maltose/glucose. Digest protein into amino acids. Digest fat or lipids into fatty acids and glycerol. Digestive enzymes are also used in washing powders. They break down and remove stains caused by organic matter e.g. sweat, blood, curry and plant material. Enzymes Have Active Sites Enzymes are proteins. Their molecules have a very precise three dimensional shape. This shape includes a 'dent' which is exactly the right size and shape for the enzyme substrate to fit into. This 'dent' is known as the active site. When a substrate molecules bind into an enzyme at the active site, they are known collectively as the 'enzyme-substrate complex'. At this stage, the enzyme tweaks the substrate molecule, pulling it out of shape and making it split into product molecules. The 3

4 product molecules then leave the active site, which is now ready to do the same to another substrate molecule. Explaining How Enzymes Work (Lock and Key Hypothesis) Characteristics of Enzymes 1. All enzymes are globular proteins. They have molecules with a precise threedimensional shape, containing an active site. 2. Enzymes are catalysts. They increase the rate of chemical reactions that would otherwise occur very slowly, and remain unchanged at the end of the reaction. Each enzyme molecule can be used over and over again. This means that a small amount of enzyme can catalyse the conversion of a lot of substrate into a lot of product. 3. Enzymes are specific. Each enzyme can only convert one kind of substrate molecule into one kind of product molecule. This is because the active site of the enzyme molecule has to be exactly the right shape to allow a substrate molecule to fit into. For example, sucrase will only act on sucrose and maltase will only act on maltase. 4. Enzyme activity is affected by temperature. The temperature, which produces the greatest activity, is called the optimum temperature. Temperatures above or below the optimum decrease the rate of enzyme activity. Enzymes being proteins are destroyed, inactivated or denatured permanently by temperatures above 50ºC. 4

5 In man, the normal body temperature, 37ºC is the optimum temperature where enzymes are most effective. If the temperature is low, the reaction is slow since enzyme-substrate complexes are formed slowly. As the temperature increases, the kinetic energy of the molecules increases. This causes more bombardments between enzyme and substrate molecules and so the rate of reaction also increases. The rate of reaction doubles with an increase in temperature of 10ºC. This increase continues until a certain maximum temperature after which the rate drops rapidly. With high temperatures protein molecules break down by a process called denaturation. They lose their particular shapes so that the substrate molecules can no longer fit the active sites (denaturation is irreversible). The effectiveness of the enzymes is then lost. 5. Enzyme activity is affected by ph. Enzymes are sensitive to changes in ph. The ph at which an enzyme works best is often called its optimum ph. Enzyme Amylase Ptyalin Pepsin Intestinal enzymes Optimum ph 7 (neutral) (slightly acidic) 2 (acidic) 8.5 (alkaline) Although changes in ph affect the activity of enzymes, these effects are usually reversible i.e. an enzyme which is inactivated by a low ph will resume its normal activity when its optimum ph is restored. Extremes of ph, however may denature some enzymes irreversibly. 5

6 6. Enzyme activity is affected by concentrations of substrate and enzyme. The more enzyme molecules produced by the cell, the faster the reaction will proceed, provided there are enough substrate molecules available. Similarly, an increase in the substrate concentration will speed up the reaction if there are enough enzyme molecules to cope with the additional substrate. Certain substances known as activators can enhance activity, while other called inhibitors may repress an enzyme. Enzymes In The Germination of Seeds Seeds store food in complex, insoluble forms such as proteins, starch and oils. They are stored in the cotyledons or in special storage tissue called endosperm. Enzymes only work in an aqueous medium. So, germination begins with the absorption of water by seeds. When sufficient water is absorbed, hydrolytic enzymes or hydrolases present in the seed are activated. These enzymes break down or digest the food stored in the seed into simpler, soluble substances by hydrolysis. The digested food also dissolves in water before being taken to the regions of growth and development at the radicle (root tip) and plumule (shoot tip) of the new plant. Complex food Enzyme Digested food Starch Carbohydrase Simple sugars Proteins Protease Amino acids Oils Lipase Fatty acids + glycerol At the plumule and radicle, the simple food molecules are used to synthesise complex substances like proteins in the protoplasm, synthesise cellulose in cell wall or oxidised to provide energy. 6

7 Examples of enzyme actions during seed germination Enzymes In Industry Brewing, baking and cheese making are examples of industries, which, for many years, have made use of enzymes in a large scale. In the case of brewing and baking it is the sugar fermenting enzymes in living yeast cells, which are exploited. These enzymes convert sugar to alcohol and carbon dioxide. It is bubbles of carbon dioxide, which make the dough rise before baking and so give the bread a light texture. In brewing, it is mainly the alcohol, which is wanted, but the carbon dioxide gives a sparkle to beers and sparkling wine. Brewing and baking exploit enzymes in living cells but for cheese making, an enzyme was extracted from the calves' stomachs (it is now produced by genetically engineered bacteria). The enzyme is called rennin. The genetically engineered enzyme is call chymosin. It clots milk in the first stages of making cheese. 7

8 Of about 2000 known enzymes, about 50 are used commercially in large quantities. The advantages of using enzymes are: They work at low temperatures. They are not corrosive, as are the strong acids and alkalis that would otherwise be needed to carry out the chemical changes. They are specific i.e. they act on only one substrate, so the process can be carefully controlled, and unwanted by products avoided. Two thirds of all commercial enzyme production is taken up by protein-digesting enzymes (proteases) for detergents, and starch-digesting enzymes (amylases) for the food and textile industries. The proteases in detergents help remove organic stains such as blood, gravy and fruit juice. The amylases are used to convert starch to glucose and fructose for sweeteners, or to destarch fabrics in the course of processing. 8