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Key Knowledge -Cell Functioning: General role of enzymes in biochemical activities of cells ENZYMES … Definitions Enzyme (E): protein catalyst Catalysts: speed up reactions without being changed by the reaction Substrate (S): reactant(s) in the enzyme-catalyzed reaction Active site: area of enzyme where substrate(s) bind(s) Enzymes Are involved in every biochemical reaction and thereby control metabolism Enzymes are globular proteins with a specific threedimensional conformation Increase the rate of reaction without altering the enzyme Enzymes A single enzyme molecule can catalyse thousands or more reactions a second. Enzymes are unaffected by the reaction and are reusable. Cofactors Many enzymes need cofactors (or coenzymes) to work properly. These can be metal ions (such as Fe2+, Mg2+, Cu2+) or organic molecules (such as haem, biotin, FAD, NAD or coenzyme A). Many of these are derived from dietary vitamins, which is why they are so important. Enzymes at work Each enzyme can usually only be the catalyst for a single reaction. For example, the enzyme maltase is the catalyst for changing maltose into glucose. Enzymes have the ending -ase. The action of maltase on this reaction is usually written as: maltase Maltose Glucose If there is too much glucose the enzyme can help the reaction to go in the opposite direction: maltase Glucose Maltose Because this reaction can go in either direction it is called a reversible reaction and it is shown with the arrows drawn in both directions: maltase Maltose Glucose Enzymes at work Proteases and peptidases - A protease is any enzyme that can break down a long protein into smaller chains called peptides . Peptidases break peptides down into individual amino acids. Proteases and peptidases are often found in laundry detergents -- they help remove things like blood stains from cloth by breaking down the proteins. Amylases - Amylases break down starch chains into smaller sugar molecules. Your saliva contains amylase and so does your small intestine. Maltase, lactase, sucrase finish breaking the simple sugars down into individual glucose molecules. Lipases - Lipases break down fats. Cellulases - Cellulases break cellulose molecules down into simpler sugars. Bacteria in the guts of cows and termites excrete cellulases, and this is how cows and termites are able to eat things like grass and wood. Specificity Enzymes are highly specific with each enzyme acting on only one kind of substrate Induced Fit Model - there is some flexibility to the active site which can expand or contract to accommodate the substrate Enzyme-Substrate complex The substrate (S) binds to the active site of the enzyme (E) to form an enzyme-substrate complex (ES) The enzyme and the substrate are held together by weak bonds Then the substrate is converted into product while attached to the enzyme, and finally the product is released. The enzyme (E) is unchanged at the end of the reaction - it returns to its original shape after releasing P How do enzymes speed up reactions? Enzymes lower the activation energy (EA=amount of energy that reactant molecules require to start a reaction) of the chemical reactions that they catalyze E-S binding causes a strain on the bonds in the substrate Factors affecting enzyme activity 1. TEMPERATURE Increases rate of reaction by increasing kinetic energy so that molecules collide more frequently Eventually at a certain temperature enzymes are denatured and will not function • Denaturation - a structural change in a protein that results in a loss (usually permanent) of its biological properties Optimum temperature Enzyme denatures Factors affecting enzyme activity 2. pH Can change enzyme shape by changing the charge on amino acids Enzymes have an optimal pH at which they function best e.g. pepsin pH optimum is 2 e.g. trypsin pH optimum is 8 pH can also denature enzymes by changing state of ionization of R groups Factors affecting enzyme activity 3. SUBSTRATE CONCENTRATION Reaction rate increases with increasing substrate concentration There is a limit to this increase however once all active sites are occupied (saturated), adding more substrate will not increase the reaction rate 4. ENZYME CONCENTRATION As the enzyme concentration increases the rate of the reaction increases linearly, because there are more enzyme molecules available to catalyse the reaction. At very high enzyme concentration the substrate concentration may become rate-limiting, so the rate stops increasing. Normally enzymes are present in cells in rather low concentrations. Factors affecting enzyme activity 5. Inhibitors molecules that bind to the substrate and change it so that the enzyme cannot bind to it • e.g. cyanide binds to copper ions in cytochrome oxidase (final enzyme in respiration) irreversibly • e.g. sarin (nerve gas) binds to acetylcholinesterase irreversibly Enzymes in biotechnology At present, cellulases and related enzymes are used in food, brewery and wine, animal feed, textile and laundry, pulp and paper industries, as well as in agriculture and for research purposes. Indeed, the demand for these enzymes is growing more rapidly than ever before, and this demand has become the driving force for research on cellulases and related enzymes. Brewing Brewing uses the living organism yeast. Enzymes within yeast catalyse anaerobic respiration, which converts glucose solution into ethanol (alcohol) and carbon dioxide. This reaction is called fermentation. glucose → carbon dioxide + ethanol Bread making Fermentation is also used in bread making. Flour, water, sugar and yeast are the main ingredients in bread. The ingredients are mixed together to make a dough. This dough is left in a warm place for an hour or two. During this time, fermentation takes place and carbon dioxide is produced. It is the carbon dioxide that helps to make the bread rise when the dough is cooked in an oven. Biological washing powders One common use in the home is in biological washing powders. The enzymes are supposed to digest protein stains from the clothes. Since enzymes work at a low temperature this saves electricity and makes them good for delicate fabrics. Wider uses of enzymes Enzymes are used in many industrial processes. Some of the enzymes and the products are: Amylases- used in textile and paper production Ficin- used in photographic processes Pepsin - used in the pharmaceutical industry Bacterial proteases- used in making leather, textiles and in laundry Catalase- used in rubber production Enzyme deficiency Most genetic disorders are due to a deficiency in enzyme function PKU Phenylketnonuria is an inborn error of metabolism in which a missing enzyme causes the amino acid phenylalanine to build up, with devastating effects on the nervous system unless the individual follows a restrictive diet. Enzyme deficiency Lactose intolerance - The inability to digest lactose (the sugar in milk) is caused by a missing lactase gene. Without this gene, no lactase is produced by intestinal cells. Albinism - Cystic fibrosis In albinos, the gene for the enzyme tyrosinase is missing. This enzyme is necessary for the production of melanin, the pigment that leads to suntans, hair color and eye color. Without tyrosinase, there is no melanin. - In cystic fibrosis, the gene that manufactures the protein called cystic fibrosis transmembrane conductance regulator is damaged. Quick Quiz 1. What is an enzyme? 2. One characteristic of an enzyme is that it is unchanged at the end of a reaction. Give two more characteristics of an enzyme. 3. Draw a graph to show the effect of changing the temperature on the rate of an enzyme-catalysed reaction. 4. Draw a graph to show the effect of changing the pH on the rate of an enzyme-catalysed reaction. 5. What happens to an enzyme when it is denatured? 6. Give two conditions that will denature an enzyme. 7. List four industries that use enzymes.