Topic 3 The Chemistry of Life 3.6 Enzymes Topic 3 The Chemistry of Life

Enzymes Catalysts are substances which speed up chemical reactions. Enzymes are Biological Catalysts. They speed up reactions that occur inside living systems. Enzymes are Proteins Macromolecules. Enzymes are globular proteins which have a unique shape. Enzymes remain unchanged in the chemical reaction to be used over and over again. Without enzymes, many chemical reactions occur very slowly. By making some enzymes, cells can control what chemical reactions occur in their cytoplasm.

Enzymes are Specific for their Substrate The reactants in enzyme catalysed reactions are called substrates. An important aspect of enzyme function is that enzymes are highly specific for a particular reaction. They have a specific active site for binding with substrates. Substrates are the reacting molecules in the reaction. The Active Site of the enzyme is a groove or cleft on the surface of the enzyme into which the substrate molecule can bind. Many enzyme names end in –ase Eg: amylase, catalase, sucrase, lipase

Enzymes – Lock and Key This is an example of the Lock and Key model of Enzyme action.

Factors Affecting Enzyme Activity. Most chemical reactions in a cell would occur very slowly or not at all if it were not for special catalysts called enzymes. Catalysts are substances that alter the rate of a chemical reaction without being altered themselves. The main factors affecting enzyme activity include: temperature. pH. substrate concentration.

Temperature & Enzymes At low temperatures, there is little enzyme activity because the molecules have little energy and little movement. At high temperatures (above 50°C) the delicate 3D structure of the protein can be altered. The enzyme is denatured. This happens when the tertiary structure is changed, altering the active site. The substrate molecules can no longer bind to the active site of the enzyme. Enzymes have optimum temperatures in which they work best.

Temperature & Enzymes Enzyme activity increases as temperature increases. This is because collisions between substrate and enzyme active sites happen more frequently at higher temperatures due to faster molecular motion. At high temperatures enzymes are denatured and stop working. This is because heat causes vibrations inside enzymes which break bonds needed to maintain the structure of the enzyme. Ref: Biology, Allott

pH & Enzymes Most enzymes work best around neutral, ph 6-8. Some enzymes work best in acidic conditions: Pepsin – a digestive enzyme in your stomach pH-2 Some enzymes work best in alkaline conditions: Trypsin – enzyme made in the pancreas for breaking down protein pH- 8 In the wrong pH conditions for an enzyme, the tertiary structure of the enzyme changes, affecting the shape of the active site (denatured). Once again, substrate molecules will not be able to bind to the active site of the enzyme. Different enzymes have their optimum operating pH.

pH & Enzymes Optimum pH at which the enzyme activity is fastest. As pH increases or decreases from the optimum, enzyme activity is reduced. Both acids and alkalis can denature enzymes. Ref: Biology, Allott

Substrate Concentration & Enzymes At low substrate concentrations, enzyme activity is directly proportional to substrate concentration. This is because random collisions between substrate and active site happen more frequently with higher substrate concentration At high substrate concentrations, all the active sites of the enzymes are fully occupied, so increasing substrate concentration has no effect. Ref: Biology, Allott

Denaturation Denaturation is changing the structure of an enzyme (or other protein) so that it can no longer carry out its function. Denaturation is usually permanent. Denaturation can be caused by high temperatures and extremes of pH. Denaturing is caused when the bonds that form the tertiary structure of a protein are broken. The protein loses its shape and hence cannot function normally.

Use of Pectinase in Fruit Juice Production Pectin is a complex carbohydrate found in the cell walls of plants. Pectinase is the enzyme that breaks down pectin. Pectinase is obtained by culturing a fungus (Aspergillus niger). The fungus grows on fruits where it uses pectinase to soften the cell walls of the fruit so that it can grow through it. Fruits juices are produced by crushing the fruit to separate the liquid juice from the solid pulp. When ripe fruits are crushed, pectin forms links between the cell wall and the cytoplasm of the fruit cells, making the juice viscous and more difficult to separate from the pulp. Pectinase is added during crushing of fruit to break down the pectin and increase the volume of juice obtained. Pectinase also makes the juice less cloudy by helping solids suspended in the juice to settle and be separated from the fluid.

Use of Protease in Biological Washing Powder. Protease enzymes break down proteins into soluble peptides or amino acids. Laundry washing powders that contain protease are called biological washing powders. Protease is obtained by culturing a bacterium (Bacillus licheniformis), that is adapted to grow in alkaline conditions. The protease has a high optimum pH of between 9 and 10. Detergents in laundry washing powders remove fats and oils during the washing of clothes, but much of the dirt is made of proteins. If protease is added to the washing powder, this protein is digested during the wash. If protease is not used, protein stains can only be removed by using a very high temperature wash. Protease allows much lower temperatures to be used, with lower energy use and less risk of shrinkage of clothes or loss of coloured dyes.