enzymes

• Enzymes are globular proteins that increase the rate of reaction by lowering the activation energy of the reaction they catalyse.
• The active site is the area of the enzyme where the reaction with the substrate takes place.
• Each enzyme has a specific shape that must be complementary to the substrate, meaning that only one type of substrate fits into the active site of each enzyme (enzyme specificity).
• When the enzyme and substrate form a complex, the structure of the enzyme is altered so that the active site of the enzyme fits around the substrate, a process called the induced fit model.
• Enzymes can be intracellular, functioning inside cells, such as DNA polymerase, or extracellular, like the enzymes used in digestion.
• The lock and key theory, proposed by Fischer in 1894, states that the active site and substrate have complementary shapes prior to binding.
• In the induced fit theory, proposed by Koshland in 1958, the enzyme has an active site and is moulded around the substrate as it enters to become complementary, forming an enzyme-substrate complex.
• Factors affecting the rate of enzyme-controlled reactions include enzyme concentration, substrate concentration, temperature, and pH.
• Enzyme concentration: the rate of reaction increases as enzyme concentration increases as there are more active sites for substrates to bind to, however increasing the enzyme concentration beyond a certain point has no effect on the rate of reaction as there are more active sites than substrates so substrate concentration becomes the limiting factor.
• Substrate concentration: as concentration of substrate increases, rate of reaction increases as more enzyme-substrate complexes are formed.
• Temperature: rate of reaction increases up to the optimum temperature as kinetic energy increases.
• pH: as the pH moves away from the enzymes optimum, rate of reaction decreases.
• Each enzyme has an optimum pH: the wrong pH alters the charges on the amino acids which make up the active site, breaking the bonds in the enzyme's tertiary structure and leading to denaturation.
• When the enzyme is not in its optimum pH, the substrate can no longer become attached to the active site and the enzyme-substrate complex cannot form.
• Catalase catalyses the breakdown of hydrogen peroxide, producing oxygen and water.
• The rate of oxygen produced can be measured over a period of time.
• Inhibitors are substances which stop the enzyme from binding to its substrate, controlling the progress of a reaction.
• Immobilising enzymes in alginate involves attaching them to an insoluble, inert material such as calcium alginate which forms a gel capsule around them, holding them in place during the reaction.
• Feedback inhibition occurs when the end product binds to the enzyme at the start of the reaction/pathway and this stops the pathway until the concentration of the end product decreases.
• The Michaelis-Menten equation can be used to calculate the maximum rate of reaction (Vmax) by relating the velocity of enzyme reactions (V) to concentration of a substrate [S].
• Immobilised enzymes are used in industry because they enable the reaction to flow continuously.
• The darker the colour, the higher the starch concentration hence a higher absorbance.
• Competitive inhibition is when an inhibitor molecule binds to the active site of the enzyme and stops the substrate from binding to it; it can be reversed by increasing the substrate concentration as the inhibitor is diluted.
• Concentration of competitive reversible inhibitors increases, the rate of reaction decreases as the active sites are temporarily blocked by inhibitors so substrates cannot bind to them.
• Amylase catalyses the breakdown of starch, producing maltose.
• Reversible inhibition can be competitive or non-competitive.
• Amylase is added to the starch samples, and at regular timed intervals, samples are taken for testing using iodine/KI solution.
• A graph of time vs absorbance can be plotted.
• Concentration of non-competitive reversible inhibitors increases, the rate of reaction decreases as the shape of the enzyme (not the active site) is altered by the inhibitors.
• Non-competitive inhibition is when an inhibitor doesn’t bind to the active site but binds to a different part of the enzyme which changes the shape of the enzyme; it decreases the reaction rate as the substrate cannot bind to the enzyme.
• Once reversible inhibition is removed from the enzyme, inhibition stops and the enzyme can work again.