1.4-enzymes

• Anabolic reactions


  Building up molecules
• Catabolic reactions


  Breaking molecules down
• Metabolic pathway


  A sequence of enzyme-controlled reactions in which a product of one reaction is a reactant in the next reaction.
• Enzymes and chemical catalysts share properties in the reaction they catalyse because

  hey speed up reaction

  They are not used up

  They are not changed

  They have a high turnover number
• Describe an enzyme


  Enzymes are proteins with a tertiary structure and the protein chain folds into a spherical or globular shape with hydrophilic R groups on the outside of the molecule, making enzymes soluble.

  Each enzyme has a particular sequence of amino acids, and the elements in the R group determine the bonds the amino acids will make with each other. These are hydrogen, ionic and disulphide bonds and they hold the enzyme molecule in its tertiary form.
• Extracellular


  Some enzymes are secreted from cells by exocytosis and catalyse extracellular reactions.
• Intracellular, in solution


  These enzymes act in a solution inside the cell.
• Intracellular, membrane-bound


  These enzymes are attached to membranes inside the cell.
• Induced fit model

  
• What doe the term activation energy mean

  It is the minimum energy required for molecules to react, breaking existing bonds in the reactants and making new ones. One way of making chemicals react is to increase their kinetic energy, to make successful collisions between them more likely.
• The effect of temperature on the rate of enzyme action

  Above 40c the molecules have more kinetic energy but the reaction rate goes down because the increasing vibration breaks hydrogen bonds, changing the tertiary structure. This alters the shape of the active site and the substrate will not fit. The enzyme is denatured, a permanent change in structure. At low temperatures, the enzyme is inactive as the molecules have very low kinetic energy. However the shape is unchanged so the enzyme will work if the temperature is raised.
• The effect of pH on the rate of enzyme action

  Most enzymes have an optimum pH, at which the rate of reaction is highest. Small pH changes around the optimum cause small reversible changes in enzyme structure and reduce its activity. Extremes of pH denature enzymes.
• Buffer


  It maintains a constant pH even when the products of the reaction are alkaline or acid.
• Substrate concentration


  If the enzyme concentration is constant, the rate of reaction increase as the substrate concentration increases. At low substrate concentration the enzyme molecules have only a few substrate molecules to collide with so the active sites are not working to full capacity.
• Enzyme concentration


  Once a product leaves the active site, the enzyme molecule can be reused, so only a low enzyme concentration is needed to catalyse a large number of reactions. As the enzyme concentration increases, there are more active sites available and therefore the rate of reaction increases.
• Enzyme inhibition


  It is the decrease in the rate of enzyme controlled action by another molecule, an inhibitor. Inhibitors combined with the enzyme and prevent it forming an enzyme substrate complex.
• Competitive inhibition


  They have a molecular shape that is complementary to the active site and similar to the substrate.
• Noncompetitive inhibition


  They bind to the enzyme at another site other than the active site. They affect bonds within the enzyme molecule and alter its overall shape. This changes the shape of the active site. The substrate

  cannot bind therefore no enzyme-substrate complex.
• Immobilised enzymes


  Enzymes are immobilised when they are fixed, bound or trapped on an inert matrix such as sodium alginate beads.
• Advantages of immobilised enzymes

  Increased stability and function over a wider range of temperature and pH than enzymes free in solution.

  Products are not contaminated with solution.

  Enzymes are easily recovered for reuse

  Enzymes can be easily added or removed giving greater control over the reaction
• Some uses of immobilised enzymes

  Lactose-free milk, biosensors and high fructose corn syrup.