1.4 Enzymes

• Enzymes combine with substrate molecules at the active site to produce a product.
• All

  enzymes are tertiary proteins where the polypeptide chain is folded back on itself into a

  spherical globular shape.
• Enzymes are biological catalysts that speed up the rate of

  metabolic reactions.
• Each enzyme reacts with particular substrate molecules – enzymes are specific.
• Each enzyme has its own special 3D globular shape maintained by tertiary protein

  bonding.
• The original lock and key hypothesis suggests that there is an exact fit between the

  substrate and the active site of the enzyme; X-ray diffraction studies of the enzyme

  lysozyme support this.
• Anabolic enzymes build larger products from smaller substrate molecules.
• Catabolic

  enzymes break large substrate molecules into smaller products.
• Lysozyme is an enzyme found in tears and other secretions. Its function is to destroy

  pathogenic bacteria by breaking down their cell walls. Lysozyme destroys the cell wall by

  breaking glycosidic bonds between the amino sugars in the bacterial cell wall.
• The substrate is held in the active site by hydrogen and

  ionic bonds.
• Scientists believe that the substrate molecule changes the shape of the active

  site; the active site changes to fit the substrate molecule perfectly. This is called the

  induced fit hypothesis.
• Enzymes are very efficient and have a high turnover number; this means that they

  can convert many molecules of substrate into product per unit time.
• The

  activation energy is the energy needed to break existing chemical bonds inside molecules.
• In the body enzymes lower the activation energy of a reaction. This reduces the input of

  energy needed to allow reactions to take place; which means they can take place at lower

  temperatures.
• Changing the following factors can affect enzyme activity:

   Temperature

   pH

   Substrate concentration

   Enzyme concentration
• An increase in temperature gives molecules greater kinetic energy. Enzyme and substrate

  molecules move around more quickly, increasing the chance of molecules colliding; this

  leads to the formation of more successful enzyme-substrate complexes.
• Small changes in pH can

  affect the rate of reaction without affecting enzyme structure. Small changes outside the

  optimum range can cause reversible changes in enzyme structure; this results in

  inactivation.
• Extreme changes of pH can denature an enzyme.
• Different enzymes have

  different pH optimums
• To form an enzyme-substrate complex the charges on the amino acid side-chains of the

  active site must attract charges on the substrate molecule. The charges of the enzyme’s

  active site are affected by free hydrogen (H+) and hydroxyl (OH-) ions.
• If, for example,

  there are too many H+ ions (too acidic) the active site and substrate may end up with the

  same charge. The enzyme active site and substrate would repel one another.
• If the enzyme concentration remains constant, the rate of reaction will increase as the

  substrate concentration increases. The reaction will level off once all the active sites are

  occupied; the number of available active sites becomes a limiting factor at higher substrate

  concentrations.
• Once a product leaves the active site, the enzyme molecule can be re-used, so only a low

  enzyme concentration is needed to catalyse a large number of reactions.
• The number of

  substrate molecules that one enzyme molecule can turn into products in a given time is

  called the turn-over number.
• As the enzyme concentration increases, there are more active sites available and

  therefore the rate of reaction increases.
• If temperature and

  pH are optimal and there is

  an excess of substrate, the

  rate of reaction is directly

  proportional to the enzyme

  concentration.
• Catalase is an enzyme found in all living cells; catalase breaks down the toxic

  waste product hydrogen peroxide into water and oxygen. It has one of the highest turnover rates
• An enzyme inhibitor is any substance which decreases the rate of an enzyme catalysed

  reaction or stops it.
• Enzyme inhibiters are either competitive inhibitors or non-

  competitive inhibitors.
• Competitive inhibitors are

  structurally similar to the

  substrate molecule; it can fit in

  the active site instead of the

  substrate molecule. A competitive

  inhibitor prevents enzyme-

  substrate complexes forming.
• Increasing the substrate concentration will

  decrease the effect of a competitive inhibitor as the

  enzyme is more likely to collide with a substrate

  molecule and form a successful enzyme-

  substrate complex.
• Non-competitive inhibitors do not

  bind to the active site; they bind to

  any other part of the enzyme (allosteric site). This

  alters the overall shape of the

  enzyme molecule, including the

  active site. The substrate molecule

  can no longer fit into the active site.
• Increasing the substrate

  concentration will not increase the

  rate of reaction in non-competitive inhibitors as the

  substrate can no longer fit into the

  enzyme’s active site.
• Immobilised enzymes are fixed, bound or trapped on an inert

  matrix. An example is alginate beads.
• Enzymes can also be immobilised on

  a membrane. This is often preferable

  to using alginate beads as the enzyme

  can make direct contact with the

  substrate allowing the reaction to

  take place more quickly.
• Advantages of using immobilised enzymes

  1 The enzyme does not contaminate the product.

  2 The immobilised enzymes can be recovered and reused.

  3 Only a small quantity of enzyme is needed.

  4 The enzymes have greater stability and denature at higher temperatures and can take place over a wider pH range.

  5 More than one enzyme can be used

  6 Greater control over the process.

  7 They can be used in a continuous process.
• The lactose content of milk can be reduced by using the enzyme

  lactase. Lactase breaks the disaccharide lactose into glucose and galactose.
• creating lactose-free milk: As the milk flows through the column

  the substrate (lactose) diffuses into

  the alginate matrix and forms an

  enzyme-substrate complex with the

  lactase. The monosaccharides glucose

  and galactose diffuse out of the

  alginate beads and leave the column

  with the rest of the milk.
• improving experiments with immobilsed enzymes: Flow rate can be decreased to allow

  more contact time between enzyme

  and substrate, allowing more

  successful enzyme-substrate

  complexes to form. Smaller beads can

  be used to increase the surface area

  allowing diffusion to take place

  quicker.
• Remember immobilised enzymes cannot move. This reduces the frequency of

  successful collision as the substrate is the only molecule moving. Free enzymes will

  therefore always have greater activity provided the temperature is not greater than the

  optimum.