2.1.4 Enzymes

• What is an enzyme?

  A protein that acts as a biological catalyst, affecting the metabolism of cells and the whole body. They have a specific tertiary structure and active site, complementary to a specific substrate.
• How do enzymes catalyse reactions?

  The formation of enzyme-substrate complexes lowers the activation energy of reactions.
• Intracellular enzymes?

  Catalyse reactions in cells. For example, catalase; catalyses the decomposition of hydrogen peroxide into water and oxygen.
• Extracellular enzymes?

  Enzymes that work outside of the cell it was made in. For example, amylase; catalyses the digestion of starch into maltose in the saliva and small intestine OR trypsin; catalyses the hydrolysis of peptide bonds in lumen.
• What is the induced fit hypothesis for enzyme function?

  The substrate is not exactly complementary to the active site; conformational changes occur to allow ESC's to form. The change puts a strain on the substrate bond and lowers the activation energy.
• What is the lock and key hypothesis for enzyme function?

  Suggests that the active site has a fixed shape, complementary to only one substrate. The formation of ESC's lowers the activation energy.
• What factors affect the rate of enzyme activity?

  Enzyme concentration, substrate concentration, inhibitor concentration, pH and temperature.
• How does the substrate concentration affect enzyme activity?

  If the enzyme concentration is fixed, the rate of enzyme activity increase and levels off when the maximum number of ESC's have formed.
• How does the enzyme concentration affect enzyme activity?

  If the substrate concentration is fixed, the rate of enzyme activity increase and levels off when the maximum number of ESC's have formed.
• How does temperature affect the rate of enzyme activity?

  The rate increases as the temperature increases because of an increase in kinetic energy. The rate peaks at an optimum temperature.
• What happens to the enzyme above optimum temperature?

  Above optimum temperature, hydrogen bonds and ionic bonds in the tertiary structure of the enzyme break and the active site is no longer complementary to the substrate.
• How does pH affect the rate of enzyme activity?

  Enzymes have a narrow optimum pH range; outside the range, H+ and OH- ions interact with the hydrogen and ionic bonds in the enzyme's tertiary structure causing denaturation.
• What is Q10?

  Q10 measures the change in rate of reaction per 10*C temperature increase. (R2 / R1 = Q10, where R is the rate of reaction, R1 is the first reading taken, e.g. 10*C and R2 is 10*C above, e.g. 20*C)
• How does a competitive inhibitor affect enzyme activity?

  They bind to the active site of the enzyme as they are similar to the substrate. They form temporary bonds and prevent ESC formation. Increasing substrate concentration reverses their affect.
• How does a non-competitive inhibitor affect enzyme activity?

  They bind to the allosteric site, causing a conformational change to the active site so the substrate is no longer complementary, making it so that ESC's can no longer form. Increasing substrate concentration has no effect.
• How does an irreversible inhibitor work?

  Permanently prevents the formation of ESC's. Heavy metal ions, such as mercury or silver, break disulphide bonds in the tertiary structure, changing the shape of the active site. They bind to the enzyme by strong covalent bonds.
• How do reversible inhibitors work?

  They can be both competitive and non-competitive. They bind to the enzyme temporarily by hydrogen or ionic bonds. ESC's can form once the inhibitor is released.
• End product inhibition?

  Products of a reaction acts as an inhibitor for the enzyme involved in the reaction pathway. This prevents further formation of products.
• Metabolic Poisons?

  Substances that damage cells by interfering with metabolic reactions.
• How does cyanide act as a metabolic poison?

  It is a non competitive, irreversible inhibitor.
• Malonate and Arsenic?

  Both act as competitive inhibitors.
• Penicillin?

  Acts as a non-competitive inhibitor; prevents the formation of peptidoglycan cross links in the bacterial cell wall.
• Ritonavir?

  Inhibits HIV protease to prevent assembly of new vinions.
• What is a cofactor?

  A small, non-protein molecule that catalyses reactions when attached to a specific enzyme, increasing the rate of reaction. They are inorganic and bind away from the active and allosteric site. They can be attached permanentally or non-permanentally to enzymes.
• Permanent cofactors/prosthetic groups?

  Form strong bonds with the enzyme, e.g. ionic or covalent bonds. For example, carbonic anhydrase; enzymes that catalyses the reaction between CO2 and H2O to make H2CO3 (carbonic acid). Has the prosthetic group Zn2+. Reaction allows CO2 to be carried from respiring tissues to the lungs.
• Non-permanent cofactors?

  Ions may form temporary, single, ionic bond with enzyme or substrate to make the two more complementary. Some act as co-substrates; ion and substrate bind to make complementary shape for enzyme. Some change the charges on the substrate/active site to form temporary bonds, making ESC's easier to form. E.g. amylase only functions in the presence of Cl-
• Coenzymes?

  Small, organic non-protein molecules. Bind temporarily to the active site of the enzyme, just before/the same time as the substrate. They are chemically changed in the reaction - have to be recycled to their original state. Many are derived from water soluble vitamins; diet.
• Coenzyme example?

  Vitamin B3 (nicotinamide) is used to form coenzymes NAD; for respiration - accepts protons for the reversible reaction of NAD —> NADH. AND NADP; also accepts protons (is reduced), also chemically changed, and is used for photosynthesis for the reversible reaction NADP —> NADPH
• NAD, NADP used to treat?

  Pellagra (diarrhoea, dermatitis, dementia)
• Conezyme A?

  Comes from Pantothenate (B6); treats elevated blood-plasma triglyceride levels.
• Precursors?

  Some enzymes are produced in inactive precursor form so have to have some aa's removed before the active site is the correct shape.
• Precursor Example?

  Trypsinogen is the precursor of trypsin (converted by an enzyme). Pepsinogen is the precursor of pepsin (converted by HCl in the stomach). May also be activated by the addition of a cofactor.