1.4 3

• Enzyme inhibition


  How enzymes can be affected by competitive, non-competitive and end-product inhibition
• Competitive inhibition


  Inhibitor molecule is similar in shape to the substrate molecule and competes with it for the active site of the enzyme
• Non-competitive inhibition


  Inhibitor may form a complex with either the enzyme itself or with the enzyme/substrate complex, not competing for the active site
• End-product inhibition


  Regulatory enzyme near the beginning of a pathway is inhibited by one of the end products of the chain
• Reversible inhibition of enzymes

  1. Inhibitor affects enzyme in a way that does not permanently damage it

  2. When reversible inhibitor is removed, enzyme can function normally again
• Irreversible inhibition of enzymes

  1. Inhibitor combines with enzyme by permanent covalent bonding to one of the groups vital for catalysis to occur

  2. Changes the shape and structure of the molecule in such a way that it cannot be reversed
• Arsenic, cyanide and mercury are poisonous because they exert irreversible inhibition on enzyme systems
• Some nerve gases used in chemical warfare also work by irreversible inhibition, combining with and completely inactivating enzymes such as acetylcholinesterase
• When acetylcholinesterase is inhibited


  The impulse continues, the muscles go into prolonged spasms causing death because breathing and swallowing become impossible
• Regulatory enzymes


  Have a site separate from the active site where another molecule can bind to have either an activating or inhibitory effect
• Regulatory enzymes are widely found in complex metabolic pathways such as photosynthesis and respiration
• End-product inhibition


  1. Regulatory enzyme near the beginning of the pathway is inhibited by one of the end products of the chain

  2. Phosphofructokinase (PFK) is inhibited by ATP, which binds non-competitively and changes the shape of the active site

  3. As ATP levels fall, ATP molecules detach from PFK and the enzyme becomes active again, increasing rates of cellular respiration and ATP production
• Membrane compartments, variations in pH, and amount of substrate available are other mechanisms used by cells to control reactions
• End-product inhibition provides a simple and effective way of controlling the rate of several reactions at once