Biology A-Level

• What are enzymes?


  biological catalysts
• What type of molecule is an enzyme?

  globular protein
• Why are enzymes important?

  speed up chemical reactions without the need for harsh environmental conditions
• How do enzymes increase the rate of reaction?

  decrease activation energy
• lock and key hypothesis


  older hypothesis, active sites fit specific substrates and form enzyme-substrate complex
• induced fit hypothesis


  currently accepted theory, initial enzyme-substrate interaction relatively weak but leads to change in active site to form stronger enzyme-substrate complex
• intracellular enzyme example


  catalase, catalyses breakdown of toxic hydrogen peroxide formed from cellular reactions into H₂O and O₂
• amylase


  extracellular enzyme in saliva that breaks down starch into maltose
• trypsin


  extracellular enzyme that catalyses hydrolysis of peptide bonds, produced in pancreas and secreted into small intestine
• Effect of temperature on enzyme activity

  increased temperature increases rate of reaction until sharp drop when enzymes denatured
• temperature coefficient (Q₁₀)


  measure of change of rate of reaction when temperature increased by 10°C
• Effect of pH on enzyme activity

  enzymes work best at optimal pH, can be denatured by extreme variations from optimum
• Effect of enzyme concentration on enzyme activity

  increased concentration leads to increased rate of reaction until all substrate molecules have formed enzyme-substrate complexes
• Effect of substrate concentration on enzyme activity

  increased concentration leads to increased rate of reaction until all enzymes have formed enzyme-substrate complexes
• What happens when an enzyme is denatured?

  Some bonds holding protein together broken, change in tertiary structure changes shape of active site, no longer complementary
• competitive inhibition


  substance competes with substrate molecules for active site, closely resembles substrate molecule, effect dependent on substrate concentration
• non-competitive inhibition


  substance binds at allosteric site, changes shape of active site, unaffected by changes in substrate concentration
• irreversible inhibition


  covalent bonds formed between enzyme and inhibitor, inhibitor cannot be removed easily
• reversible inhibition


  hydrogen or ionic bonds formed between enzyme and inhibitor, inhibitor can be removed
• How do statins work?

  inhibit enzyme involved in cholesterol synthesis
• How do antiviral drugs work?

  inhibits reverse transcriptase, prevents virus from replicating
• How does penicillin work?

  inhibits transpeptidase, stops formation of proteins in bacterial cell walls, cell wall weakens and bursts
• How does cyanide work?

  irreversible inhibition of cytochrome oxidase, stops respiration
• How does malonate work?

  inhibits succinate dehydrogenase, stops respiration
• How does arsenic work?

  inhibits pyruvate dehydrogenase, stops respiration
• End-product inhibition


  final product in a metabolic pathway inhibits an enzyme that acts earlier on in the pathway
• Example of end-product inhibition

  phosphofructokinase catalyses initial breakdown of glucose molecule in metabolic pathway to produce ATP, ATP competitively inhibits phosphofructokinase
• cofactor


  non-protein substance, helps enzyme and substrate bind together
• cofactor example


  Cl- ions act as a cofactor for amylase
• coenzymes


  organic molecules acting like cofactors, often derived from vitamins
• prosthetic groups in enzymes


  cofactors that are tightly and permanently bound to the protein
• example of prosthetic group in enzymes

  Zn²⁺ ions in carbonic anhydrase
• What is an apoenzyme?

  inactivated precursor enzyme
• Why are some enzymes produced in an inactive form?

  They can cause damage to the cells where they are produced or the tissues where they are released, action needs to be controlled and only activated under certain conditions
• What often needs to occur to activate apoenzymes?

  Change in tertiary structure
• How can the tertiary structure of an apoenzyme be changed?

  cofactor addition, action of another enzyme e.g. a protease, change to environment
• How is prothrombin activated?


  clotting factor X cleaves certain bonds to transform it into thrombin
• How is pepsinogen activated?

  enters stomach, low pH transforms molecule into pepsin
• What is an enzyme known as after it has been activated?

  Holoenzyme