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Catalysis

  • Catalyst: chemical agent that changes the rate of a reaction without being consumed by the reaction
    • Catalysts decrease a reaction’s activation energy by providing an alternate pathway to the transition state
A decrease of the uphill climb means that the transition state (the turning point) is reached more frequently and easily, thus the reaction proceeds faster
  • Enzymes are biological catalysts: all reactions with enzymes are theoretically reversible but in practice, many are irreversible
    • Thus, when used, it is just practically irreversible
  • General form of reaction:

Catlysis.jpg

Examples
  • An enzyme is a biological catalytic protein
    • Highly specific, and have great catalytic power: >106x
    • Increase the rate of reaction such that equilibrium is reached faster, but not changed
    • Many require other components for activity: eg: cofactors/coenzymes (ions?)
    • Without enzymes, chemical reactions in cells would not occur rapidly enough to retain life
  • Enzymes and the Laws of Thermodynamics
    • Enzymes lower the activation energy of a thermodynamically favourable reaction and thus speed up the rate of reaction
    • Enzymes don’t alter the equilibrium of the reaction or change the free energy, or make an unfavourable thermodynamic reaction proceed

Enzyme Substrate (ES) complex

Active Site Binding
  • 'Active Site: Place on an enzyme at which catalysis takes place
    • At these sites active sites, is a binding site that is often specific for particular substrates
      • Enzymes are often very specific
    • Substrate binding in reality may result in conformational changes in the enzyme so as to fit the substrate or in the substrate to fit the enzyme
      • This is known as an ‘induced fit’
  • Active site provides a microenvironment for catalysis
    • Enzyme is not altered in the process
    • A single enzyme can catalyse thousands of reactions/second

Factors that affect enzyme activity

  • Enzyme concentration: Ie: the more enzymes present, the faster the reaction will proceed
    • Too many enzymes will present a limit to rate of reaction
  • Temperature
    • Optimum temperature varies
    • Normally dependent on the host body’s normal temperature
      • If temperature gets too high, enzyme can denature and breakdown
  • pH
    • optimum pH also varies
    • also depends on the host body’s normal environment
  • Substrate concentration
    • Has a non-linear, rectangular hyperbola relationship
    • Michaelis-Menten equation:
      • Reaction velocity = Maximum velocity/(substrate concentration + substrate concentration at 1⁄2 max velocity)
Michaelis-Menten Equation
  • Enzymes may require a non-protein component (a co-factor) for activity.
    • Can be a metallic ion or a more complex organic molecule
  • Inhibitors
    • Types of Inhibition:
      • Competitive inhibitors: mimic the substrate and bind at the active site, thus blocking substrate binding
      • Non competitive inhibitors: bind at separate site distorting binding site, thus prevents catalysis
    • Inhibition is reversible (like substrate binding), but may be irreversible if a covalent bond is formed between inhibitor and amino acid side chain
  • Enzyme inhibition forms the basis of action of many pharmaceutical drugs
    • Methotrexate is a potent inhibitor of the enzyme dihydrofolate reductase
      • Close structural analogue of the substrate,
      • binds tightly to active sites
    • Penicillin irreversibly inhibits an enzyme in bacterial cell-wall synthesis
      • Contains a reactive chemical group that forms a covalent bond at the active site
    • Crixivan inhibits the HIV protease enzyme that splits the HIV polyprotein
      • Thus, the protein can’t be split and thus isn’t infectious