Enzymes - Basic Concepts and Kinetics

Enzymes as Catalysts

• Enzyme rate enhancement HERE / Enzyme specificity HERE
• Enzyme cofactors HERE

Free Energy

• G determines the direction a reaction proceeds
  • If G <0, the reaction proceeds forward as written
  • If G >0, the reaction proceeds backward as written
  • If G = 0, the reaction is at equilibrium
  • Note that equilbrium DOES NOT mean equal concentrations of reactants and products. Rather, equilbrium means that the concentration of reactants and products does not change over time.
• G⁰'
  • G⁰' is related to G. It is the same as G under standard conditions
  • Thus, the sign of G⁰' determines the direction of a reaction ONLY under standard conditions. G determines the direction of a reaction under any conditions, including standard conditions.
• Calculations
  • G = G⁰' + RT ln[Products]/[Reactants], where R is the gas constant and T is the temperature in Kelvin
  • At equilibrium, G = 0, so G⁰' = -RT ln[Products]/[Reactants] = -RTln K'eq
  • Thus, G⁰' is related to the equilibrium constant for a reaction
  • Relation between G⁰' and K'eq at 25°C HERE

Mechanisms

• Enzymes speed reactions HERE
• Enzymes act by decreasing activation energy HERE
• Reaction velocity versus substrate concentration HERE
• Residues at active site HERE / Hydrogen bonds with substrate HERE
• Fischer lock&key model of catalysis HERE / Koshland induced fit model HERE

Michaelis-Menten Model

• Initial velocity determination HERE
• K_M determination HERE
• LineWeaver-Burk plot HERE
• K_M values of some enzymes HERE / Turnover numbers HERE
• Substrate preferences of chymotrypsin HERE
• Diffusion-controlled enzymes HERE

Multiple Substrate Reactions

Sequential displacement

• Ordered example HERE / Schematic HERE
• Random example HERE / Schematic HERE

Double displacement HERE

Allosteric enzyme kinetics HERE

Enzyme Inhibition

• Uninhibited vs. Competitive vs. Uncompetitive vs. Non-Competitive HERE
• Methotrexate and Tetrahydrofolate HERE
• Kinetics of a competitive inhibitor HERE (V vs. [S]) and HERE (Lineweaver-Burk)
  
• Kinetics of a non-competitive inhibitor HERE (V vs. [S]) and HERE (Lineweaver-Burk)
• Serine modification by DIPF HERE
• Suicide inhibition
  • Triose phosphate isomerase by bromoacetal phosphate HERE
  • Glycopeptide transpeptidase by penicillin HERE