3-L1

Enzyme Characteristics

• Enzymes are biological molecules, mostly proteins, that catalyze chemical reactions
• They are highly specific, interacting with one or a few substrates and catalyzing one type of reaction
• Enzymes increase the rate of a reaction by factors of 1 million or more, but do not affect the equilibrium of a reaction
• They remain unchanged after the reaction has occurred, providing a place for the reaction to occur

Enzyme Nomenclature

• Enzymes are named based on the type of reaction they catalyze
• The suffix –ase is usually added to the name of the substrate or reaction they catalyze
• Examples: Lactase (hydrolyzes lactose into glucose and galactose), DNA polymerase (polymerizes deoxynucleotides to form DNA)

Classification of Enzymes

• Enzymes can be grouped into six main classes:
  • Oxidoreductases (oxidation-reduction reactions)
  • Transferases (transfer of C-, N-, or P-containing groups)
  • Hydrolases (catalyze cleavage of bonds by the addition of water)
  • Lyases (addition or removal of groups to form double bonds)
  • Isomerases (transfer of groups within molecules to form isomers)
  • Ligases (formation of bonds between C and O, S, or N at the expense of ATP)

Properties of Enzymes

• Virtually all enzymes are proteins
• Some enzymes require cofactors (inorganic ions like Fe2+, Mn2+) or coenzymes (organic compounds like NAD, CoA) to catalyze reactions
• Prosthetic groups are cofactors or coenzymes tightly or covalently linked to the enzyme protein

Mechanism of Enzyme-Catalyzed Reactions

• Enzyme-catalyzed reactions involve the formation of a complex between the substrate and enzyme (ES complex)
• The active site of an enzyme is the place where the reaction occurs
• Substrate binding to the active site increases the local concentration of reactants and stabilizes the formation of the high-energy transition state
• Enzymes work by lowering the activation energy needed for a reaction to occur

Michaelis-Menten Model

• Describes how the reaction velocity (V0) varies with substrate concentration ([S])
• The Michaelis-Menten equation can be rearranged to give a linear plot (Lineweaver-Burk equation)
• Km (Michaelis constant) is the substrate concentration that gives half the maximal rate (Vmax)
• Low Km means high affinity of the enzyme to the substrate, while high Km means low affinity

Effect of Enzyme Concentration

• The rate of a reaction is directly proportional to the concentration of enzyme
• Doubling the amount of enzyme doubles the amount of product produced per minute

Effect of Temperature

• The reaction velocity increases with temperature until a peak velocity is reached
• Further elevation of the temperature results in a decrease in reaction velocity due to temperature-induced denaturation of the enzyme
• The optimum temperature for most human enzymes is between 35 and 40°C

Effect of pH

• The catalytic process usually requires specific chemical groups in either an ionized or unionized state
• Extreme pH values can lead to denaturation of the enzyme

Inhibition of Enzyme Activity

• Two types of enzyme inhibition: irreversible and reversible
• Irreversible inhibitors bind covalently to the enzyme molecule to destroy enzyme function
• Reversible inhibitors bind non-covalently to the enzyme, with two types:
  • Competitive inhibitors (e.g., allopurinol, statins) compete with the substrate for binding at the active site
  • Non-competitive inhibitors (allosteric regulation) bind to a site other than the active site, affecting Vmax but not Km

International Unit of Enzyme Activity

• 1 unit of enzyme activity is defined as the amount of enzyme causing transformation of 1.0 micromol of substrate per minute under optimal conditions of measurement