Enzymes

• Enzyme catalysis

  Provides an alternate pathway with lower activation energy, increasing reaction rate
• Lock and key hypothesis

  Enzyme structure allows substrate to fit exactly into active site
• Enzymatic reaction

  • E + S ⇌ ES → E + P
• Factors affecting enzyme rate

  Temperature, pH, substrate concentration
• Michaelis-Menten curves

  • Vmax, Km (Michaelis constant)
• Enzyme inhibitors

  Competitive, non-competitive
• Enzyme regulation

  Feedback inhibition, allosteric regulation, phosphorylation/dephosphorylation, proteolysis
• Feedback reduction - product of reaction binds allosterically to enzyme

  Affects catalytic activity
• The higher the concentration of final product

  The more likely the product will bind allosterically, shutting down the pathway
• Feedback inhibition

  Cell responds to the amount of regulation product in order to regulate its further production
• Feedback regulation

  Can either be positive or negative
• Positive feedback

  Results in activation of enzymes, increasing production of product
• Negative feedback

  Inhibition of enzyme in biochemical pathway, decreases activity of earlier enzymes, stopping pathway
• Phosphorylation - dephosphorylation

  1. Addition of a phosphate to a serine, threonine or tyrosine amino acid

  2. Acts as an on/off switch for enzyme activity

  3. Phosphorylation controlled by kinase enzymes

  4. Dephosphorylation controlled by phosphatase enzymes
• Proteolysis

  Enzymes can be switched on / off by proteolytic enzymes
• Gene expression

  Amount of enzyme can vary by altering gene expression
• Cofactors

  • Non-protein substance required for enzyme reaction (often metal ions like Cu2+)
  • Can be easily removed from enzyme
• Coenzymes

  Cofactors that can behave as electron donors or acceptors (like NAD+ and FAD)
• Vitamins

  Can act as cofactors
• Biomarkers

  Enzymes released into the blood due to physical/functional damage to cells (caused by disease, trauma or side effects from drugs)
• Enzymes expressed in different tissues

  • Liver tissue: ALT, AST
  • Bile duct tissue: ALP, GGT
  • Muscle tissue: CK-MB