chapter 4

• Biocatalyst

  Enzymes, Enzyme Kinetics, Enzyme Inhibition, Proenzymes and Isoenzymes
• Catalyst


  A substance that speeds up a chemical reaction by providing an alternative pathway with a lower activation energy, without being consumed or permanently altered in the process
• Enzyme


  Specialized proteins that act as biological catalysts, accelerating chemical reactions in living organisms without being consumed in the process
• Enzymes

  • Primarily composed of protein molecules, though some RNA molecules called ribozymes also exhibit catalytic activity
  • Each enzyme typically catalyzes a specific reaction or a group of similar reactions, owing to its unique three-dimensional structure and active site
  • Have a region called the active site where the substrate binds, undergoes a chemical reaction, and then releases the product
  • Lower the activation energy required for a chemical reaction to occur, thereby increasing the rate of the reaction
  • Are not consumed in the reactions they catalyze; they remain unchanged after the reaction and can be reused multiple times
  • Enzyme activity can be influenced by factors such as temperature, pH, substrate concentration, and the presence of inhibitors or activators
  • Enzyme activity is tightly regulated within cells to maintain metabolic pathways and cellular homeostasis
  • Are characterized by their remarkable efficiency and specificity in catalyzing chemical reactions, often increasing reaction rates by factors of millions to billions compared to uncatalyzed reactions
• How enzymes work

  1. Lowering activation energy (free energy)

  2. Role of active site of enzyme
• Enzyme specificity

  Enzymes catalyse only one type of reaction and act on only one or one related group of substrates
• Lock and key model

  The enzyme is like a lock, and its active site is the keyhole. The substrate molecule is compared to a key that fits into the lock (the enzyme's active site).
• Induced fit model


  Initially, the enzyme and substrate exist in different shapes. When the substrate binds to the enzyme's active site, the enzyme undergoes a conformational change to better accommodate the substrate.
• Enzyme classification

  • Oxidoreductase
  • Transferase
  • Hydrolase
  • Lyase
  • Isomerase
  • Ligase
• Enzyme kinetics


  The study of the rate at which an enzyme works
• Maximal velocity (Vmax)


  The maximum velocity of the reaction, when 100% of active sites of enzymes are saturated with substrate
• Effect of substrate concentration on enzyme activity


  Initially the velocity of reaction is proportional to the substrate concentration, then the velocity increases, and finally the velocity becomes independent of substrate concentration because all enzyme molecules are saturated with substrate molecules
• Michaelis-Menten constant (Km)


  The substrate concentration at which half the enzyme's active sites are occupied by substrate, or 50% of active sites of enzymes are saturated with substrates. Inversely proportional to affinity of enzyme for substrates.
• What affects enzyme activity


  • Environmental conditions
  • Cofactors and coenzymes
  • Enzyme inhibitors
• Environmental conditions


  • Rate of reaction increases when temperature is increased, but extreme temperatures may denature the enzyme
  • Enzyme is most active at pH near neutral (6-8)
• Cofactors and coenzymes


  Non-protein chemical compounds or ions that are necessary for the proper functioning of enzymes, assisting them in catalyzing biochemical reactions
• Km


  A low Km means only a small amount of substrate is needed to saturate the enzyme, indicating a high affinity for substrate
• Factors affecting enzyme activity


  • Environmental conditions
  • Cofactors and coenzymes
  • Enzyme inhibitors
• Environmental conditions


  1. Rate of reaction increases when temperature is increased

  2. Extreme temperatures are the most dangerous

  3. High temps may denature (unfold) the enzyme

  4. Enzyme is inactive at low temperature

  5. pH (most like 6 - 8 pH near neutral)
• Cofactors


  Non-protein chemical compounds or ions that are necessary for the proper functioning of enzymes
• Types of cofactors


  • Inorganic cofactors (metal ions)
  • Organic cofactors (coenzymes)
• Apoenzyme


  The inactive form of an enzyme, consisting of only the protein portion
• Holoenzyme


  The complete, catalytically active form of an enzyme, consisting of both the protein portion and the required cofactor or coenzyme
• Types of enzyme inhibition


  • Reversible
  • Competitive
  • Non-competitive
  • Irreversible
• Competitive inhibitors


  Chemicals that resemble an enzyme's normal substrate and compete with it for the active site
• Competitive inhibitors


  • Increase the Km of an enzyme
  • Decrease the apparent affinity of the enzyme for its substrate
  • Do not affect the maximum velocity (Vmax) of the enzyme
• Non-competitive inhibitors


  Inhibitors that do not enter the active site, but bind to another part of the enzyme (allosteric site) causing the enzyme to change its shape, which in turn alters the active site
• Non-competitive inhibitors


  • Do not affect the Km (Michaelis constant) of the enzyme
  • Reduce the maximum velocity (Vmax) of the enzyme-catalyzed reaction
• Antimetabolites


  A class of drugs that interfere with the normal metabolic processes of cells by mimicking or antagonizing endogenous metabolites
• Antimetabolite: Allopurinol


  • Mechanism of action: Inhibits the enzyme xanthine oxidase, reducing the production of uric acid
  • Antimetabolite properties: Lowers uric acid levels, preventing the formation of uric acid and reducing the risk of gout attacks
• Proenzymes (Zymogens)


  Inactive forms of enzymes that require activation before they can catalyze a reaction
• Activation of proenzymes


  Typically through proteolytic cleavage by other enzymes called proteases
• Role of proenzymes


  Serve as a safety mechanism to prevent premature enzyme activity, activated in response to specific signals or conditions
• Examples of proenzymes


  • Trypsinogen (inactive) and trypsin (active)
  • Pepsinogen (inactive) and pepsin (active)
• Isoenzymes


  Enzymes that occur in more than one molecular form, catalyzing the same reaction but with different physical and biochemical properties
• Examples of isoenzymes


  • Cardiac lactate dehydrogenase and hepatic lactate dehydrogenase
  • Cytosolic alcohol dehydrogenase and mitochondrial alcohol dehydrogenase
• Lactate dehydrogenase (LDH)


  Present in most animal tissues as 5 isomers, with 2 subunits of monomers: H (Heart) polypeptide and M (Muscle) polypeptide
• Clinical enzymology


  • Enzymes produced within cells remain inside the cells, but elevation of such enzymes in the plasma indicate tissue destruction
  • Measurement of enzyme levels in plasma can help in diagnosis and prognosis of diseases
• Diseases where enzyme estimations are helpful


  • Myocardial infarction
  • Liver disease
  • Muscle disease
  • Bone disease
  • Cancers
  • GI tract disease