MIC310 (Microbial Physiology) - Chapter 4 : Enzymes


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MIC310 (Microbial Physiology) - Chapter 4 : Enzymes

  1. 1. Enzymes Regulatory enzymes are usually the enzymes that are the rate-limiting, or committed step, in a pathway, meaning that after this step a particular reaction pathway will go to completion There are five primary forms of enzyme regulation: substrate availability, allosteric, post-translational modification, interaction with control proteins
  2. 2. Properties of Enzymes In general, chemical reactions that release energy can occur without input of energy The oxidation of glucose releases energy, but the reaction does not occur without an input of energy Activation energy: the energy required to start such a reaction Enzymes lower the activation energy so reactions can occur at mild temperatures in living cells
  3. 3. Enzymes Provide a surface on which reactions take place Active site: the area on the enzyme surface where the enzyme forms a loose association with the substrate Substrate: the substance on which the enzyme acts Enzyme-substrate complex: formed when the substrate molecule collides with the active site of its enzyme Enzymes generally have a high degree of specificity Endoenzymes (intracellular)/exoenzymes (extracellular)
  4. 4. Energy Requirements of a ChemicalReaction Figure 5.2
  5. 5. Enzyme Components Biological catalysts  Specific for a chemical reaction; not used up in that reaction Apoenzyme: Protein Cofactor: Nonprotein component  Coenzyme: Organic cofactor Holoenzyme: Apoenzyme plus cofactor
  6. 6. The Parts of an Enzyme
  7. 7. Properties of Coenzymes and Cofactors Many enzymes can catalyze a reaction only if substances called coenzymes, or cofactors are present Apoenzyme: protein portion of such enzymes Holoenzyme: nonprotein coenzyme or cofactor that is active when combined with apoenzyme Coenzyme: nonprotein organic molecule bound to or loosely associated with an enzyme Cofactor: an inorganic ion (e.g. magnesium, zinc) that often improve the fit of an enzyme with its substrate
  8. 8. Components of a Holoenzyme Figure 5.3
  9. 9. Important Coenzymes NAD+ NADP+ FAD Coenzyme A
  10. 10. mechanism1. Substrate binding2. Formation enzyme substrate complex3. Production formation and dissociation4. Enzyme recovery
  11. 11. The Mechanism of EnzymaticAction Figure 5.4a
  12. 12. Each substrate binds to an active site, producingan enzyme-substrate complex. The enzyme helpsa chemical reaction occur, and one or moreproducts are formed
  13. 13. Enzyme Classification Oxidoreductase: Oxidation-reduction reactions Transferase: Transfer functional groups Hydrolase: Hydrolysis Lyase: Removal of atoms without hydrolysis Isomerase: Rearrangement of atoms Ligase: Joining of molecules, uses ATP
  14. 14. Factors Influencing Enzyme Activity Temperature pH Substrate concentration Inhibitors
  15. 15. Temperature and pH Enzymes are affected by heat and extremes of pH Even small pH changes can alter the electrical charges on various chemical groups in enzyme molecules, thereby altering the enzyme’s ability to bind its substrate and catalyze a reaction Most enzymes have an optimum temperature, near normal body temperature, and an optimum pH, near neutral, at which they catalyze a reaction most rapidly The rate at which an enzyme catalyzes a reaction increases with temperature up to the optimum T
  16. 16. Effect of Temperature on EnzymeActivity Figure 5.5a
  17. 17. Effect of pH on Enzyme Activity Figure 5.5b
  18. 18. Effect of Substrate Concentrationon Enzyme Activity Figure 5.5c
  19. 19. Enzyme Inhibition Competitive inhibitor: A molecule similar in structure to a substrate can bind to an enzyme’s active site and compete with substrate Noncompetitive inhibitors: attach to the enzyme at an allosteric site, which is a site other than the active site noncompetitive inhibitors: distort the tertiary protein structure and alter the shape of the active site Feedback inhibition: regulates the rate of many metabolic pathways when an end product of a pathway accumulates and binds to and inactivates the first enzyme in the metabolic pathway
  20. 20. Enzyme Inhibitors: CompetitiveInhibition Figure 5.7a–b
  21. 21. Competitive inhibition of enzymes
  22. 22. Allosteric regulation of enzymeactivity Allosteric regulation = the activation or inhibition of an enzyme’s activity due to binding of an effectors molecule at a regulatory site that is distinct from the active site of the enzyme Allosteric regulators generally act by increasing or decreasing the enzyme’s affinity for the substrate
  23. 23. Enzyme Inhibitors: NoncompetitiveInhibition Figure 5.7a, c
  24. 24. Noncompetitive (allosteric) inhibition of enzymes
  25. 25. modification enzymes Can either activate it or inhibit it by altering the conformation of the enzyme or by serving as a functional group in the active site
  26. 26. denaturation
  27. 27. denaturation
  28. 28. Enzyme Inhibitors: FeedbackInhibition Figure 5.8