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Enzymes

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

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Enzymes

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  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. 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. 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. 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

  5. The Parts of an Enzyme

  6. 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: nonproteinorganic 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

  7. Components of a Holoenzyme Figure 5.3

  8. Important Coenzymes • NAD+ • NADP+ • FAD • Coenzyme A

  9. mechanism • Substrate binding • Formation enzyme substrate complex • Production formation and dissociation • Enzyme recovery

  10. The Mechanism of Enzymatic Action Figure 5.4a

  11. Each substrate binds to an active site, producing an enzyme-substrate complex. The enzyme helps a chemical reaction occur, and one or more products are formed

  12. 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

  13. Factors Influencing Enzyme Activity • Temperature • pH • Substrate concentration • Inhibitors

  14. 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

  15. 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

  16. Enzyme Inhibitors: Competitive Inhibition Figure 5.7a–b

  17. Competitive inhibition of enzymes

  18. Allosteric regulation of enzyme activity • 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

  19. Enzyme Inhibitors: Noncompetitive Inhibition Figure 5.7a, c

  20. Noncompetitive (allosteric) inhibition of enzymes

  21. 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

  22. denaturation

  23. denaturation

  24. Enzyme Inhibitors: Feedback Inhibition Figure 5.8

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