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  • 1Copyright 2004 Pearson Education Inc., publishing as Benjamin Cummings.

    19.1 Biological Catalysts19.2 Names and Classification of Enzymes19.3 Enzymes as Catalysts19.4 Factors Affecting Enzyme Activity

    Chapter 19Enzymes and Vitamins

  • 2Copyright 2004 Pearson Education Inc., publishing as Benjamin Cummings.

    19.1 Catalysis by Enzymes Enzymes are proteins that:

    Increase the rate of reaction by lowering the energy of activation.Catalyze nearly all the chemical reactions taking place in the cells of the body.Have unique three-dimensional shapes that fit the shapes of reactants.

  • 3Copyright 2004 Pearson Education Inc., publishing as Benjamin Cummings.

    Enzymes are specific with respect to stereochemistry catalyze reaction of only one of the pair of enantiomers. For example, the enzyme lactate dehydrogenase catalyzes the removal of hydrogen from L-lactate but not from D-lactate.

  • 4Copyright 2004 Pearson Education Inc., publishing as Benjamin Cummings.

    The specificity of an enzyme The specificity of an enzyme for one of two for one of two enantiomersenantiomersis a matter of fit. One is a matter of fit. One enantiomerenantiomer fits better into fits better into the active site of the enzyme the active site of the enzyme than the other than the other enantiomerenantiomer. . Enzyme catalyzes reaction Enzyme catalyzes reaction of the of the enantiomerenantiomer that fits that fits better into the active site of better into the active site of the enzyme.the enzyme.

  • 5Copyright 2004 Pearson Education Inc., publishing as Benjamin Cummings.

    The name of an enzyme:Usually ends in ase. Identifies the reacting substance. For example, sucrase catalyzes the reaction of sucrose.Describes the function of the enzyme. For example, oxidases catalyze oxidation.Could be a common name, particularly for the digestion enzymes such as pepsin and trypsin.

    Names of Enzymes

  • 6Copyright 2004 Pearson Education Inc., publishing as Benjamin Cummings.

    19.2 Enzyme CofactorsPlease see section 19.8 of these slides.

  • 7Copyright 2004 Pearson Education Inc., publishing as Benjamin Cummings.

    Enzymes are classified according to the reaction they catalyze.

    Class Reactions catalyzedOxidoreductases Oxidation-reductionTransferases Transfer groups of atomsHydrolases HydrolysisLyases Add atoms/remove atoms

    to/from a double bondIsomerases Rearrange atomsLigases Use ATP to combine

    molecules

    19.3 Enzyme Classification

  • 8Copyright 2004 Pearson Education Inc., publishing as Benjamin Cummings.

    Classification of Enzymes: Oxidoreductases and Transferases

  • 9Copyright 2004 Pearson Education Inc., publishing as Benjamin Cummings.

    Classification: Hydrolases and Lyases

  • 10Copyright 2004 Pearson Education Inc., publishing as Benjamin Cummings.

    Classification: Isomerases and Ligases

  • 11Copyright 2004 Pearson Education Inc., publishing as Benjamin Cummings.

    Match the type of reaction with an enzyme:1) aminase 2) dehydrogenase3) isomerase 4) synthetase

    A. Converts a cis-fatty acid to a trans-fatty acid.B. Removes 2 H atoms to form double bond.C. Combine two molecules using ATP.D. Adds NH3.

    Learning Check

  • 12Copyright 2004 Pearson Education Inc., publishing as Benjamin Cummings.

    Match the type of reaction with an enzyme:1) aminase 2) dehydrogenase3) isomerase 4) synthetaseA. 3 Converts a cis-fatty acid to a trans-fatty acid.B. 2 Removes 2 H atoms to form double bond.C. 4 Combine two molecules using ATP.D. 1 Adds NH3.

    Solution

  • 13Copyright 2004 Pearson Education Inc., publishing as Benjamin Cummings.

    The active site: Is a region within an enzyme that fits the shape of molecules called substrates. Contains amino acid R groups that align and bind the substrate. Releases products when the reaction is complete.

    19.4 How Enzymes Work

  • 14Copyright 2004 Pearson Education Inc., publishing as Benjamin Cummings.

    Hydrolysis of a peptide bond by Chymotripsin

  • 15Copyright 2004 Pearson Education Inc., publishing as Benjamin Cummings.

    Enzymes may recognize and catalyze:A single substrate.A group of similar substrates.A particular type of bond.

    Enzyme Specificity

  • 16Copyright 2004 Pearson Education Inc., publishing as Benjamin Cummings.

    Lock-and-Key ModelIn the lock-and-key model of enzyme action:

    The active site has a rigid shape.Only substrates with the matching shape can fit.The substrate is a key that fits the lock of the active site.

  • 17Copyright 2004 Pearson Education Inc., publishing as Benjamin Cummings.

    Induced-fit ModelIn the induced-fit model of enzyme action:

    The active site is flexible, not rigid.The shapes of the enzyme, active site, and substrate adjust to maximize the fit, which improves catalysis.There is a greater range of substrate specificity.

  • 18Copyright 2004 Pearson Education Inc., publishing as Benjamin Cummings.

    Enzyme Catalyzed Reaction The proper fit of a substrate (S) in an active site forms an enzyme-substrate (ES) complex.

    E + S ESWithin the ES complex, the reaction occurs to convert substrate to product (P).

    ES E + PThe products, which are no longer attracted to the active site, are released. Overall, substrate is converted to product.

    E + S ES E + P

  • 19Copyright 2004 Pearson Education Inc., publishing as Benjamin Cummings.

    Example of An Enzyme Catalyzed Reaction

  • 20Copyright 2004 Pearson Education Inc., publishing as Benjamin Cummings.

    A. The active site is:1) The enzyme shape.2) A section of the enzyme.3) The substrate.

    B. In the induced-fit model, the shape of the enzyme when substrate binds:1) Stays the same.2) Adapts to the shape of the substrate.

    Learning Check

  • 21Copyright 2004 Pearson Education Inc., publishing as Benjamin Cummings.

    A. The active site is:2) A section of the enzyme.

    B. In the induced-fit model, the shape of the enzyme when substrate binds:2) Adapts to the shape of the substrate.

    Solution

  • 22Copyright 2004 Pearson Education Inc., publishing as Benjamin Cummings.

    Diagnostic Enzymes The levels of diagnostic enzymes determine the amount of damage in tissues.

  • 23Copyright 2004 Pearson Education Inc., publishing as Benjamin Cummings.

  • 24Copyright 2004 Pearson Education Inc., publishing as Benjamin Cummings.

    Enzymes:Are most active at an optimum temperature (usually 37C in humans).Show little activity at low temperatures.Lose activity at high temperatures as denaturation occurs.

    19.6 Temperature and Enzyme Action

  • 25Copyright 2004 Pearson Education Inc., publishing as Benjamin Cummings.

    Enzymes:Are most active at optimum pH.Contain R groups of amino acids with proper charges at optimum pH.Lose activity in low or high pH as tertiary structure is disrupted.

    pH and Enzyme Action

  • 26Copyright 2004 Pearson Education Inc., publishing as Benjamin Cummings.

    Optimum pH ValuesMost enzymes of the body have an optimum pH of about 7.4.In certain organs, enzymes operate at lower and higher optimum pH values.

  • 27Copyright 2004 Pearson Education Inc., publishing as Benjamin Cummings.

    Optimum pH Values

  • 28Copyright 2004 Pearson Education Inc., publishing as Benjamin Cummings.

    Enzyme Concentration

    The rate of reaction increases as enzyme concentration increases (at constant substrate concentration). At higher enzyme concentrations, more substrate binds with enzyme.

  • 29Copyright 2004 Pearson Education Inc., publishing as Benjamin Cummings.

    Substrate Concentration

    The rate of reaction increases as substrate concentration increases (at constant enzyme concentration).Maximum activity occurs when the enzyme is saturated.

  • 30Copyright 2004 Pearson Education Inc., publishing as Benjamin Cummings.

    Sucrase has an optimum temperature of 37C and an optimum pH of 6.2. Determine the effect of the following on its rate of reaction.1) no change 2) increase 3) decrease A. Increasing the concentration of sucroseB. Changing the pH to 4C. Running the reaction at 70C

    Learning Check

  • 31Copyright 2004 Pearson Education Inc., publishing as Benjamin Cummings.

    Sucrase has an optimum temperature of 37C and an optimum pH of 6.2. Determine the effect of the following on its rate of reaction1) no change 2) increase 3) decrease A. 2 Increasing the concentration of sucraseB. 3 Changing the pH to 4C. 3 Running the reaction at 70C

    Solution

  • 32Copyright 2004 Pearson Education Inc., publishing as Benjamin Cummings.

    Inhibitors: Are molecules that cause a loss of catalytic activity.Prevent substrates from fitting into the active sites.E + S ES E + PE + I EI no P

    19.8 -19.9Enzyme Regulation: Inhibition

  • 33Copyright 2004 Pearson Education Inc., publishing as Benjamin Cummings.

    Reversible Competitive Inhibition

    A competitive inhibitor:Has a structure like the substrate.Competes with the substrate for the active site.Has its effect reversed by increasing substrate concentration.

  • 34Copyright 2004 Pearson Education Inc., publishing as Benjamin Cummings.

    A noncompetitive inhibitor:Has a structure different than the substrate.Distorts the shape of the enzyme, which alters the shape of the active site.Prevents the binding of the substrate. Cannot have its effect reversed by adding more substrate.

    Noncompetitive, irreversible inhibi

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