Chapter 23: Enzymes PDF

23 General, Organic, and Biochemistry, 8e Bettelheim, Brown, Campbell, and Farrell © 2006 Thomson Learning, Inc. All rights reserved 23-1 23 Chapter 23 Enzymes © 2006 Thomson Learning, Inc....

23 General, Organic, and Biochemistry, 8e Bettelheim, Brown, Campbell, and Farrell © 2006 Thomson Learning, Inc. All rights reserved 23-1 23 Chapter 23 Enzymes © 2006 Thomson Learning, Inc. All rights reserved 23-2 23 Enzyme Catalysis Enzyme: a biological catalyst. With the exception of some RNAs that catalyze their own self-cleavage, all enzymes are proteins. Enzymes can increase the rate of a reaction by a factor of 109 to 1020 over an uncatalyzed reaction. Some catalyze the reaction of only one compound. Others are stereospecific; for example, enzymes that catalyze the reactions of only L-amino acids. Others catalyze reactions of specific types of compounds or bonds; for example, trypsin that catalyzes hydrolysis of peptide bonds formed by the carboxyl groups of Lys and Arg. © 2006 Thomson Learning, Inc. All rights reserved 23-3 23 Classification of Enzymes Enzymes are commonly named after the reaction or reactions they catalyze. example: lactate dehydrogenase, acid phosphatase. Enzymes are classified into six major groups. Oxidoreductases: oxidation-reduction reactions. Transferases: group transfer reactions. Hydrolases: hydrolysis reactions. Lyases: addition of groups to a double bond, or removal of groups to create a double bond. Isomerases: isomerization reactions. Ligases: the joining to two molecules. © 2006 Thomson Learning, Inc. All rights reserved 23-4 23 Classification of Enzymes 1. Oxidoreductase: 2. Transferase: 3. Hydrolase: © 2006 Thomson Learning, Inc. All rights reserved 23-5 23 Classification of Enzymes 4. Lyase: 5. Isomerase: 6. Ligase: © 2006 Thomson Learning, Inc. All rights reserved 23-6 23 Terms in Enzyme Chem Apoenzyme: the protein part of an enzyme. Cofactor: a nonprotein portion of an enzyme that is necessary for catalytic function; examples are metallic ions such as Zn2+ and Mg2+. Coenzyme: a nonprotein organic molecule, frequently a B vitamin, that acts as a cofactor. Substrate: the compound or compounds whose reaction an enzyme catalyzes. Active site: the specific portion of the enzyme to which a substrate binds during reaction. © 2006 Thomson Learning, Inc. All rights reserved 23-7 23 Schematic of an Active Site Figure 23.2 Schematic diagram of the active site of an enzyme and the participating components. © 2006 Thomson Learning, Inc. All rights reserved 23-8 23 Terms in Enzyme Chem Activation: any process that initiates or increases the activity of an enzyme. Inhibition: any process that makes an active enzyme less active or inactive. Competitive inhibitor: any substance that binds to the active site of an enzyme thereby preventing binding of substrate. Noncompetitive inhibitor: any substance that binds to a portion of the enzyme other than the active site and thereby inhibits the activity of the enzyme. © 2006 Thomson Learning, Inc. All rights reserved 23-9 23 Enzyme Activity Enzyme activity: a measure of how much a reaction rate is increased. We examine how the rate of an enzyme-catalyzed reaction is effected by: enzyme concentration substrate concentration temperature pH © 2006 Thomson Learning, Inc. All rights reserved 23-10 23 Enzyme Activity Figure 23.3 The effect of enzyme concentration on the rate of an enzyme-catalyzed reaction. Substrate concentration, temperature, and pH are constant. © 2006 Thomson Learning, Inc. All rights reserved 23-11 23 Enzyme Activity Figure 23.4 The effect of substrate concentration on the rate of an enzyme-catalyzed reaction. Enzyme concentration, temperature, and pH are constant. © 2006 Thomson Learning, Inc. All rights reserved 23-12 23 Enzyme Activity Figure 23.5 The effect of temperature on the rate of an enzyme-catalyzed reaction. Substrate and enzyme concentrations and pH are constant. © 2006 Thomson Learning, Inc. All rights reserved 23-13 23 Enzyme Activity Figure 23.6 The effect of pH on the rate of an enzyme- catalyzed reaction. Substrate and enzyme concentrations and temperature are constant. © 2006 Thomson Learning, Inc. All rights reserved 23-14 23 Mechanism of Action Lock-and-key model of enzyme mechanism. The enzyme is a rigid three-dimensional body. The enzyme surface contains the active site. © 2006 Thomson Learning, Inc. All rights reserved 23-15 23 Mechanism of Action Induced-fit model The active site becomes modified to accommodate the substrate. © 2006 Thomson Learning, Inc. All rights reserved 23-16 23 Mechanism of Action Figure 23.9 The mechanism of competitive inhibition. the inhibitor fits into the active site, thereby preventing the substrate from entering. © 2006 Thomson Learning, Inc. All rights reserved 23-17 23 Mechanism of Action Figure 23.10 Mechanism of noncompetitive inhibition. the inhibitor binds to a site other than the active site, thereby changing the conformation of the active site.The substrate no longer fits. © 2006 Thomson Learning, Inc. All rights reserved 23-18 23 Mechanism of Action Figure 23.11 Enzyme kinetics in the presence and absence of inhibitors. © 2006 Thomson Learning, Inc. All rights reserved 23-19 23 Mechanism of Action Both the lock-and-key model and the induced-fit model emphasize the shape of the active site. However, the chemistry of the active site is the most important. Just five amino acids participate in the active sites in more than 65% of the enzymes studies to date. These five are His > Cys > Asp > Arg > Glu. Four these amino acids have either acidic or basic side chains; the fifth has a sulfhydryl group (-SH). © 2006 Thomson Learning, Inc. All rights reserved 23-20 23 Catalytic Power Enzymes provide an alternative pathway for reaction, one with a significantly lower activation energy and, therefore, a faster rate. © 2006 Thomson Learning, Inc. All rights reserved 23-21 23 Enzyme Regulation Feedback control: an enzyme-regulation process where the product of a series of enzyme- catalyzed reactions inhibits an earlier reaction in the sequence. The inhibition may be competitive or noncompetitive. © 2006 Thomson Learning, Inc. All rights reserved 23-22 23 Enzyme Regulation Proenzyme (zymogen): an inactive form of an enzyme that must have part of its polypeptide chain hydrolyzed and removed before it becomes active. An example is trypsin, a digestive enzyme. It is synthesized and stored as trypsinogen, which has no enzyme activity. It becomes active only after a six-amino acid fragment is hydrolyzed and removed from the N-terminal end of its chain. Removal of this small fragment changes in not only the primary structure but also the tertiary structure, allowing © 2006 Thomson Learning, the molecule to achieve its active form. Inc. All rights reserved 23-23 23 Enzyme Regulation Allosterism: enzyme regulation based on an event occurring at a place other than the active site but that creates a change in the active site. An enzyme regulated by this mechanism is called an allosteric enzyme. enzyme Allosteric enzymes often have multiple polypeptide chains. Negative modulation: inhibition of an allosteric enzyme. Positive modulation: stimulation of an allosteric enzyme. Regulator: a substance that binds to an allosteric enzyme. © 2006 Thomson Learning, Inc. All rights reserved 23-24 23 The Allosteric Effect Figure 23.14 The allosteric effect. Binding of the regulator to a site other than the active site changes the shape of the active site. © 2006 Thomson Learning, Inc. All rights reserved 23-25 23 Enzyme Regulation Figure 23.15 Effects of binding activators and inhibitors to allosteric enzymes. © 2006 Thomson Learning, Inc. All rights reserved 23-26 23 Enzyme Regulation Protein modification: the process of affecting enzyme activity by covalently modifying it. The best known examples of protein modification involve phosphorylation/dephosphorylation. Example: pyruvate kinase (PK) is the active form of the enzyme; it is inactivated by phosphorylation to pyruvate kinase phosphate (PKP). © 2006 Thomson Learning, Inc. All rights reserved 23-27 23 Enzyme Regulation Isoenzyme: an enzyme that occurs in multiple forms; each catalyzes the same reaction. Example: lactate dehydrogenase (LDH) catalyzes the oxidation of lactate to pyruvate. The enzyme is a tetramer of H and M chains. H4 is present predominately in heart muscle. M4 is present predominantly in the liver and in skeletal muscle. H3M, H2M2, and HM3 also exist. H4 is allosterically inhibited by high levels of pyruvate while M4 is not. H in serum correlates with the severity of heart attack. © 2006 Thomson Learning, 4 Inc. All rights reserved 23-28 23 Enzymes in Medicine Enzyme assays useful in medical diagnosis. © 2006 Thomson Learning, Inc. All rights reserved 23-29 23 Transition State Analogs Transition state analog: analog a molecule whose shape mimics the transition state of a substrate. © 2006 Thomson Learning, Inc. All rights reserved 23-30 23 Transition State Analogs Absyme: an antibody that has catalytic activity because it was created using a transition state analog as an immunogen. © 2006 Thomson Learning, Inc. All rights reserved 23-31 23 Enzymes End Chapter 23 © 2006 Thomson Learning, Inc. All rights reserved 23-32