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Enzymes
 

Enzymes

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Bettleheim

Bettleheim

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Enzymes Enzymes Presentation Transcript

  • Chapter 23: Enzymes Chem 104 K. Dunlap
  • Enzymes Ribbon diagram of cytochromecoxidase, the enzyme that directly uses oxygen during respiration.
  • 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 stereoselective; 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 catalyzes hydrolysis of peptide bonds formed by the carboxyl groups of Lys and Arg.
  • Enzyme Catalysis Trypsincatalyzes the hydrolysis of peptide bonds formed by the carboxyl group of lysine and arginine.
  • 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 according to the type of reaction catalyzed: – – – – Oxidoreductases: Oxidation-reduction reactions. Transferases: Group transfer reactions. Hydrolases: Hydrolysis reactions. Lyases: Addition of two groups to a double bond, or removal of two groups to create a double bond. – Isomerases:Isomerization reactions. – Ligases: The joining to two molecules.
  • Classification of Enzymes 1.Oxidoreductase: 2. Transferase: 3. Hydrolase:
  • Classification of Enzymes 4. Lyase: COO CH2 C-COOCH + H2O Aconitase COOCH2 C-COOHO C-H - COO- COO cis-Aconitate Isocitrate 5. Isomerase: CH2 OPO3 2Phosphohexose O isomerase OH OH HO OH a-D- Glucose-6-phosphate 6. Ligase: ATP + L-tyrosine + t-RNA CH2 OPO3 2O H HO H H HO CH2 OH OH a-D-Fructose-6-phosphate Tyrosine-tRNA synthetase L-tyrosyl-tRNA + AMP + PPi
  • Enzyme Terminology 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.
  • Schematic of an Active Site Schematic diagram of the active site of an enzyme and the participating components.
  • Terms in Enzyme Chemistry 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: A 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.
  • Enzyme Activity Enzyme activity: A measure of how much a reaction rate is increased. We examine how the rate of an enzymecatalyzed reaction is affected by: – Enzyme concentration. – Substrate concentration. – Temperature. – pH.
  • The effect of enzyme concentration on the rate Substrate concentration, temperature, and pH are constant.
  • The effect of substrate concentration on the rate Enzyme concentration, temperature, and pH are constant.
  • The effect of temperature on the rate Substrate and enzyme concentrations and pH are constant.
  • The effect of pH on the rate Substrate and enzyme concentrations and temperature are constant.
  • Lock-and-key model - The enzyme is a rigid three-dimensional body. – The enzyme surface contains the active site.
  • Induced Fit The active site becomes modified to accommodate the substrate.
  • Competitive Inhibition When a competitive inhibitor enters the active site, the substrate cannot enter.
  • Noncompetitive Inhibition The inhibitor binds itself to a site other than the active site (allosterism), thereby changing the conformation of the active site. The substrate still binds but there is no catalysis.
  • Enzyme kinetics in the presence and the absence of inhibitors.
  • 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 site in more than 65% of the enzymes studied to date. – These five are His > Cys > Asp > Arg > Glu. – Four of these amino acids have either acidic or basic side chains; the fifth has a sulfhydryl group (-SH).
  • Catalytic Power • Enzymes provide an alternative pathway for reaction. (a) The activation energy profile for a typical reaction. (b) A comparison of the activation energy profiles for a catalyzed and uncatalyzed reactions.
  • Enzyme Regulation Feedback control: An enzyme-regulation process where the product of a series of enzymecatalyzed reactions inhibits an earlier reaction in the sequence. – The inhibition may be competitive or noncompetitive.
  • 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 not only the primary structure but also the tertiary structure, allowing the molecule to achieve its active form.
  • 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. – 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.
  • Enzyme Regulation • The allosteric effect. Binding of the regulator to a site other than the active site changes the shape of the active site.
  • Enzyme Regulation Effects of binding activators and inhibitors to allosteric enzymes. The enzyme has an equilibrium between the T form and the R form.
  • 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: Pyruvatekinase (PK) is the active form of the enzyme; it is inactivated by phosphorylation to pyruvatekinase phosphate (PKP).
  • Enzyme Regulation Isoenzyme (Isozymes): 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. – H4 in serum correlates with the severity of heart attack.
  • Enzyme Regulation The isozymes of lactate dehydrogenase (LDH). The electrophoresis gel depicts the relative isozyme types found in different tissues.
  • Enzymes Used in Medicine Insert Table 23.2, page 648
  • Transition-State Analogs • Transition state analog: A molecule whose shape mimics the transition state of a substrate. • Figure 23.17 The prolineracemase reaction. Pyrrole-2-carboxylate mimics the planar transition state of the reaction (next screen).
  • Transition-State Analog
  • Transition-State Analogs • Abzyme: An antibody that has catalytic activity because it was created using a transition state analog as an immunogen. (a) The molecule below is a transition analog for the reaction of an amino acid with pyridoxal-5’-phosphate. (b) The abzyme is then used to catalyze the reaction on the next screen.
  • Transition-State Analogs