Exam 2 Notes Lecture # 3 ? Ch 19: 592-622 Differences between reactions in a livening organism compared to a lab: An organism must maintain physiological conditions of: Constant temperature Solvent pH of about 7.4 Lab conditions can be varied extremely Enzyme: A protein Water-soluble globular protein Powerful and specific biological catalysts *All enzymes are proteins but not all proteins are enzymes Several ways of interacting with reactants Speed up reactions Be controlled 5 Functions of Enzymes: Accelerate the rates of reactions Do not undergo change themselves at the end of the reaction Do not affect the equilibrium point of a reaction Cannot catalyze a reaction that is energetically unfavorable Decrease the time it takes to reach equilibrium by lowering the activation energy *Most enzymes end with ?ASE? Methods used for screening: Electrophoresis DNA sequencing DNA chips Active site: A region where a reaction takes place Specific shape Chemical reactivity needed to catalyze the reaction *What is the relevance of shape to specificity? Very specific requirements on how a reactant comes in and fits with amino acids *Amino acid side chains in the active site; why? Important in interaction with reactants Turnover rate: Rate at which an enzyme works Enzyme Cofactors: Increase the versatility of enzymes Metal cofactors are present at active sites Coenzymes: Specific sub-set of cofactors Non-protein organic molecules 1) Oxidoreductases: Catalyze oxidation-reduction reactions Addition or removal of O or H 2) Transferases: Catalyze transfer of one group from one molecule to another E.g. add or loose phosphate 3) Hydrolases: Catalyze the hydrolysis: the breaking of bonds with addition of water 4) Isomerases: Catalyze the isomerization or rearrangement of atoms within a molecule 5) Lyases: Catalyze addition of a molecule (H20, C02, NH3) across a double bond 6) Ligases: Catalyze the bonding together of two molecules Requires energy in the form of ATP Biocatalysis: Examples: When tea is exposes to oxygen a polyphenoloxidase breaks up polyphenols to darken the leaves 3 Enzyme Models: Lock and key model Induced fit model A substrate enters the active site, and the enzyme changes shape *Enzymes are specific due to amino acids in their active site Hydrolysis of a peptide bond by chymotrypsin: Polypeptide enters active site H transfer Peptide bond is cleaved [ ] = concentration Substrate Concentration: Low substrate concentration rate: Directly proportional to concentration Increasing substrate concentration rate: Drops off with more and more active sites being filled Makes a curve As [sub] rate increases, the enzyme is all used up Enzyme Concentration: In an excess of [substrate] the reaction rate varies directly to [enzyme] Creates a linear, upward line A lot of substrate in enzymes so it is linear Temperature: Reaction rate increases until the point at which the enzyme denatures Unwinds enzyme as the temperature increases Optimal temperature: where active site occurs Bell curve pH: Reaction rate depends on the amino acids involved in the active site and structure Waves Extremozymes: From organisms which live in conditions that would inactivate human enzymes How they work: Chaperonins, proteins which re-nature heat-denatured enzymes Tightly folded, highly hydrophobic cores which resist denaturing A useful test is to measure ENZYME activity to diagnose heart attacks: Measure LDH isozymes Feedback Control: (Enzyme regulation) A B C D turns off Regulates enzyme activity Allosteric Control: (Enzyme regulation) Binding of a molecule at a site (NOT the active site) on the enzyme which affects substrate binding Allosteric site: separate from active site Affects substrate binding (doesn?t have to look like substrate or active site) Negative Allosteric Control: CLOSES active site ?doesn?t work? Positive Allosteric Control: OPENS active site ?does work? Inhibition: Enzyme Regulation Reversible noncompetitive inhibition: The inhibitor does not compete with the substrate for the active site Reversible competitive inhibition: The inhibitor does compete with the substrate for the active site Irreversible inhibition: An inhibitor forms a bond in the active site and prevents the substrate from entering Binds permanently Covalent Modification: Enzyme Regulation Zymogen: an enzyme precursor activated by processing Inactive enzyme that is then activated thru processing (part of the enzyme is removed Activation: 1)Processing 2)Add Phosphate group *Reversible addition of a phosphoryl is a mode of covalent mod. Add a phosphate group to make inactive enzyme change to active Genetic Control: Enzyme Regulation Amount of enzymes Regulate activity affects the supply of the enzymes Regulated by its synthesis Mechanisms of Enzyme Control (SUMMARY): Feedback control Exerted on a reactant by a product thru allosteric control Inhibition Covalent bond: irreversible inhibition Precursors Create an active enzyme Covalent modification Genetic control VITAMINS: Vitamin C: Water-soluble Can form coenzymes Vitamins A, D, E, K: Fat soluble vitamins Stored in body?s fat deposits Excess is hazardous to body because body does not readily eliminate them Antioxidant: Prevents oxidation Vitamin E: Protect cell membranes from damage by free radicals Vitamin C: Because it is water soluble, it is an antioxidant in blood
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