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HL Enzymes! topics 3.6 and 7.6
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There are 2 types of chemical reactions
a) exothermic (exergonic): the amount of energy released is greater than the activation energy Energy released
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b) endothermic (endergonic):
the amount of energy released is less than the activation energy Energy must be supplied
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Biological catalysts Proteins Specific Enzymes!
A substance that speeds up a chemical reaction without being used up in the reaction
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Enzymes ... are catalysts Every reaction involves breaking bonds, that requires energy. The rate of a particular reaction depends on the activation energy necessary to initiate it. Catalysts reduce the activation of energy increasing reaction rates.
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Enzymes ... are specific LOCK AND KEY MODEL
chemist Emil Fischer. Proposed the lock and key model. KEY (substrate) has a specific shape (arrangement of functional groups and other atoms) that allows it and no other key to fit into the LOCK (the enzyme).
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The substrate and enzyme complement each other
Therefore, they can fit together, like a lock and key. Different molecules do not complement the enzyme's active site.
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Enzymes ... are specific INDUCED FIT MODEL
Daniel E. Koshland Jr. modified the lock-and-key model He proposed that binding of the substrate to the enzyme alters the configuration of both, providing a better fit.
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Before binding, the substrate and enzyme do not exactly fit each other
Binding of the substrate to the enzyme changes the configuration of both so that they fit together. Different molecules cannot induce a fit with the enzyme
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Enzymes ... are affected by
CONCENTRATION Increase substrate or enzyme concentration…Molecules more likely to collide…More reactions…Rates of reaction increases…Up to a point where the rxn reaches a maximum – saturation point.
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Enzymes ... are affected by
TEMPERATURE Increasing temperature – Increases molecular movement … more likely for enzyme and substrate molecules to meet… Increases reaction rate. Low temp: H-bonds and interactions that give the enzyme its shape aren’t flexible enough to permit the induced fit for optimum reaction rates High temp: H-bonds and interactions are too weak to maintain enzymes shape due to increased movement of atoms making up the enzyme
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Enzymes ... are affected by
pH Enzymes function at an optimum pH Changing pH results in a change of balance of the hydrogen ion concentration and therefore the balance between positively and negatively charged amino acids making up the enzyme Changes in the charges of the aa’s results in a change in the shape of the enzyme Different enzymes have different optimal pH’s
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Enzymes ... can be denatured
Denaturation = a structural change in a protein that results in a loss (usually permanent) of its biological properties. When exposed to high temperature or extremes of pH
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Inhibition Enzyme activity can be inhibited which slows the rate of reaction Competitive inhibition Noncompetitive inhibition Examples of inhibitors…. Aspirin, Ibuprofin, DDT, Sarin nerve gas, Penicillin Protease inhibitors as a possible cure for AIDS
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Competitive Inhibition
Inhibiting molecule is structurally similar to the substrate It binds to the active site preventing the substrate from binding Enzyme inhibition
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Penicillin This antibiotic binds to active site of glycoprotein peptidase, an enzyme that forms the peptide bonds in peptidoglycan, a component of the bacterial cell wall (so is a competitive inhibitor.) With a weaker cell wall, bacteria are more susceptible to rupture by osmotic lysis and so fail to survive Penicillin G Penicillin V
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Noncompetitive Inhibition
Allosteric Inhibition – Inhibitor molecule binds to enzyme (not the active site) Causes a conformational change to active site Resulting decrease in activity Regulates metabolic pathways thru end product inhibition
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Sarin Nerve Gas Inhibits the enzyme acetyl-cholinesterase, an enzyme in the body that plays a critical role in maintaining nerve function and control. When the enzyme is inhibited, a build-up of acetylcholine occurs at the nerve endings, causing symptoms that include blurred vision, profuse sweating and loss of motor function control. Paralysis many times follows.
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Type of reaction catalyzed
Classification of Enzymes Enzyme Group Type of reaction catalyzed Examples Oxidoreductases Transfer of O & H atoms between substances, ie. all oxidation-reduction reactions Dehydrogenases Oxidases Transferases Transfer of a chemical group from 1 substance to another Transaminases Phophorylases Hydrolases Hydrolysis reactions Peptidases Lipases Phosphatases Lyases Addition or removal of a chemical group other than by hydrolysis Decarboxylases Isomerases The rearrangement of grops within a molecule Mutases Ligases Formation of bonds between 2 molecules using energy derived from the breakdown of ATP Synthestases
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Naming of Enzymes Start with the name of the substrate upon which the enzyme acts, ie. succinate Add the name of the type of the reaction which it catalyzes, ie. dehydrogenation Convert the end of the last word to an –ase suffix, ie. dehydrogenase Thus: succinic dehydrogenase
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