Enzymes PDF

ENZYMES  Classification of enzymes 1. Oxidoreductases 2. Transferases 3. Hydrolases 4. Lyases 5. Isomerases 6. Ligases 7. Translocases 1. Oxidoreductases catalyze oxidation/reduction reactions; transfer of H and O atoms or electrons from one substance to...

ENZYMES  Classification of enzymes 1. Oxidoreductases 2. Transferases 3. Hydrolases 4. Lyases 5. Isomerases 6. Ligases 7. Translocases 1. Oxidoreductases catalyze oxidation/reduction reactions; transfer of H and O atoms or electrons from one substance to another 2. Transferases catalyze transfer of a functional group from one substance to another; the group may be methyl-, acyl-, amino- or phosphate group 3. Hydrolases catalyze formation of two products from a substrate by hydrolysis peptidase 4. Lyases catalyze non-hydrolytic addition or removal of groups from substrates; C-C, C-N, C-O or C-S bonds may be cleaved 5. Isomerases catalyze intramolecule rearrangement of atoms 6. Ligases (synthetases) join together two molecules by synthesis of new C-O, C-S, C-N or C-C bonds with simultaneous breakdown of ATP 7. Translocases catalyze the movement of ions or molecules across mambranes or their separation within membranes  Classification of enzymes according to localization of activity Intracellular enzymes (nonfunctional plasma enzymes) Extracellular enzymes – Secretory enzymes (functional plasma enzymes) – Excretory enzymes (nonfunctional plasma enzymes)  Diagnostic value of enzymes (examples) Amylase Lipase Alanine aminotransferase Aspartate aminotransferase Alkaline phosphatase Creatine kinase (MM, BB, MB)  Action of enzymes Without enzyme substrates (reactants) need a lot of potential energy to reach a transition state, which then decays into products. The enzyme stabilizes the transition state, reducing the energy needed to form products.  Enzyme specificity 1. Reaction specificity 2. Substrate specificity  Enzyme – substrate interaction A „lock and key” model Emil Fisher 1894 Both the enzyme and the substrate possess specific complementary geometric shapes that fit exactly into one another  Induced fit model of enzyme-substrate interaction Daniel Koshland 1958 Structure of the active site The induced conformational change in hexokinase The effect of substrate concentration on reaction velocity Competitive inhibition of enzyme activity Effects of competitive inhibition on enzyme kinetics Noncompetitive inhibition Effects of noncompetitive inhibition on enzyme kinetics Zymogen activation by proteolytic cleavage  Covalent modifications that control the activities of enzymes