CHAPTER 6 (Figures) - David L Nelson and Michael M Cox...

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1 Lehninger Principles of Biochemistry Fourth Edition Chapter 6: Enzymes Copyright  ©  2004 by W. H. Freeman & Company David L. Nelson and Michael M. Cox
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2 Introduction Living things must Self-replicate Catalyze chemical reactions efficiently and selectively. With exceptions of ribozymes , all enzymes are proteins. The catalytic activity of enzymes depends on the primary, secondary, tertiary and quaternary structures. Some proteins require either or both Cofactors –inorganic ions (Fe 2+ , Mg 2+ , Mn 2+ , Zn 2+ ) [ TABLE 6-1 ] Coenzymes – complex metalloorganic molecules ( Table 6-2 ). Holoenzymes – complete, catalytically active enzyme together with its bound coenzyme or cofactor. Apoenzyme or apoprotein - protein part of holoenzyme. Many enzymes have been named by adding the sufix “-ase ” to the name of their substrate or a to a word or phrase describing their activity. For Example : Urease – catalyzes hydrolysis of urea DNA polymerase –catalyzes polymerization of nucleotides to form DNA.
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5 Introduction Due to ambiguities (trypsin, pepsin) and increasing number of newly discovered enzymes, an international classification system has been developed which classifies enzymes into SIX major classes (TABLE 6-3) . For Example : ATP + D-glucose ADP + D-glucose –6-P Systematic name is ATP:glucose phosphotransferase Enzyme commission number (E.C. number) is 2.7.1.1 2 – class name –transferase 7- the subclass – phosphotransferase 1- Phosphotransferase with hydroxyl group as the acceptor 1- D-glucose as the phosphoryl-group acceptor.
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7 How Enzyme Works Enzymes speed up biological reactions by providing specific environment with which a given reaction are energetically more favored . Active site – a pocket on an enzyme where enzyme-catalyzed reaction occurs. Substrate – Molecule bound to the active site which is acted upon by enzyme. Enzymes affect the rate of the reaction by lowering the activation energy – but no effect on equilibrium . When several steps occur in a reaction, the overall rate is determined by the step (or steps) with the highest activation energy- hence rate- limiting step . The catalytic power and specificity of enzymes depend on the binding energy (energy derived from enzyme-substrate interactions). Binding energy is a major source of free energy used by enzymes to lower activation energy. Two
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8 How Enzyme Works Two fundamental principles governs the catalysis and specificity of enzymes: Much of the catalytic power of enzymes is ultimately derived from free energy released by forming multiple weak bonds and interactions between an enzyme and its substrate. This binding energy contributes to specificity as well as catalysis .
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