RelativeResourceManager.pdf1 - Chapter 4 Enzymes...

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Unformatted text preview: Chapter 4 Enzymes Enzymes •  Special types of receptors •  Most are soluble and found in cytosol of cells •  Proteins ­or polypep<des (polymers of amino acids) valine his<dine leucine proline threonine glutamic acid hAp:// ­2148/5/22/figure/F4 Enzymes (cont.) •  Form enzyme ­substrate complexes •  Catalyze reac<ons •  Release products (rather than intact ligands) hAp:// hAp:// A. What are enzymes? •  Naturally ­occurring proteins that catalyze rxns. •  Can be crystallized to obtain a crystal structure •  Can be sequenced to determine amino acid sequence •  Molecular masses of 1,000s to 1,000,000s •  Turnover numbers ­ # of molecules of substrate converted to product per <me Enzymes work by lowering the ac<va<on energy for a reac<on. Ac<ve site:  ­site for binding and catalysis  ­cle\s and crevices  ­three ­dimensional hAp://<ve.html Enzymes must be able to: 1) recognize a substrate (specificity) 2) catalyze a rxn of the substrate (rate accelera<on) hAp://www.bioinforma< hAp://www.chemicalconnec< B. Specificity 1)  Binding (same as Chap. 3) covalent ionic H ­bonding charge ­transfer hydrophobic van der Waals Maximum binding occurs at the transi<on state. Enzymes are a) absolute (only one substrate can bind) example: lactase b) broad (many substrates can bind) example: cytochrome P450 2) Reac<vity (specificity) Acidic, basic and nucleophilic func<onal groups in the amino acid side chains can show specificity for certain protons. Trypsin (in blue) actually hydrolyzes proteins hAp:// ­<onen/averhoff ­petra ­2006 ­05 ­24/HTML/chapter1.html C. Mechanisms 1)  Approxima<on  ­proximity  ­enzyme is a template holding substrates close to each other hAp:// 2)  Covalent Catalysis  ­an amino acid side chain (usually a nucleophile) forms a covalent bond with the substrate. This allows a second substrate to react with the E•S intermediate. hAp:// 3) General acid ­base catalysis  ­acids or bases, other than H+ or OH ­, accelerate the rate 4)  Electrosta<c catalysis  ­the correct charges stabilize the transi<on state hAp:// Lipid binding Lipids are new sources for cancer, arthri<s and heart disease therapeu<cs. Red ­nega<ve areas Blue ­posi<ve areas hAp:// 5)  Desolva<on  ­removal of water forces charged groups to aAract hAp:// 6)  Strain or distor<on  ­aArac<on of the enzyme to the transi<on state is greater than that of the substrate itself Hexokinase and D ­glucose Lehninger Biochemistry (Nelson & Cox) Fig. 8 ­21 D. Enzymes as drugs •  Enzyme therapy •  Usually catalyze hydrolysis rxns •  Examples: –  Stomach diges<on –  Blood clot diges<on –  Necro<c <ssue diges<on –  Lactose intolerance Urukinase aids in dissolving blood clots. hAp:// ­nk ­blood ­clot.jpg ...
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