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chapter 5 students - From Protein Structure to Function...

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From Protein Structure to Function 1. Hemoglobin and myoglobin: Principles of reversible ligand binding 1. (Antibodies: Principles of specific, high affinity ligand binding) 2. Myosin and actin: Protein activity modulated by ATP 3. Enzymes 1 MCDB310 – Chapter 5: Protein Function
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Ligand Binding usually transient and reversible interaction with others molecule(= ligands) such as metals, hormones often involves “ molecular breathing ” of theprotein, i.e. ability to undergo small conformational changes often induces molecular rearrangements in theprotein ligand binding sites are - highly conserved - complementary in size, shape, and charge 2 MCDB310 – Chapter 5: Protein Function
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model proteins to study reversibleligand binding Mb: First protein whose structurewas solved (1958) very important for oxygen binding in muscle(Mb) and oxygen transport in blood (Hb) metal ions (Fe, Co) aregood oxygen binders, but freemetal ions are very reactive O 2 binding component in Mb and Hb is heme -protoporphyrine ring with a central Fe 2+ , which reversibly binds oxygen 3 MCDB310 – Chapter 5: Protein Function Myoglobin (Mb) & Hemoglobin (Hb)
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prosthetic group of Mb and Hb incorporated into Hb and Mb during folding responsiblefor reversible O 2 binding responsiblefor red color of blood and muscles 4 MCDB310 – Chapter 5: Protein Function Heme
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4 pyrrolerings linked by methenebridges (= protoporphyrine) pyrrole ring X: additional functional groups methenebridge 5 MCDB310 – Chapter 5: Protein Function Heme – Basic Structure
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central Fe 6 MCDB310 – Chapter 5: Protein Function Heme – Structure 2 vinyl groups (buried in protein) 4 methyl groups 2 propionate groups (exposed)
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Fe + + Fe 2+ has 6 coordination sites (4 with N of pyrrole rings, and 2 perpendicular to ring system) Hb: 5 th coordination site is occupied with proximal His 6 th coordination site: O 2 oxyhemoglobin none deoxyhemoglobin CO carboxyhemoglobin 7 MCDB310 – Chapter 5: Protein Function Heme – Iron Coordination
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8 MCDB310 – Chapter 5: Protein Function Heme – Binding of CO vs. O 2 free heme binds C0 10 5 times better than O 2 kinked binding topology in Mb favors O 2 100fold
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Myoglobin (Mb) primarily found in muscle(highly abundant in marine mammals such as whales) singlepolypeptide(153 aa) with onebound heme very simpleoxygen binder: Binds oxygen at high pO 2 , releases it at low pO 2 Mb + O 2 MbO 2 typical globin fold 9 MCDB310 – Chapter 5: Protein Function
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8 helices (A-H) and loops in between 10 MCDB310 – Chapter 5: Protein Function The Globin Fold
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Binding/Association Constant K a Quantitatively describes the affinity of a protein P for its ligand L P + L PL the higher the binding affinity, thehigher K a 11 MCDB310 – Chapter 5: Protein Function [L] [P] ] PL [ = a K
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