lecture_12legge - The Backbone structure of Myoglobin 2...

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1 Globins Lecture 10/01/2009 The Backbone structure of Myoglobin 2 Myoglobin: 44 x 44 x 25 Å single subunit 153 amino acid residues 121 residues are in an a helix. Helices are named A, B, C, …F. The heme pocket is surrounded by E and F but not B, C, G, also H is near the heme. Amino acids are identified by the helix and position in the helix or by the absolute numbering of the residue. By introducing steric hindrance on one side of the heme plane interaction can be prevented and oxygen binding can occur. Role of the Globin Modulate oxygen binding affinity Make reversible oxygen binding possible Helix E Fe 3+ - O O - Fe 3+ A heme dimer is formed which leads to the formation of Fe(III) Helix F Distal His Proximal His The Heme group Each subunit of hemoglobin or myoglobin contains a heme. - Binds one molecule of oxygen - Heterocyclic porphyrin derivative - Specifically protoporphyrin IX The heme prosthetic group in Mb ad Hb: protoporphyrin IX + Fe(II) The iron must be in the Fe(II) form or reduced form (ferrous oxidation) state. The Heme complex in myoglobin E7 F8
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2 Hemoglobin Spherical 64 x 55 x 50 Å two fold rotation of symmetry α and β subunits are similar and are placed on the vertices of a tetrahedron. There is no D helix in the α chain of hemoglobin. Extensive interactions between unlike subunits α 2- β 2 or α 1- β 1 interface has 35 residues while α 1- β 2 and α 2- β 1 have 19 residue contact. Oxygenation causes a considerable structural conformational change Oxygenation rotates the α 1 β 1 dimer in relation to α 2 β 2 dimer about 15° The conformation of the deoxy state is called the T state The conformation of the oxy state is called the R state individual subunits have a t or r if in the deoxy or oxy state. What causes the differences in the conformation states? The Fe iron is about 0.6 Å out of the heme plane in the deoxy state. When bi d it ll th The positive cooperativity of O 2 binding to Hb The effect of the ligand-binding state of one heme on the ligand-binding affinity of another. oxygen binds it pulls the iron back into the heme plane. Since the proximal His F8 is attached to the Fe this pulls the complete F helix like a lever on a fulcrum. Binding of the O 2 on one heme is more difficult but its binding causes a shift in the α 1- β 2 (& α 2- β 1) contacts and moves the distal His E7 and Hemoglobin structure β -monomers are related by 2-fold symmetry (same is true for α ) Note changes in structure: between β -monomers – see big double-headed arrows at points of contact – see small arrows DeoxyHb Val E11 out of the oxygen’s path to the Fe on the
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lecture_12legge - The Backbone structure of Myoglobin 2...

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