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Biochemistry Sixth Edition Berg • Tymoczko • Stryer Chapter 7: Hemoglobin: Portrait of a Protein in Action Copyright © 2007 by W. H. Freeman and Company Erythrocytes (Red cells) Hemoglobin and Myoglobin • These are conjugated proteins. A simple protein has only a polypeptide chain. A conjugated protein has a non-protein part in addition to a polypeptide component. Both myoglobin and hemoglobin contain heme. • Myoglobin 17000 daltons (monomeric) 153 amino acids • Hemoglobin - 64500 daltons ( tetrameric) a-chain has 141 amino acids b-chain has 146 amino acids Hemoglobin O2 carrying capability • Erythrocytes/ml blood: 5 billion • Hemoglobin/red cell: ( 5 x 109 ) 280 million ( 2.8 x 108 ) • O2 molecules/hemoglobin: 4 • O2 ml blood: (5 x 109)(2.8 x 108)(4) = (5.6 x 1018) or (5.6 x 1020) molecules of O2/100 ml blood Myoglobin, monomeric Aromatic Heme Iron in Hemoglobin binding O2 Hemoglobin, a2b2 tetramer O2 binding: Hemoglobin & Myoglobin P50 = 2 torr P50 = 26 torr O2 transport capability, a comparison Resting state vs exercise O2 Binding Changes 4o Structure Decreasing O2 affinity 2,3-bisphosphoglycerate (2,3-BPG) • Lowers the affinity of oxygen for Hemoglobin 2,3-bisphosphoglycerate (2,3-BPG) The binding pocket for BPG contains 4 His and 2 Lys Binding of bisphosphoglycerate Oxygen Affinity of Fetal Red Blood Cells Fetal red blood cells have a higher oxygen affinity than that of maternal red blood cells because fetal hemoglobin does not bind 2,3-BPG as well as maternal hemoglobin does The Bohr Effect Bohr Effect: • Lowering the pH decreases the affinity of oxygen for Hb Loss of O2 from Hemoglobin Carbamate: • CO2 combines with NH2 at the N-terminus of globins Carbamate formation Covalent binding at the N-terminus of each subunit Sickle Cell due to Glu 6 Val 6