5-423-08HemoglobinStudent - Hemoglobin and Myoglobin...

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Hemoglobin and Myoglobin: Oxygen-binding proteins OBJECTIVES -Be able to compare and contrast the structure, overall function, and oxygen binding properties of hemoglobin and myoglobin. -Be able to draw and interpret oxygen-binding curves -Be able to describe how hemoglobin structure is perfect for the binding and, just as important, the releasing of oxygen -What does allosteric mean? What is cooperativity? -Describe the basis of the Bohr effect; know how CO 2 , H+ interact with hemoglobin -Describe the role of BPG in the function of hemoglobin OUTLINE I Overview Myoglobin, Hemoglobin Oxygen-carrying proteins Provide specific examples of relationship between structure (individual amino acids, functional groups) and function (noncovalent interactions, binding) II Myoglobin in detail Structure Single polypeptide protein One heme Ligand binding in proteins; in general Binding curves Heme group Prosthetic group Porphyrin derivative Four N atoms bind central iron atom Two other Fe orbitals available: in Mb and Hb, one of these binds His of polypeptide, one for binding O2 Held in place in proteins by covalent, noncovalent bonds Surrounding part of protein is nonpolar cleft Oxygen binding in myoglobin Oxygen-binding curve: hyperbolic Steep, low Kd, means high affinity for O2 Reversible binding!!!
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III Hemoglobin in detail Structure Four subunit protein: 2 α , 2 β subunits, which interact through hydrophobic and ionic ints. Four heme groups Oxygen binding in Hemoglobin Oxygen-binding curve: sigmoidal because of allosteric interactions Affinity for O 2 , affected by O 2 itself, and by CO 2 , H+, BPG Reversible binding!!! Cooperativity Quaternary structure of Hb changes with oxygen binding Garden hose analogy Movement of Fe w/in heme at molecular level Changes in subunit interfaces, noncovalent interactions Importance to physiological function of Hb Factors that influence O 2 binding in Hb BPG binding 2,3 Bisphosphoglycerate Negatively charged Fits in central, positively-charged cavity of Hb: stabilizes deoxy form Oxygen binding ejects BPG BPG lowers O 2 affinity; which is good! High altitude increases [BPG] Bohr effect O 2 binding in Hb is affected by H+, CO2 Increased CO 2 = increased H+; carbonic anhydrase helps H+ binds to Hb (not at same place as O 2 ), decreases Hb’s O 2 affinity CO 2 also directly affects O 2 binding; carbamate formation IV Summary -O 2 , H+, CO 2 , BPG all bind to different sites on Hb -These sites communicate with one another, i.e. are allosterically linked by conformational changes that occur within the protein. Allosteric interactions of hemoglobin : 1. O 2 binding to hemoglobin enhances binding of more O 2 . 2. O
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5-423-08HemoglobinStudent - Hemoglobin and Myoglobin...

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