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INTRODUCTION

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OBJECTIVES

After studying this chapter, you should be able to:

  • Describe the most important structural similarities and differences between myoglobin and hemoglobin.

  • Sketch binding curves for the oxygenation of myoglobin and hemoglobin.

  • Identify the covalent linkages and other close associations between heme and globin in oxymyoglobin and oxyhemoglobin.

  • Explain why the physiologic function of hemoglobin requires that its O2-binding curve be sigmoidal rather than hyperbolic.

  • Explain the role of a hindered environment on the ability of hemoglobin to bind carbon monoxide.

  • Define P50 and indicate its significance in oxygen transport and delivery.

  • Describe the structural and conformational changes in hemoglobin that accompany its oxygenation and subsequent deoxygenation.

  • Explain the role of 2,3-bisphosphoglycerate (BPG) in oxygen binding and delivery.

  • Outline the role of hemoglobin in CO2 and proton transport, and describe accompanying changes in the pKa of the relevant imidazolium group.

  • Describe the structural consequences to HbS of lowering pO2.

  • Identify the metabolic defect that occurs as a consequence of α- and β-thalassemias.

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BIOMEDICAL IMPORTANCE

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The efficient delivery of oxygen from the lungs to the peripheral tissues and the maintenance of tissue reserves to protect against anoxic episodes are essential to health. In mammals, these functions are performed by the homologous heme proteins hemoglobin and myoglobin, respectively. Myoglobin, a monomeric protein of red muscle, binds oxygen tightly as a reserve against oxygen deprivation. The multiple subunits of hemoglobin, a tetrameric protein of erythrocytes, interact in a cooperative fashion that enables this transporter to offload a high proportion of bound O2 in peripheral tissues while simultaneously retaining the capacity to bind it efficiently in the lungs. In addition to delivering O2, hemoglobin scavenges the waste products of respiration, CO2 and protons, for transport to and ultimate disposal in the lungs. Oxygen delivery is enhanced by the binding of 2,3-bisphosphoglycerate (BPG), which stabilizes the quaternary structure of deoxyhemoglobin. Hemoglobin and myoglobin illustrate both protein structure–function relationships and the molecular basis of genetic disorders such as sickle cell disease and the thalassemias. Cyanide and carbon monoxide kill because they disrupt the physiologic function of the heme proteins cytochrome oxidase and hemoglobin, respectively.

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HEME & FERROUS IRON CONFER THE ABILITY TO STORE & TO TRANSPORT OXYGEN

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Myoglobin and hemoglobin contain heme, a cyclic tetrapyrrole consisting of four molecules of pyrrole linked by methyne bridges. This planar network of conjugated double bonds absorbs visible light and colors heme deep red. The substituents at the β-positions of heme are methyl (M), vinyl (V), and propionate (Pr) groups arranged in the order M, V, M, V, M, Pr, Pr, M (Figure 6–1). The atom of ferrous iron (Fe2+) resides at the center of the planar tetrapyrrole. Other proteins with metal-containing tetrapyrrole prosthetic groups include the cytochromes (Fe and Cu) and chlorophyll (Mg) (see Chapter 31). Oxidation and reduction of ...

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