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Chapter 7 Hemoglobin: Portrait of a Protein in Action

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Chapter 7 Hemoglobin: Portrait of a Protein in Action Chapter 7 Hemoglobin: Portrait of a Protein in Action Hemoglobin gives blood its red color responsible for the transport of oxygen Hemoglobin was one of the first proteins to have its structure determined 1 The transition from anaerobic to aerobic life was a major step in evolution because it uncovered a rich reservoir of energy. Fifteen times as much energy is extracted from glucose in the presence of oxygen than in its absence. For single-celled and other small organisms, oxygen can be absorbed into actively metabolizing cells directly from the air or surrounding water. Vertebrates evolved two principal mechanisms for supplying their cells with an adequate supply of oxygen. The first is a circulatory system that actively delivers oxygen to cells throughout the body. The second is the use of the oxygen-transport and oxygen-storage proteins, hemoglobin and myoglobin. Hemoglobin, which is contained in red blood cells, is a fascinating protein, efficiently carrying oxygen from the lungs to the tissues while also contributing to the transport of carbon dioxide and hydrogen ions back to the lungs. Myoglobin, located in muscle, provides a reserve supply of oxygen available 2in time of need. These two evolutionarily related proteins employ nearly identical structures for oxygen binding (Chapter 6). Hemoglobin is able to use as much as 90% of its potential oxygen-carrying capacity. Myoglobin would be able to use only 7% of its potential capacity. What accounts for this dramatic difference? Myoglobin exists as a single polypeptide, Hemoglobin comprises four polypeptide chains. The four chains in hemoglobin bind oxygen cooperatively, binding of oxygen in one chain increases the likelihood that the remaining chains will bind oxygen. Furthermore, the oxygen-binding properties of hemoglobin are modulated by the binding of hydrogen ions and carbon dioxide in a manner that enhances oxygen- carrying capacity. Both cooperativity and the response to modulators are made possible by variations in the quaternary structure of hemoglobin when different combinations of 3 molecules are bound. Hemoglobin and myoglobin have played important roles in the history of biochemistry. They were the first proteins for which three-dimensional structures were determined by X-ray crystallography. 1962 Chemistry Furthermore, the possibility that variations in protein sequence could lead to disease was first proposed and demonstrated for sickle-cell anemia, a blood disease caused by a change in a single amino acid in one hemoglobin chain. Hemoglobin has been and continues to be a valuable source of knowledge and insight, both in itself and as a prototype for many other proteins that we will encounter throughout our study of biochemistry. 4 7.1 Myoglobin and Hemoglobin Bind Oxygen at Iron Atoms in Heme Sperm whale myoglobin was the first protein for which the three-dimensional structure was determined. X-ray crystallographic studies pioneered by John Kendrew revealed the structure of this protein in the 1950s (Figure 7.1). Myoglobin consists largely of α helices that are linked to one another by turns to form a globular structure. Myoglobin can exist in an oxygen-free form called deoxymyoglobin or in a form with an oxygen molecule bound called oxymyoglobin. The ability of myoglobin, and hemoglobin as well, to bind oxygen depends on the presence of a bound prosthetic group called heme. The heme group gives muscle and blood 5 their distinctive red color. Heme consists of an organic component and a central iron atom. The organic component, called protoporphyrin, is made up of four pyrrole rings linked by methine bridges to form a tetrapyrrole ring. Four methyl groups, two vinyl groups, and two propionate side chains are attached. The iron atom lies in the center of the protoporphyrin, bonded to the four pyrrole nitrogen atoms. Although the heme-bound iron can be in either the ferrous (Fe2+) or ferric (Fe3+) oxidation states, only the Fe2+ state is capable of binding oxygen. The iron ion can form two additional bonds, one on each side of the heme plane. These binding sites are called the fifth and sixth coordination sites. Fifth coordination site is occupied by a histidine residue from the protein. This histidine is referred to as the proximal histidine. 6 Oxygen binding occurs at the sixth coordination site. In deoxymyoglobin, the sixth coordination site remains unoccupied. The iron ion is slightly too large to fit into the well-defined hole within the porphyrin ring; it lies approximately 0.4 Å outside the porphyrin plane (Figure 7.2, left). Binding of the oxygen molecule at the sixth coordination site of the iron ion substantially rearranges the electrons within the iron so that the ion becomes effectively smaller, allowing it to move into the plane of the porphyrin (Figure 7.2, right). proximal histidine 7 Changes in heme electronic structure upon oxygen binding are the basis for functional imaging studies The change in electronic structure that occurs when the iron ion moves into the plane of the porphyrin is paralleled by alterations in the magnetic properties of hemoglobin; these changes are the basis for functional magnetic resonance imaging (fMRI), one of the most powerful methods for examining brain function. MRI detects signals that originate primarily from the protons in water molecules and are altered by the magnetic properties of hemoglobin. With the use of appropriate techniques, images can be generated that reveal differences in the relative amount of deoxy- and oxyhemoglobin and thus the relative activity of various part of the brain. When a specific part of the brain is active, blood vessels relax to allow more blood flow to that region. Thus more-active region of the brain will be richer in oxyhemoglobin. These noninvasive methods identify area of the brain that 8 process sensory information. The Structure of Myoglobin Prevents the Release of Reactive Oxygen Species Oxygen binding to iron in heme is accompanied by the partial transfer of an electron from the ferrous ion to oxygen. In many ways, the structure is best described as a complex between ferric ion (Fe3+) and superoxide anion (O2-), as illustrated in Figure 7.4. It is crucial that oxygen, when it is released, leaves as dioxygen rather than superoxide, for two important reasons First, superoxide itself another species that can be generated from it are reactive oxygen species that can be damaging to many biological materials. Second, the release of superoxide leaves the iron ion in the ferric state. This species, termed metmyoglobin, does not bind oxygen. Thus, potential oxygen- storage capacity is lost. 9 Features of myoglobin stabilize the oxygen complex such that superoxide is less likely to be released. In particular, the binding pocket of myoglobin includes an additional histidine residue (termed the distal histidine) that donates a hydrogen bond to the bound oxygen molecule (Figure 7.5). The superoxide character of the bound oxygen species strengthens this interaction. Thus, the protein component of myoglobin controls the intrinsic reactivity of heme, making it more suitable for reversible oxygen binding. 10 Human Hemoglobin Is an Assembly of Four Myoglobin-like Subunits The three-dimensional structure of hemoglobin from horse heart was solved by Max Perutz shortly after the determination of the myoglobin structure. Since then, the structures of hemoglobins from other sources including human beings have been determined. Hemoglobin consist of four polypeptide chains, two identical α chains and two identical β chains (Figure 7.6). The recurring structure is called a globin fold. Consistent with this structural Hemoglobin A (HbA) similarity, the amino acid sequences of the α1 β1 α2 β2 α and β chains of human hemoglobin are readily aligned with the amino acid sequence of sperm whale myoglobin with 25% and 24% identity, respectively, and good conservation of key residues such as the proximal and distal histidine residues. Thus, the α and β chains are related to each other and to myoglobin by divergent evolution. 11 7.2 Hemoglobin Binds Oxygen Cooperatively Mb+O2 MbO2 Oxygen-binding curve: a plot of the fractional saturation versus the concentration of oxygen. pO The fractional saturation, Y, is defined as the Y = 2 fraction of possible binding sites that contain ( pO2 + P50 ) bound oxygen. The concentration of oxygen is measured by its partial pressure, pO2 For myoglobin, a binding curve indicating a simple chemical equilibrium is observed (Figure 7.7). Mb+O2 MbO2 Notice that the fraction of myoglobin with bound oxygen rises sharply as pO2 increases and then levels off. Half-saturation (P50 for 50% saturated) is at the relatively low value of 2 torr (mmHg), indicating high affinity 12 In contrast, the oxygen-binding curve for hemoglobin in red blood cells shows some remarkable features (Figure 7.8). pO n Y = 2 ( pO n P n ) It does not look like a simple binding curve such as 2 + 50 that for myoglobin; instead, it resembles an "S." Such curves are referred to as sigmoid because of their S-like shape. Hb+nO2 Hb(O2)n Notice that oxygen binding is significantly weaker than that for myoglobin. Half-saturation is at the higher value of P50 = 26 torr. A sigmoid binding curve indicates that a protein shows a special binding behavior. The shape of the curve reveals that the binding of oxygen at one site within the hemoglobin molecule increases the likelihood that oxygen binds at the remaining unoccupied sites. Conversely, the unloading of oxygen at one heme facilitates the unloading of oxygen at the others. This sort of binding behavior is cooperative, because the binding reactions at individual sites in each hemoglobin molecule are not independent of one another. 13 What is the physiological significance of the cooperative binding of oxygen by hemoglobin? Oxygen must be transported in the blood from the lungs, where the partial pressure of oxygen is relatively high (approximately 100 torr), to the actively metabolizing tissues, where the partial pressure of oxygen is much lower (typically, 20 torr).
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