Symposium on a Physiologic Approach to Critical Care

The Oxyhemoglobin Dissociation Curve in Acute Disease Rita McConn, PhD. *

While clinicians have always regarded the red cell as an important link in the oxygen transport chain, they tend to evaluate it in terms of the concentration of hemoglobin. From the therapeutic viewpoint, comparatively little attention has been paid to the physiologic significance of the position of the dissociation curve other than those changes induced by the patient's condition, such as the presence of fever or an acidotic or alkalotic pH. In 1967, it was found that the organic phosphate, 2,3 diphosphoglyceric acid, could influence the affinity of oxygen for hemoglobin. 20 ,34 Three years later, Perutz described the structural changes which occur in the hemoglobin molecule upon oxygenation.95 These findings initiated a new look at the role of the dissociation curve in oxygen transport and unloading by the red cell in health and disease. An important aspect of this "new interest" in the dissociation curve was the re-evaluation of current methods of blood storage in terms of the respiratory function of the blood. This article aims to review these recent findings in terms of their relevance to oxygen transport in the acutely ill patient. The oxyhemoglobin dissociation curve is an expression of the reaction of oxygen with hemoglobin in terms of the per cent saturation of hemoglobin versus the partial pressure of oxygen. Classically one is taught that the oxyhemoglobin dissociation curve has a sigmoid shape as first described by Bohr in 1892.25 It is only with the advent of sophisticated techniques in comparatively recent times that the structure of hemoglobin has been unravelled and the'S-shaped dissociation curve explained in terms of the change in the stereochemistry of the respiratory protein on oxygenation. Proteins have a primary structure, i.e., amino acid composition and order of sequence, which is determined genetically, while the secondary and tertiary structure are influenced by both the intrinsic components and the environment. Human hemoglobin consists of two unlike pairs of polypeptide chains, each chain containing one binding site for oxygen, the heme group. Over 90 per cent of the hemoglobin in adult erythrocytes is hemoglobin A, which is composed of two alpha chains and two "'Associate Professor of Surgery, Albert Einstein College of Medicine, Bronx, New York

Surgical Clinics of North America- Vol. 55, No.3, June 1975

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Figure 1. Tertiary structure of the '" chain of human hemoglobin A. A, B, C, D, E, F, G, and H indicate the alphabetical parts of the molecule. The porphyrin ring is shown as a disk. The oxygen molecule is bound between the disk and the E-helix. (Figs. 1 and 2 reproduced from Rorth, M.: Hemoglobin Interactions and Red Cell Metabolism, Series Haematologica V, 1972. Used with permission.)

beta chains. In adult man, hemoglobin A 2, two alpha and two delta chains, accounts for only about 2.5 per cent while there is less than 1 per cent of hemoglobin F, two alpha and two gamma chains. The alpha chain has 141 amino acids and the beta chain 146 amino acids, and in 1961 the primary structure of these chains was determined. 27,67 The secondary structure of these chains has a relatively large proportion of alpha helices-there are seven distinct segments of 7 to 21 amino acids in the alpha helix conformation in the alpha chains (named A, B, C, E, F, G, and H), and eight segments of alpha helix in the beta chains (named A, B, C, D, E, F, G, and H). The helices are linked by stretches of amino acids in a random coil arrangement. The helical segments have clearly defined position in space-the tertiary structure-thus, e.g., the A helix is close to the G and H helices. The E and F helices form a hydrophobic cleft into which the heme moiety is inserted (Fig. 1). The spatial relationship of the four subunit chains is termed the quaternary structure of hemoglobin. The fact that oxyhemoglobin crystals were different in shape from crystals of reduced hemoglobin, the former being needle-shaped and the latter hexagonal,57 gave the first clue that the reaction of hemoglobin with oxygen is accompanied by a change in the structure of the hemoglobin on oxygenation.85 Perutz proposed that combination of an oxygen molecule with a heme group alters the position of the ferrous ion in the heme ring. This in turn triggers a series of physicochemical events which alters the position of the peptide chain such that the salt bridge with a neighboring chain is broken, and this results in a change of the oxygen affinity of the heme group in this neighboring subunit chain. Each combination of an oxygen molecule with a heme group thus facilitates the binding of the next one. Studies suggest that on oxygenation of the third heme, the quaternary structure of hemoglobin changes. 54 ,115 These molecular events are the cause of the changing oxygen affinity of hemoglobin on oxygenation and cause the dissociation curve to have a sigmoid shape. In addition to the shape, the position of the dissociation curve is also an indicator of the affinity of oxygen for hemoglobin. Figure 2 shows that a monomeric hemoglobin has a much higher affinity for oxygen than a tetrameric hemoglobin. Thus the quaternary structure of the molecule superimposes constraints on the monomers and, according to

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OXYHEMOGLOBIN DISSOCIATION CURVE IN ACUTE DISEASE

Perutz these constraints are six oxygen-linked salt bridges (whose actual position has been identified), which must be broken before oxygenation of the tetramer is complete. Hence the dissociation curve of the monomer does not have a sigmoid shape and has a higher affinity for oxygen due to the absence of the salt bridges. In physiologic studies of the dissociation curve of human blood, it is generally assumed that the shape of the dissociation curve is invariant. Although studies by Roughton in 1972 show a slight deviation in shape at the top of the curve with temperature,t°6 for all practical purposes it can be regarded as insignificant in comparison to the variance which can occur in the position of the curve. Until recently, with the exception of the genetic variants, the position of the dissociation curve had been regarded as a constant physiologic parameter, only varying with temperature and pH. It is now well established that this is a gross oversimplification. The factors influencing the position of the hemoglobin dissociation curve are complex and this complexity is increased even further when the protein is enclosed within the red cell in vivo and travelling through the circulation. The advent of chronic and acute disease is an additional complicating factor, the extent of which is not fully understood. Temperature Paul Bert in 1872 found that blood was 90 per cent saturated at an oxygen tension of 15 mm Hg at room temperature whereas it was only 50 per cent saturated at the same tension at body temperature. 23 Barcroft and King (1909) confirmed these observations in hemoglobin solutions and whole blood. 14 The decrease in oxygen affinity (a right shift of the curve) with a rise in temperature is in agreement with the fact that the reaction of oxygen with hemoglobin is an exothermic reaction. A number of investigators have determined the magnitude of the shift of the dissociation curve of whole blood with temperature in terms of an equation relating the change in P0 2 with temperature at constant

HbA 25°C

pH 7.4 0.5

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The oxyhemoglobin dissociation curve in acute disease.

Symposium on a Physiologic Approach to Critical Care The Oxyhemoglobin Dissociation Curve in Acute Disease Rita McConn, PhD. * While clinicians have...
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