Oxygen dissociation curve

THE OXYGEN DISSOCIATION CURVES FOR MYOGLOBIN HEMOGLOBIN SUIT THEIR PHYSIOLOGIC ROLES... 

Goodford PI, St-Louis J, Wootton R. A quantitative analysis of the effects of 2,3-diphosphoglycerate, adenosine triphosphate and inositol hexaphosphate on the oxygen dissociation curve of human haemoglobin. J Physiol 1978 283 397. 

I. A 10-month-old child is being evaluated for the underlying cause of a hemolytic anemia. In the diagram shown below, the oxygen dissociation curve for hemoglobin in his erythrocytes is compared with the curve obtained with normal red cells. 

The oxygen-dissociation curve is steepest at the oxygen concentrations that occur in the tissues. This permits, oxygen delivery to respond to small changes in p02. 

I therefore, a shift to the right in the oxygen dissociation curve. 

Hyperbolic shape of the enzyme kinetics curve Most enzymes show Michaelis-Menten kinetics (see p. 58), in which the plot of initial reaction velocity, v0, against substrate concentration [S], is hyperbolic (similar in shape to that of the oxygen-dissociation curve of myoglobin, see p. 29). In contrast, allosteric enzymes frequently show a sigmoidal curve (see p. 62) that is similar in shape to the oxygen-dissociation curve of hemoglobin (see p. 29). 

Figure 7.11 shows the oxygen dissociation curve of one such hemoglobin, Hb Rainier, where it is seen that this hemoglobin is still 50% saturated with oxygen at a p02 of about 12 mm Hg compared to 27 mm Hg in normal hemoglobin. The value of n in the Hill equation is 1.5 for this hemoglobin. 

Figure 2-4. Oxygen dissociation curve for adult hemoglobin (HbA) demonstrating the characteristics of allosteric positive comparativity. Right shift of the curve caused by het-erotrophic ligands and increased temperature leads to decreased oxygen affinity.

Early this century, A.V. Hill derived a useful equation that describes the oxygen dissociation curve of hemoglobin fairly accurately. It is... 

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