Enzyme Regulation
Part III. Hemoglobin - Positive Cooperative
Although it is not an enzyme, Hemoglobin works for oxygen transport from lungs to muscles via the principle of Positive Cooperative. Hemoglobin binds oxygen with positive cooperative in much the same way that the allosteric enzyme responses with positive cooperative to the binding of its substrate. The impact of positive cooperative on oxygen binding to hemoglobin is most easily seen by comparing the oxygen saturation curves for myoglobin (the muscle protein with a virtually identical 3-D shape but only one subunit) to hemoglobin with its tetrameric subunit composition. Myoglobin can not respond cooperatively to oxygen binding since it has only one subunit and one oxygen binding site. While hemoglobin has one binding site for oxygen in each of its 4 subunits and can therefore have interaction among these binding sites for oxygen.
Figure 5. Comparison of the oxygen binding curves for myoglobin and hemoglobin at different [oxygen].
Figure from Lehninger Principles of Biochemistry copyright ©1983 Worth Publishers, Inc.
In the lungs where oxygen is abundant and it is transferred to hemoglobin for transport to the tissue where it will be used as an electron acceptor, hemoglobin becomes nearly saturated with oxygen (shown as 'Arterial pressure' for oxygen in the figure above). Then as the blood is pumped from the lungs to the more remote tissues of the body, the hemoglobin carries the oxygen into an environment low in oxygen and will release the bound oxygen (shown as 'Venous pressure' in the figure above). It can also be seen from the Fig. 5 that if myoglobin were used as the oxygen transport protein in blood, only about 10% of the oxygen would be released from the binding sites in transporting this protein from the conditions of the lung to the outer tissues (compare the degree of oxygen saturation of myoglobin and hemoglobin under Venous and Arterial pressures). thus, it is the tetrameric structure of hemoglobin that accounts for this difference and the positive cooperative in loading oxygen as shown by sigmodial curve for it as compared to the Michaelis-Menten type of oxygen loading curve for myoglobin. Thus, the myoglobin - hemoglobin comparison represents what one would find for comparison of a monomeric M-M type of enzyme and a multimeric enzyme with positive cooperatively response to substrate binding in catalyzing its reaction.
While a number of metabolites found in blood influence how the loading and unloading of oxygen is carried out under different physiological conditions, these discussions are beyond the scope of this course and you will encounter them in a physiology course if you take one.
X-ray crystallography of hemoglobin has demonstrated that the overall shape of the tetramer of this protein changes in the presence and absence of oxygen.
Figure 6. Comparison of the 3-D shape of Oxyhemoglobin and Deoxyhemoglobin.
Figure from Zubay et al. Principles of Biochemistry copyright ©1995 Wm. C. Brown Comm., Inc.
The observation of this large change in 3-D shape of hemoglobin when oxygen is bound led to a number of theories being offered for the molecular mechanism of positive cooperative in oxygen loading by the hemoglobin tetramer.
Figure 7. Models to explain the Molecular Mechanism of Positive Cooperativity when Hemoglobin loads oxygen.
Figure from Zubay et al. Principles of Biochemistry copyright ©1995 Wm. C. Brown Comm., Inc.
In these models, hemoglobin exits in two forms - low affinity and high affinity. However, the difference is that in the 'Symmetry' model, all subunits of hemoglobin exist in either the low or high affinity form. In the sequential model, the binding of an oxygen molecule leads to the conversion of that subunit to the high affinity form and so as each molecule of oxygen binds it converts its subunit to the high affinity form. I think that it is easier to understand positive cooperativity in oxygen binding if we use a hybrid of these two model's.
Figure 8. Hybrid model of oxygen loading mechanism to explain positive cooperative in oxygen binding to hemoglobin.
Figure modified from Voet & Voet Biochemistry copyright ©1990 John Wiley & Sons
This model also differences from the model in Fig. 7 in that the low affinity oxygen binding form is represented by a square with a blue color when oxygen is bound and the high affinity form is shown as a circle with a red color when oxygen is bound. This model is presented as a more general mechanism for positive cooperative for a tetramer, so the oxygen is shown as 'S' as if it were a substrate molecule binding to an enzyme. I think this make help you to understand how the hemoglobin model can be extended to enzymes with positive cooperativity for substrate binding.
In this model, the first oxygen molecule is very difficult to bind because all possible binding sites are in the low affinity form. But after this first molecule of oxygen is bound, the entire set of all 4 subunits shifts to high affinity form and now it is very easy to bind the 2nd and 3rd molecules of oxygen to hemoglobin. In away, all these models fail to represent the fact that the last molecule of oxygen is very difficult to get onto hemoglobin and so the tetrameric hemoglobin is very rarely ever completely saturated with oxygen. This reality might be represented by a model where the hemoglobin with 3 molecules of oxygen loaded has as its last subunit a low affinity subunit, which will load oxygen poorly.
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©Wilbur H. Campbell, 1995; wcampbel@mtu.edu