Allosteric properties of hemoglobin

In addition to enzymes, noncatalytic proteins may exhibit many of these properties hemoglobin is the classic example. The allosteric properties of hemoglobin are the subject of a Special Focus beginning on page 480. 

THE ALLOSTERIC PROPERTIES OF HEMOGLOBINS RESULT FROM THEIR QUATERNARY STRUCTURES... 

B25. Benesch, R., and Benesch, R. E., The effect of organic phosphates from the human erythrocyte on the allosteric properties of hemoglobin. Biochem. Biophys. Res. Commun. 26, 162-167 (1967). 

The properties of individual hemoglobins are consequences of their quaternary as well as of their secondary and tertiary structures. The quaternary structure of hemoglobin confers striking additional properties, absent from monomeric myoglobin, which adapts it to its unique biologic roles. The allosteric (Gk alios other, steros space ) properties of hemoglobin provide, in addition, a model for understanding other allosteric proteins (see Chapter 11). 

The indirect correlation is the major source of cooperativity in biochemical systems, such as hemoglobin (Chapter 6) or allosteric enzymes (Chapter 8). The model treated in this section is the simplest binding model having indirect correlation. We now examine some of the outstanding properties of the indirect correlation... 

In hemoglobin, the interactions between the subunits are known under the general term of allosteric properties and are of great physiological importance. They determine the cooperative binding of O2. In the deoxy form, the iron atom of each heme is in a high-spin (S =2) five-coordinate iron(II) state and lies about 0.5 A out of the heme plane in the direction of the proximal histidine [12]. The Fe-N(imida2ole) bond vector has 10° tilt off the heme normal. 

A classic illustration of the quaternary structure and its effect on protein properties is a comparison of hemoglobin, an allosteric protein, with myoglobin, which consists of a single polypeptide chain. 

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