Indirect allosteric effects of a neutral mutation. Structure of deoxyhaemoglobin north Chicago (ProC2(36)beta----Ser).

Haemoglobin North Chicago (ProC2(36 beta----Ser) has an abnormally high oxygen affinity. A survey of other abnormal human haemoglobins with high oxygen affinity has indicated that mutations leading to a cavity in the quaternary T-structure, or to the rupture of any bond in that structure, have raised oxygen affinities, because such mutations loosen the constraints of the T-structure. They do not usually affect the oxygen affinity of the R-structure. Haemoglobin North Chicago aroused our interest because the side-chain of serine is smaller than that of proline by only one carbon atom, and it was hard to conceive how such a small gap should raise the oxygen affinity significantly. Our X-ray study shows that the mutation produces unexpectedly large indirect changes in the T-structure.