High-resolution proton NMR as indicator of a silent mutation in the haem cavity of a monomeric allosteric haemoglobin.

Comparison of the high-field 1H-NMR spectra of the met-cyano complexes of allosteric haemoglobins III and IV of the insect larva Chironomus thummi thummi (CTT-III and CTT-IV) shows that, in addition to the molecular heterogeneity previously described for both Hbs as due to haem orientational disorder [La Mar et al. (1980) J. Biol. Chem. 255, 66-70], CTT-III but not CTT-IV exhibits a second heterogeneity as evidenced by splitting of numerous signals. Reconstitution of CTT-III with modified haems alters the populations of the two haem rotational isomers but leaves the secondary heterogeneity unaffected. This argues directly for locating this secondary heterogeneity solely within the polypeptide chain rather than the result of protein-haem interaction. We assign this secondary heterogeneity found solely in Hb-III to a point mutation in the haem cavity, 57E6 (Ile/Thr); CTT-IV is chemically homogeneous. The observation of significant hyperfine shift differences for the alternative substitution at 57E6, particularly for non-haem single-proton resonances thought to arise from distal residues, indicates some structural consequences in the haem cavity due to the point mutation. While a difference in the allosteric properties cannot be detected in that the point mutation appears to leave the pK for the allosteric transition unaltered, subtle influences on the function of the protein in both affinity states cannot be ruled out at this time.