Solution structure of the B-chain of insulin as determined by 1H NMR spectroscopy. Comparison with the crystal structure of the insulin hexamer and with the solution structure of the insulin monomer.

The solution structure of the isolated B-chain of bovine insulin has been determined by 1H NMR spectroscopy combined with simulated annealing calculations. Complete sequence-specific assignments for the proton resonances are reported together with a set of 309 NOEs used in the structure calculations. Chemical-shift variations from random coil values provide support for the existence of helical regions in the polypeptide chain, as do a characteristic series of d alpha beta(i, i + 3) NOEs from residues B8 to B17. While there is some evidence for a limited degree of conformational averaging over the helical region, in general the helix is relatively well defined and corresponds closely to the helical region seen in the X-ray crystal structure of the insulin hexamer. Other similarities with the crystal structure include turn-like conformations at the carboxy terminal end of the helix and extended strands at both the amino and carboxy termini of the peptide. These similarities between the crystal structure and the isolated B-chain suggest that this peptide has intrinsic folding properties, which allow it to adopt its characteristic structure in intact insulin without the need for extensive cooperative interactions with the A-chain. Despite these general similarities, an important difference between the isolated B-chain and the intact protein occurs in the carboxy terminal region. This region appears significantly more mobile in the isolated B-chain. As a conformational change involving the carboxy terminus has been implicated in receptor binding, the current study of the isolated B-chain provides valuable information on the extent of this region's intrinsic mobility.