The high resolution solution structure of the insulin monomer determined by NMR

Studies of naturally occurring and chemically modified insulins indicate that relatively few of the 51 amino acid residues may be assigned specific roles in insulin-receptor interactions. Most of the insulin X-ray structural information is derived from aggregated species (notably hexamers). Because insulin exerts its physiological effect as a 5808 Dalton monomeric species, it is necessary to consider whether crystal-packing forces have modified the structure from that required for biological action. Insulin aggregation in solution complicates high resolution NMR studies of the monomer. However, site-directed mutagenesis can be used to generate biologically active mutants (e.g., B16-Tyr--> His) that remain monomeric at millimolar concentrations in aqueous solution at low pH. The resulting homogeneous and monomeric samples are suitable for structure determination by NMR methods. The high resolution solution structure of B16--Tyr--> His insulin resembles crystal structures, notably molecule 1 of T6 insulin. Side-chain conformation in some biologically important motifs, however, shows subtle differences between solution and crystal structures.