Primary Citation of Related Structures:   1KMF
PubMed Abstract: 
The hydrophobic core of vertebrate insulins contains an invariant isoleucine residue at position A2. Lack of variation may reflect this side-chain's dual contribution to structure and function: Ile(A2) is proposed both to stabilize the A1-A8 alpha-helix and to contribute to a "hidden" functional surface exposed on receptor binding ...
The hydrophobic core of vertebrate insulins contains an invariant isoleucine residue at position A2. Lack of variation may reflect this side-chain's dual contribution to structure and function: Ile(A2) is proposed both to stabilize the A1-A8 alpha-helix and to contribute to a "hidden" functional surface exposed on receptor binding. Substitution of Ile(A2) by alanine results in segmental unfolding of the A1-A8 alpha-helix, lower thermodynamic stability and impaired receptor binding. Such a spectrum of perturbations, although of biophysical interest, confounds interpretation of structure-activity relationships. To investigate the specific contribution of Ile(A2) to insulin's functional surface, we have employed non-standard mutagenesis: inversion of side-chain chirality in engineered monomer allo-Ile(A2)-DKP-insulin. Although the analogue retains native structure and stability, its affinity for the insulin receptor is impaired by 50-fold. Thus, whereas insulin's core readily accommodates allo-isoleucine at A2, its activity is exquisitely sensitive to chiral inversion. We propose that the Ile(A2) side-chain inserts within a chiral pocket of the receptor as part of insulin's hidden functional surface.
Related Citations: 
Critical Role of the A2 Amino Acid Residue in the Biological Activity of Insulin:
[2-Glycine-A]- and [2-Alanine-A]insulins Kitagawa, K., Ogawa, H., Burke, G.T., Chanley, J.D., Katsoyannis, P.G. (1984) Biochemistry 23: 1405
Importance of Aliphatic Side-Chain Structure at Positions 2 and 3 of the Insulin A Chain in Insulin-Receptor Interactions Nakagawa, S.H., Tager, H.S. (1992) Biochemistry 31: 3204
Mapping the Functional Surface of Insulin by Design:
Structure and Function of a Novel A-Chain Analogue Hua, Q.X., Hu, S.Q., Frank, B.H., Jia, W., Chu, Y.C., Wang, S.H., Burke, G.T., Katsoyannis, P.G., Weiss, M.A. (1996) J Mol Biol 264: 390
A Cavity-Forming Mutation in Insulin Induces Segmental Unfolding of a Surrounding alpha-Helix Xu, B., Hua, Q.X., Nakagawa, S.H., Jia, W., Chu, Y.C., Katsoyannis, P.G., Weiss, M.A. (2002) Protein Sci 11: 104
Organizational Affiliation: 
Department of Biochemistry, Case Western Reserve University School of Medicine, Cleveland, OH 44106-4935, USA.