Primary Citation of Related Structures:   3BXQ
PubMed Abstract: 
The zinc insulin hexamer undergoes allosteric reorganization among three conformational states, designated T(6), T(3)R(3)(f), and R(6). Although the free monomer in solution (the active species) resembles the classical T-state, an R-like conformational change is proposed to occur upon receptor binding ...
The zinc insulin hexamer undergoes allosteric reorganization among three conformational states, designated T(6), T(3)R(3)(f), and R(6). Although the free monomer in solution (the active species) resembles the classical T-state, an R-like conformational change is proposed to occur upon receptor binding. Here, we distinguish between the conformational requirements of receptor binding and the crystallographic TR transition by design of an active variant refractory to such reorganization. Our strategy exploits the contrasting environments of His(B5) in wild-type structures: on the T(6) surface but within an intersubunit crevice in R-containing hexamers. The TR transition is associated with a marked reduction in His(B5) pK(a), in turn predicting that a positive charge at this site would destabilize the R-specific crevice. Remarkably, substitution of His(B5) (conserved among eutherian mammals) by Arg (occasionally observed among other vertebrates) blocks the TR transition, as probed in solution by optical spectroscopy. Similarly, crystallization of Arg(B5)-insulin in the presence of phenol (ordinarily a potent inducer of the TR transition) yields T(6) hexamers rather than R(6) as obtained in control studies of wild-type insulin. The variant structure, determined at a resolution of 1.3A, closely resembles the wild-type T(6) hexamer. Whereas Arg(B5) is exposed on the protein surface, its side chain participates in a solvent-stabilized network of contacts similar to those involving His(B5) in wild-type T-states. The substantial receptor-binding activity of Arg(B5)-insulin (40% relative to wild type) demonstrates that the function of an insulin monomer can be uncoupled from its allosteric reorganization within zinc-stabilized hexamers.
Related Citations: 
Design of an active ultrastable single-chain insulin analog: synthesis, structure, and therapeutic implications. Hua, Q.X., Nakagawa, S.H., Jia, W., Huang, K., Phillips, N.B., Hu, S.Q., Weiss, M.A. (2008) J Biol Chem 283: 14703
The structure of 2zn pig insulin crystal at 1.5 A resolution Baker, E.N., Blujdell, T.L., Cutfield, J.F., Cutfield, S.M., Dodson, E.J., Dodson, G.G., Hodgkin, D., Isaacs, N.W., Reynolds, C.D. (1988) Philos Trans R Soc London,ser B 319: 369
Structure of insulin in 4-zinc insulin Bentley, G., Dodson, E., Dodson, G., Hodgkin, D., Mercola, D. (1976) Nature 261: 166
Phenol stabilizes more helix in a new symmetrical zinc insulin hexamer Derewenda, U., Derewenda, Z., Dodson, E., Dodson, G., Reynold, C., Smith, G., Sparks, C., Swenson, D. (1989) Nature 338: 594
Toward the active conformation of insulin: stereospecific modulation of a structural swith in the B chain Hua, Q.X., Nakagawa, S., Hu, S.Q., Jia, W., Wang, S., Weiss, M.A. (2006) J Biol Chem 281: 24900
Organizational Affiliation: 
Departments of Biochemistry and Nutrition, Case Western Reserve University School of Medicine, Cleveland, OH 44106, USA.