The Glu(B13) carboxylates of the insulin hexamer form a cage for Cd2+ and Ca2+ ions.

Substitution of Cd2+ for Zn2+ yields a hexameric insulin species containing 3 mol of metal ion per hexamer. The Cd2+ binding loci consist of the two His(B10) sites and a new site involving the Glu(B13) residues located at the center of the hexamer [Sudmeier, J. L., Bell, S. J., Storm, M. C., & Dunn, M. F. (1981) Science (Washington, D.C.) 212, 560-562]. Substitution of Co2+ or Co3+ for Zn2+ gives hexamers containing 2 mol of metal per hexamer. Insulin solutions to which both Cd2+ and Co2+ have been added in a ratio of 6:2:1 [In]:[Co2+]:[Cd2+] followed by oxidation to the exchange-inert Co3+ state yield stable hybrid species containing both Co3+ and Cd2+ with a composition of (In)6(Co3+)2Cd2+. The kinetics of the reaction of 2,2',2"-terpyridine (terpy) with the exchange-labile (In)6(Cd2+)2 and (In)6(Co2+)2 derivatives are biphasic and involve the rapid formation of an intermediate with coordination of one terpy molecule to each protein-bound metal ion; then, in a rate-limiting step the terpy-coordinated metal ion dissociates from the protein, and a second molecule of terpy binds to the metal ion to form a bis complex. Reaction of the exchange-inert Co3+ ions of (In)6(Co3+)2 with terpy is a slow apparent first-order process (t1/2 = 13.1 h). In contrast to the kinetic behavior of (In)6(Co2+)2 and (In)6(Cd2+)2, the Cd2+ ions bound to the hybrid (In)6(Co3+)2Cd2+ react quite slowly with terpy (t1/2 = 1 h at pH 8.0).(ABSTRACT TRUNCATED AT 250 WORDS)