Binding of D-glucose to insulin.

Binding of D-glucose to insulin has been studied by equilibrium dialysis. The binding is not very specific and probably takes place in two steps. The average amount of glucose molecules bound per insulin molecule is eight, two molecules in the first and six during the second step of binding. The intrinsic binding constants for both steps are almost the same (6-10-2 M-minus 1 and 1-10-3 M-minus 1) which can be explained by assuming: (1) that after binding of the first two molecules a conformational change of insulin occurs which facilitates the binding of the next six molecules of D-glucose; or (2) that in the second step of binding the glucose binds to hydrophobic regions which are unmasked by dissociation of the insulin dimer. Using a three-dimensional model of the insulin molecule areas of the protein molecule where binding of glucose can occur were selected. The glucose-binding site very probably involves the area at the insulin surface where most of the invariant and modification-selective residues are present.