Electrostatic Modification of Protein Surfaces: Effect on Hemoglobin Ligation and Solubility

Protein amino groups can be carboxymethylated under mild conditions by the combined use of glyoxylate ion and cyanoborohydride. The amino group is converted to a zwitterionic residue where the pK of the secondary amine is only slightly altered and a carboxyl group has been added some 3 A from the nitrogen atom in the amine. Modification of hemoglobin to low levels of carboxymethylation yields derivatives specifically modified at the terminal α-amino groups. These modified hemoglobins are models for the interactions between the protein and small anions. When the extent of modification is increased by treating the protein with a higher concentration of the modifying agents, lysine residues become converted to N^∊-(carboxymethyI)lysine. In excess of 90% of lysine residues in hemoglobin and myoglobin can be modified by this technique. The increased negative charge can be adjusted to any intermediate level of modification. The change in electrostatic free energy that results from the altered distribution of charge on the protein surface can be correlated with functional properties. Thus, the increased repulsion between the hemoglobin dimers leads to dimerlike oxygen binding properties at a high degree of modification. Similarly, changes in protein solubility secondary to modification reflect altered tetramer-tetramer interactions in the solid state. This method for achieving an altered distribution of charge on the protein surface, a method which can be carried out in a specific or nonspecific fashion to achieve varying degrees of modification, represents a powerful tool for the study of electrostatic interactions in protein chemistry.