Studies on β-Lactoglobulins A, B, and C. II. Preparation of Modified Proteins by Treatment with Carboxypeptidase A^*

Interaction of dodecyl sulfate with native and modified .beta.-lactoglobulin

“…In order to determine this, carboxypeptidase A modified ß-lactoglobulin B was utilized. This modified protein contains only one histidine residue in each of the identical polypeptide chains of the protein, the other penultimate histidine having been eliminated along with carboxyl-terminal isoleucine by the enzymatic hydrolysis (Greenberg and Kalan, 1965). As pointed out in Table II, this derivative also binds 2 moles of dodecyl sulfate/mole of protein.…”

Section C. Chemistry of milk proteins

“…(a) Carboxypeptidase removes terminal isoleucine-162 and then histidine-161 to give crystallizable products from /?-lactoglobulins A, B and C (112,177). From the negligible effect of this modification on the optical rotatory dispersion spectrum of /#-lactoglobulin A, it is likely that these amino acids lie on the surface of the native molecule (471).…”

“…By these means, it has been shown that in each monomer in the normal native state the 2 tryptophans, the 4 tyrosines, one carboxylic (aspartic or glutamic) acid group and the -SH group are all either buried inside the molecule or hindered to some extent from reacting with aqueous reagents (471). On the other hand, all the other charged groups, except the extra histidine in /Mactoglobulin C, are accessible to H ions and may be assumed to lie on, or very near, the surface (36); also, the carboxyl end of the polypeptide chain is probably near or on the surface, since the terminal isoleucine and penultimate histidine are readily removed by carboxypeptidase without disruption of the rest of the molecule (112,177). Two of the 4 tyrosines can be acetylated without change of conformation, and though partly masked must lie close to the surface (471); one of the 2 tryptophans must also lie close to the surface (430).…”