Inter- and intramolecular interactions of α-lactalbumin XI. Comparison of the “exposure” of tyrosyl, tryptophyl, and lysyl side chains in the goat and bovine proteins

Abstract

The “exposure” of tyrosyl, tryptophyl and lysyl side chains in bovine and goat α-lactalbumin has been compared. Solvent perturbation measurements indicate that the environment of tryptophan residues in the two proteins must be very nearly the same; two of the four groups are “exposed” with respect to long-range perturbants for both the pH 6 and pH 2 conformers at 25 °C. At 3 °C the pH 2 conformer has an additional exposed tryptophan. Although all four tyrosyl residues ionize completely at alkaline pH, this is paralleled by a conformational change. The tyrosyl residues of goat α-lactalbumin are less reactive with respect to N- acetylimidazole than are those of the bovine protein. Two of the four groups in the goat protein remain unreactive at the relatively high reagent concentration employed, whereas those of the bovine protein are readily acylated. Acylation of amino groups likewise seems to occur less readily with the goat protein, and it seems likely that a single lysyl residue remains resistant to reaction with trinitrobenzene sulfonic acid.