Isosteric conversion of protein carboxyl groups into carboxamide groups. II. Application to lysozyme

For isosteric conversion of carboxyl groups of proteins into amide groups, ammonolysis of protein esters under mild conditions was attempted. Ammonolysis of methyl esters of lysozyme and bovine serum albumin proved to be incomplete. Highly reactive N-ethylsalicylamide esters of guanylated lysozyme were therefore prepared by subjecting the protein to reaction with N-ethylbenz-isoxazolium ion at pH 4.2, 0°. Per molecule, 5–7 ester groups were introduced, with concomitant decrease of activity of 80–90%. Only 0.3 tyrosine was modified. On hydrolysis at pH 9.2 the activity was completely restored. At pH 7.9 three classes of ester groups could be distinguished: one group of high rate of hydrolysis (k₁ = 1.5 min^-1), three groups of intermediate rate (k₂ = 0.13 min^-1) and two groups of low rate (k³ = 0.018 min^-1). The intermediate rate approximated the rate of hydrolysis of the model compound benzoylglycine N-ethyl-salicylamide ester (k = 0.15 min^-1). Ammonolysis at pH 9.2 in 2.0 M ammonia/ ammonium acetate provided complete conversion of the ester groups into amide groups without restoration of activity, confirming the essentiality of certain carboxyl groups. In particular, rearrangement of the ester groups into relatively stable imide groups by O–N acyl migration was found to be completely absent. When native lysozyme was esterified with N-ethylbenzisoxazolium ion the activity did not completely return on hydrolysis.