Intramolecular interactions of amino groups in 13C reductively methylated hen egg-white lysozyme

Reductive methylation of hen egg-white (HEW) lysozyme with [13C]formaldehyde and NaCNBH3 and subsequent 13C NMR spectroscopy reveal resonances for each of the mono- and dimethyl derivatives of the six lysyl epsilon-amino groups and the NH2 terminus. Each resonance has a unique chemical shift, pKa, and chemical shift change upon deprotonation. The assignment of the resonances arising from the N alpha,N-dimethyl and N alpha-monomethyl NH2 terminus has been made as have resonance assignments for the two lysyl residues which crystallographic studies indicate are involved in ion pair interactions (Imoto, T., Johnson, L. N., North, A. C. T., Phillips, D. C., and Rupley, J. A. (1972) in The Enzymes (Boyer, P. D., ed) 3rd Ed, Vol. 7, pp. 665-868, Academic Press, New York). One resonance, tentatively assigned to the lysine 1 (epsilon NH3+) which forms an ion pair with glutamic acid 7 (gamma COO-), has a highly perturbed chemical shift which shows a biphasic titration curve (N epsilon, N-dimethyl pKa values 10.0 and 2.6). A similar titration curve is observed for a N epsilon-monomethyl lysyl residue. The resonance for lysine 13 (epsilon NH3+), which forms an ion pair with the carboxyl terminus, leucine 129 (alpha COO-), has been assigned by removal of leucine 129 with carboxypeptidase whereupon only one pair of mono- and dimethyl lysyl resonances is greatly perturbed. The pKa of N epsilon,N-dimethyl lysine 13 is 9.3 in des-Arg-Leu-lysozyme in contrast to 9.8 for the intact enzyme. Thus, it appears that both of the intramolecular ion pairs predicted by x-ray crystallography exist in the solution structure of HEW lysozyme.