Optical activity of human lysozyme.

Abstract

The ultraviolet circular dichroism spectra of human lysozyme are presented. Effects of pH and added inhibitor (N-acetyl-D-glucosamine) were examined and the results were compared with similar measurements of hen egg-white lysozyme. The near-ultraviolet CD spectral bands are substantially different in the human and hen egg-white enzymes. In addition to marked dissimilarities in the spectral interval 260-300 nm, an unusual CD band occurs at an anomalous wavelength (313 nm) in human lysozyme. The pH dependence of the latter suggests a possible interaction, absent in hen egg-white lysozyme, between a tryptophan and a tyrosine residue. Analysis of the spectra furthermore suggests lesser net rotational strengths of tryptophan bands in hen egg-white lysozyme than in human lysozyme, although the latter has one less tryptophan residue. The relationship between the CD spectra and the sequence differences of the proteins is discussed, as well as the CD spectra (published by others) of a closely related protein, bovine alpha-lactalbumin. Contributions of cystine residues to the spectra are examined in the light of possible differences in chirality of one of the four disulfide bridges.The far-ultraviolet CD spectra of human and egg-white lysozyme are quite similar, though not identical. In view of the pronounced differences in side-chain optical activity, and of the effect of pH variation on the far-ultraviolet CD spectrum of human lysozyme, it is likely that at least part of the observed difference in spectra is due to nonpeptide optical activity, and that the proteins have a secondary structure in common.