THE EFFECTS OF TYROSINE NITRATION ON THE STRUCTURE AND FUNCTION OF HEN EGG-WHITE LYSOZYME

We have investigated the effects of tyrosine nitration (to form the we ak acid, 3-nitrotyrosine) at positions 23 or 20 plus 23, on the struct ure and function of hen egg-white lysozyme. Enzyme activity against Mi crococcus luteus cell-wall fragments or soluble substrates exhibits tw o phenomena. (a) A decrease in K-m and k(cat) for the hydrolysis of so luble oligo- and poly-saccharides, resulting in only minor changes in the catalytic efficiency (k(cat)/K-m) upon nitration. (b) The hydrolys is of M. luteus cell-wall fragments appeared to be dominated by electr ostatic interactions with the protein, giving a decrease in enzyme act ivity as the 3-nitrotyrosyl group became ionized. Removal of the cell- wall anionic polymer, teichuronic acid, from M. luteus abolished this effect. The 3-nitrotyrosine group was also found to act as a fluoresce nce quencher of exposed tryptophan residues in lysozyme.