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 weak acid, 3-nitrotyrosine) at positions 23 or 20 plus 23, on the structure and function of hen egg-white lysozyme. Enzyme activity against Micrococcus luteus cell-wall fragments or soluble substrates exhibits two phenomena. (a) A decrease in Km and kcat for the hydrolysis of soluble oligo- and poly-saccharides, resulting in only minor changes in the catalytic efficiency (kcat/Km) upon nitration. (b) The hydrolysis of M. luteus cell-wall fragments appeared to be dominated by electrostatic interactions with the protein, giving a decrease in enzyme activity 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 fluorescence quencher of exposed tryptophan residues in lysozyme.