Studies on the function of tryptophan-108 on lysozyme

Chemical studies of selective modification of Trp-108 of lysozyme gave ambiguous results concerning its function on the catalytic activity, since the oxyndole derivative obtained with N-bromosuccinimide is inactive, whereas the kynurenine derivative obtained by oxidation with ozone is fully active. In order to explain this discrepancy, lysozyme has been modified with 2-nitro-4-carboxyphenylsulfenyl chloride (NCPS-Cl). This reagent reacts with the indole ring of tryptophan giving a 2-thioaryl-derivative. By chromatographic fractionation of the reaction mixture, a lysozyme derivative was isolated, that by sequence studies was proved to be modified only at Trp-108 retaining 10% of the lytic activity. Physico-chemical as well as kinetic studies indicate that the large decrease in activity following modification could be related to minor effects in the microenvironment of the active site, with a concomitant modification of the ionization constants of the groups involved in catalysis.