Essential tryptophan residues in the function of cellulase from Schizophyllum commune

The tryptophan residues of the cellulase (EC 3.2.1.4; 1,4-beta-D-glucan 4-glucanohydrolase) from Schizophyllum commune were oxidized by N-bromosuccinimide in both the presence and absence of substrates and inhibitors of the enzyme. In the absence of protective ligands, eight of the twelve tryptophan residues in the cellulase were susceptible to modification with concomitant inactivation of the enzyme. The binding of the substrates, CM-cellulose, methyl cellulose, cellohexaose or lichenan and the competitive inhibitor, cellobiose, protected one tryptophan residue from oxidation but did not prevent the inactivation. Characterization of the oxidized enzyme derivatives by ultraviolet difference absorption and by fluorescence spectroscopy indicated that two tryptophan residues are essential in the mechanism of cellulase catalysis. One residue appears to be directly involved in the binding of substrate, while the second residue is proposed to constitute an integral part of a catalytically sound active centre.