Catalytic Thiol and Carboxylate: Role of Cysteine and Glutamic Acid in the Xylosidic Activity of Endoxylanase fromChainiasp. (NCL 82-5-1)

Chemical modification of the endoxylanase fromChainiasp. with group-specific chemical modifiers in the absence and presence of substrate and kinetics of modification revealed the involvement of a thiol and a carboxylate in the catalytic function of the enzyme. The active-site peptides were chemically labeled and sequenced. The sequence alignment of the chemically labeled peptide with other family G/11 xylanases showed that the catalytic glutamate ofChainiaxylanase is located in a highly homologous region and may function as an acid/base catalyst while thiol of the Cys may function as a nucleophile.