The role of two positively charged amino acid residues located at the active site of Escherichia coli maltodextrin phosphorylase was investigated by site-directed mutagenesis. Substitution of Lys539 by an arginine caused a 600-fold reduction, substitution of Arg534 by a glutamine caused an even larger 7000-fold reduction of the catalytic rate while substrate binding remained essentially unaffected. Since the Arg534----Gln exchange reduces the catalytic rate near to inactivity and even the conservative Lys534----Arg exchange caused a marked decrease of activity, the central functional role of both positively charged residues in phosphorylase catalysis anticipated by the crystallographic analysis of the corresponding amino acid residues Arg569 and Lys574 in the catalytic site of phosphorylase b was confirmed.
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