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Biochemistry. 1990 Oct 23;29(42):9956-62.

Escherichia coli maltodextrin phosphorylase: contribution of active site residues glutamate-637 and tyrosine-538 to the phosphorolytic cleavage of alpha-glucans.

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Department of Physiological Chemistry, University of W├╝rzburg, Federal Republic of Germany.


The role of Escherichia coli maltodextrin phosphorylase (EC active site residues Glu637 and Tyr538 which line the sugar-phosphate contact region of the enzyme was investigated by site-directed mutagenesis. Substitution of Glu637 by an Asp or Gln residue reduced kcat to approximately 0.2% of wild-type activity, while the Km values were affected to a minor extent. This indicated participation of Glu637 in transition-state binding rather than in ground-state binding. 31P NMR analysis of the ionization state of enzyme-bound pyridoxal phosphate suggested that Glu637 is also involved in modulation of the protonation state of the coenzyme phosphate observed during catalysis. Despite loss of proposed hydrogen-bonded substrate contacts, the Tyr538Phe mutant enzyme retained more than 10% activity; the apparent affinity of all substrates was slightly decreased. Mutations at either site affected the error rate of the enzyme (ratio of hydrolysis/phosphorolysis). Besides a role in substrate binding, the hydrogen-bond network of Tyr538 supports the exclusion of water from the active site.

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