PMID:1980213
| Citation |
Schinzel, R and Palm, D (1990) Escherichia coli maltodextrin phosphorylase: contribution of active site residues glutamate-637 and tyrosine-538 to the phosphorolytic cleavage of alpha-glucans. Biochemistry 29:9956-62 |
|---|---|
| Abstract |
The role of Escherichia coli maltodextrin phosphorylase (EC 2.4.1.1) 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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| Keywords |
Amino Acid Sequence; Bacterial Proteins/genetics; Bacterial Proteins/metabolism; Base Sequence; Binding Sites; Escherichia coli/enzymology; Escherichia coli/genetics; Glucans/metabolism; Glucosyltransferases/genetics; Glucosyltransferases/metabolism; Glutamates; Glutamic Acid; Kinetics; Molecular Sequence Data; Mutagenesis, Site-Directed; Phosphorylation; Protein Conformation; Pyridoxal Phosphate/metabolism; Sequence Homology, Nucleic Acid; Tyrosine |
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