Engineering a New Magnesium Binding Site in the Subunit Contact Region of Escherichia coli Inorganic Pyrophosphatase

A. N. Parfenyev¹, A. Salminen², A. A. Baykov^1*, and R. Lahti²

¹Belozersky Institute of Physico-Chemical Biology and School of Chemistry, Lomonosov Moscow State University, Moscow, 119899 Russia; fax: (095) 939-3181; E-mail: baykov@genebee.msu.su

²Department of Biochemistry, University of Turku, Turku, FIN-20500, Finland; fax: (358-2) 633-6860; E-mail: reila@utu.fi

^* To whom correspondence should be addressed.

Received October 12, 1999

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Three Gln-80 residues belonging to different subunits of homohexameric Escherichia coli pyrophosphatase are separated by only one water molecule to which they are hydrogen bonded. Substitution of Glu for Gln-80 stabilizes quaternary structure of the enzyme but has only a small effect on enzyme activity. The substitution stimulates Mg²⁺ binding and changes the appearance of the Mg²⁺ concentration dependence of the rate constant for the trimer --> hexamer transition. These data suggest that a new Mg²⁺ binding site is formed in the intersubunit contact region as a result of the substitution. Three-dimensional modeling of the mutated protein showed that a chelate complex might form involving two of the three Glu-80 residues.

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KEY WORDS: pyrophosphatase, site-directed mutagenesis, magnesium

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