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Biosynthetic Arginine Decarboxylase from Escherichia coli

SUBUNIT INTERACTIONS AND THE ROLE OF MAGNESIUM ION

From the ¹ From the Department of Biochemistry, University of Washington, Seattle, Washington 98195

Biosynthetic arginine decarboxylase of Escherichia coli undergoes reversible association and dissociation dependent on the buffer conditions. The apoenzyme underwent dissociation which was pH-dependent, with apparent molecular weights of 140,000 at pH 6 and 80,000 at pH 8. At pH 8, in the presence of pyridoxal 5'-phosphate alone, the enzyme remained fully dissociated, while magnesium ion by itself induced association of the enzyme to a form which was indistinguishable from the tetrameric holoenzyme (molecular weight 275,000 to 300,000). The negative effector, putrescine, caused the formation of more rapidly sedimenting species, but somewhat less efficiently than magnesium ion. In the presence of pyridoxal 5'-phosphate, putrescine much more efficiently induced the formation of tetramer.

At pH 8, but not at pH 6, magnesium ion was required for the formation of an adduct between pyridoxal 5'-phosphate and arginine decarboxylase which, on the basis of its spectral properties, was interpreted to a Schiff base. At pH 6, the Mg²⁺ requirement for enzymatic activity was not as stringent as at higher pH values. The requirement of Mg²⁺ for both Schiff base formation and formation of tetramer was proposed to be directly related to the requirement of this metal ion for the catalytic activity of this enzyme.

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