ATP and phosphate reciprocally affect subunit association of human recombinant High Km 5′‐nucleotidase

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Abstract
IMP‐specific, High Km 5′‐nucleotidase ( EC 3.1.3.5) is an ubiquitous enzyme, the activity of which is highly regulated by substrate, ATP, and inorganic phosphate. The cDNA encoding this enzyme has recently been cloned and found to contain a unique stretch of nine glutamic and four aspartic acid residues at the C‐terminus. To study the effects of this acidic tail, and of ATP and inorganic phosphate on enzyme function, we generated several structural modifications of the 5′‐nucleotidase cDNA, expressed the corresponding proteins in Escherichia coli and compared their molecular and kinetic properties. As with the enzyme purified from human placenta, all recombinant proteins were activated by ATP and inhibited by inorganic phosphate. Although the S0.5‐values were higher, the specific activities of the purified protein variants (except that truncated at the C‐terminus) were similar. The molecular mass of the full‐length enzyme subunit has been estimated at 57.3 kDa and the molecular mass of the native protein, as determined by gel‐filtration chromatography, was estimated to be 195 kDa. Increasing the concentration of NaCl to 0.3 m promoted oligomerization of the protein and the formation of aggregates of 332 kDa. ATP induced further oligomerization to 715 kDa, while inorganic phosphate reduced the estimated molecular mass to 226 kDa. In contrast to the truncation of 30 amino acids at the N‐terminus, which did not alter enzyme properties, the removal of the polyglutamic/aspartic acid tail of 13 residues at the C‐terminus caused profound kinetic and structural changes, including a 29‐fold decrease in specific activity and a significant increase in the sensitivity to inhibition by inorganic phosphate in the presence of AMP. Structurally, there was a dramatic loss of the ability to form oligomers at physiological salt concentration which was only partially restored by the addition of NaCl or ATP. These data suggest an important function of the polyglutamic acid tract in the process of association and dissociation of 5′‐nucleotidase subunits.

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