Biochemical Properties of a cAMP Phosphodiesterase in the Cyanobacterium Anabaena sp. strain PCC 7120

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A gene for cAMP phosphodiesterase, designated cpdA, was identified in the nitrogen-fixing cyanobacterium Anabaena sp. PCC 7120. The predicted amino acid sequence of the gene was similar to the sequences of cAMP phosphodiesterases from Thermosynechococcus elongatus, Escherichia coli and Haemophilus influenzae. The recombinant protein was purified by sequential column chromatography and its biochemical properties were determined. The Anabaena cAMP phosphodiesterase hydrolyzed cAMP and cGMP with similar levels of activity. The K_m value for cAMP was 45 μM and the Vmax was 4.9 mmol min^-1 mg protein^-1. These values are similar to those of Escherichia coli cAMP phosphodiesterase. The enzyme was activated by divalent cations such as Fe²⁺ and Mn²⁺. The tertiary structure of this enzyme was predicted by homology modeling. The deduced structure has two metal-binding sites in the catalytic domain.