Arginine294Is Essential for the Inhibition ofAnabaenaPCC 7120 ADP-Glucose Pyrophosphorylase by Phosphate

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Abstract
Treatment of ADP-glucose pyrophosphorylase (EC 2.7.7.27) from the cyanobacterium Anabaena PCC 7120 with phenylglyoxal in 50 mM Hepes, pH 8.0, at 25 °C resulted in a time- and concentration-dependent loss of enzyme activity. Phosphate, the inhibitor, protected the enzyme from inactivation most effectively, while 3-P-glycerate, fructose-1,6-P2, pyridoxal-P, and ATP plus magnesium were also good protectors. After incubation with 2 mM phenylglyoxal for 1 h, the modified enzyme had a 10-fold lower apparent affinity for phosphate in the absence of the activator, 3-P-glycerate, than that of the wild-type enzyme. This result has implicated the involvement of an arginine residue at the allosteric sites, most probably the inhibitor-binding site, of ADP-glucose pyrophosphorylase from the cyanobacterium Anabaena PCC 7120. In order to identify the arginine residue, five arginine residues, which are conserved in all higher-plant and cyanobacterial enzymes but not in enteric bacterial enzymes, were individually converted to alanine by site-directed mutagenesis. The mutant enzymes, R66A, R105A, R294A, and R385A, were purified, and the properties of these mutants were compared with the wild-type enzyme. Substitution of arginine294 with alanine resulted in an enzyme with more than 100-fold or 40-fold lower affinity for the inhibitor, phosphate, in the absence or presence of 3-P-glycerate, respectively. This mutation had no or lesser impact on the kinetic constants for the substrates and the activator, 3-P-glycerate.