Complex formation and catalytic activation by the PII signaling protein of N-acetyl-L-glutamate kinase from Synechococcus elongatus strain PCC 7942

The signal transduction protein P-II from the cyanobacterium Synechococcus elongatus strain PCC 7942 forms a complex with the key enzyme of arginine biosynthesis, N-acetyl-L-glutamate kinase (NAGK). Here we report the effect of complex formation on the catalytic properties of NAGK. Although pH and ion dependence are not affected, the catalytic efficiency of NAGK is strongly enhanced by binding of P-II, with K-m decreasing by a factor of 10 and V-max increasing 4-fold. In addition, arginine feedback inhibition of NAGK is strongly decreased in the presence of P-II, resulting in a tight control of NAGK activity under physiological conditions by P-II. Analysis of the NAGK-P-II complex suggests that one P-II trimer binds to one NAGK hexamer with a K-d of similar to3 nM. Complex formation is strongly affected by ATP and ADP. ADP is a strong inhibitor of complex formation, whereas ATP inhibits complex formation only in the absence of divalent cations or in the presence of Mg2+ ions, together with increased 2-oxoglutarate concentrations. Ca2+ is able to antagonize the negative effect of ATP and 2-oxoglutarate. ADP and ATP exert their adverse effect on NAGK-P-II complex formation through binding to the P-II protein.