Unique N-acetyl-L-glutamate kinase of Prasinoderma coloniale: new features of an old enzyme

N-Acetyl-L-glutamate kinase of Prasinoderma coloniale is an example of an evolutionary hack that allows this alga survival without the protein PII. N-Acetyl-L-glutamate kinase (NAGK) catalyzes the phosphorylation of N-acetyl-L-glutamate to N-acetyl-L-glutamyl-phosphate, which is the rate-limiting step in the ornithine/arginine biosynthesis pathway. In cyanobacteria and Archaeplastida, NAGK activity is strictly regulated by the end product, arginine (Arg), and is controlled by the PII signaling protein. During evolution, Prasinoderma coloniale, a phytoplankton species from an early-branching lineage within the Archaeplastida, lost the gene encoding PII protein. PcNAGK shows a unique feature: it has evolved to be an enzyme that is virtually insensitive to intracellular levels of Arg with an IC₅₀ of 9.5 mM. Using molecular and biochemical characterization, we demonstrated that substitution of amino acids Lys24 with Gln and Ile282 with Glu restored the sensitivity of the recombinant PcNAGK protein to Arg. Physiological studies have shown that NAGK activity in P. coloniale cells is among the lowest reported to date, which may contribute to energy conservation in this alga. These results not only indicate that NAGK characteristics may vary more across oxygenic phototrophs than previously thought, but also expand our understanding of metabolic adaptation to deep-sea oligotrophic environments.