Photosynthetic organisms take advantage of an abundance of nitrogen by storing it as arginine (whose side chain contains a nitrogen-rich guanidinium group), a reaction that is catalyzed by the dimeric protein N-acetylglutamate kinase (NAGK)-an enzyme that is, not surprisingly, subject to feedback inhibition by arginine. P_(II) proteins are one of the central metabolic coordinators of carbon and nitrogen fluxes; this protein is a homotrimer and has been shown previously to regulate ammonia influx into cells by inserting its T loop into the cytoplasmic vestibule of the trimeric ammonia channel; too much of a good thing can be hazardous to one's acid/base equilibrium.
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