AAK_G5K_ProB: Glutamate-5-kinase (G5K) catalyzes glutamate-dependent ATP cleavage; G5K transfers the terminal phosphoryl group of ATP to the gamma-carboxyl group of glutamate, in the first and controlling step of proline (and, in mammals, ornithine) biosynthesis. G5K is subject to feedback allosteric inhibition by proline or ornithine. In microorganisms and plants, proline plays an important role as an osmoprotectant and, in mammals, ornithine biosynthesis is crucial for proper ammonia detoxification, since a G5K mutation has been shown to cause human hyperammonaemia. Microbial G5K generally consists of two domains: a catalytic G5K domain and one PUA (pseudo uridine synthases and archaeosine-specific transglycosylases) domain, and some lack the PUA domain. G5K requires free Mg for activity, it is tetrameric, and it aggregates to higher forms in a proline-dependent way. G5K lacking the PUA domain remains tetrameric, active, and proline-inhibitable, but the Mg requirement and the proline-triggered aggregation are greatly diminished and abolished, respectively, and more proline is needed for inhibition. Although plant and animal G5Ks are part of a bifunctional polypeptide, delta 1-pyrroline-5-carboxylate synthetase (P5CS), composed of an N-terminal G5K (ProB) and a C-terminal glutamyl 5- phosphate reductase (G5PR; ProA); bacterial and yeast G5Ks are monofunctional single-polypeptide enzymes. In this CD, all three domain architectures are present: G5K, G5K+PUA, and G5K+G5PR.
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