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Ppx/GppA phosphatase, domain III
Ppx/GppA phosphatases belong to ASKHA (acetate and sugar kinases, Hsp70, actin) which has an N-terminal (Pfam:PF02541) nucleotide-binding domain (domains I/II), domain III which corresponds to the catalytic phosphohydrolase domain with the common helical fold and domain IV [1-3]. This entry represents domain III. Paper describing PDB structure 1u6z. [1]. 16905100. Origin of exopolyphosphatase processivity: Fusion of an ASKHA phosphotransferase and a cyclic nucleotide phosphodiesterase homolog. Alvarado J, Ghosh A, Janovitz T, Jauregui A, Hasson MS, Sanders DA;. Structure. 2006;14:1263-1272. Paper describing PDB structure 2flo. [2]. 16678853. The structure of the exopolyphosphatase (PPX) from Escherichia coli O157:H7 suggests a binding mode for long polyphosphate chains. Rangarajan ES, Nadeau G, Li Y, Wagner J, Hung MN, Schrag JD, Cygler M, Matte A;. J Mol Biol. 2006;359:1249-1260. Paper describing PDB structure 6pbz. [3]. 31679177. Structure and activity of PPX/GppA homologs from Escherichia coli and Helicobacter pylori. Song H, Dharmasena MN, Wang C, Shaw GX, Cherry S, Tropea JE, Jin DJ, Ji X;. FEBS J. 2020;287:1865-1885. (from Pfam)
Ppx/GppA phosphatase family
This family consists of the N-terminal region of exopolyphosphatase (Ppx) EC:3.6.1.11 and guanosine pentaphosphate phospho-hydrolase (GppA) EC:3.6.1.40. [1]. 8212131. Exopolyphosphate phosphatase and guanosine pentaphosphate phosphatase belong to the sugar kinase/actin/hsp 70 superfamily. Reizer J, Reizer A, Saier MH Jr, Bork P, Sander C;. Trends Biochem Sci 1993;18:247-248. (from Pfam)
exopolyphosphatase
exopolyphosphatase catalyzes the degradation of inorganic polyphosphates (polyP) and releases orthophosphate processively from the ends of the polyP chain
It appears that a single enzyme may act as both exopolyphosphatase (Ppx) and guanosine pentaphosphate phosphohydrolase (GppA) in a number of species. Members of the seed alignment use to define this exception-level model are encoded adjacent to a polyphosphate kinase 1 gene, and the trusted cutoff is set high enough (425) that no genome has a second hit. Therefore all members may be presumed to at least share exopolyphospatase activity, and may lack GppA activity. GppA acts in the stringent response.
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