Exopolyphosphatase Ppx (VPARA_10490) in Variovorax paradoxus strain TBEA6 is a 506-amino-acid cytosolic hydrolase (EC 3.6.1.11; InterPro IPR030673, PANTHER PTHR30005:SF14, PIRSF PIRSF001267) that catalyzes the processive hydrolytic cleavage of terminal orthophosphate (Pi) residues from linear inorganic polyphosphate (polyP) chains. Ppx comprises an N-terminal Ppx/GppA phosphatase catalytic domain of the ASKHA (actin-like ATPase) superfamily (Pfam PF02541, CDD cd24053, residues 23-307) preserving the P1 (11-DLGSNSFR-18) and P2 (142-DIGGRSTE-149) phosphate- and Mg(2+)-binding loops, followed by a C-terminal polyP-binding domain (Pfam PF21447, residues 315-480), and acts together with polyphosphate kinase Ppk (VPARA_28600) to control intracellular inorganic polyphosphate turnover and phosphate homeostasis.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004309 exopolyphosphatase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Accurate core molecular function of the Ppx exopolyphosphatase (EC 3.6.1.11). Reason: VPARA_10490 (A0A0H2MBK5) contains both the N-terminal Ppx/GppA catalytic domain (Pfam PF02541, residues 23-307) with conserved P1 (11-DLGSNSFR-18) and P2 (142-DIGGRSTE-149) active-site motifs and the C-terminal Ppx/GppA domain (Pfam PF21447, residues 315-480), belonging to PANTHER subfamily PTHR30005:SF14 (EXOPOLYPHOSPHATASE, EC 3.6.1.11). Supporting Evidence: UniProtKB:A0A0H2MBK5 FT DOMAIN 23..307 FT /note="Ppx/GppA phosphatase N-terminal" FT /evidence="ECO:0000259|Pfam:PF02541" PMID:23687963 In addition to Ppk, there is another polyP-modifying enzyme: the so-called exopolyphosphatase (Ppx), that is in charge of polyP hydrolysis to Pi (Figure 1A). |
| GO:0006798 polyphosphate catabolic process | IEA GO_REF:0000118 | ACCEPT | Summary: Accurate core biological process; Ppx hydrolyzes inorganic polyphosphate polymers to orthophosphate. Reason: Exopolyphosphatase Ppx catalyzes the hydrolytic breakdown of inorganic polyphosphate chains ((phosphate)n + H2O -> (phosphate)n-1 + Pi) in the bacterial polyphosphate catabolic pathway. Supporting Evidence: UniProtKB:A0A0H2MBK5 DR PANTHER; PTHR30005:SF14; EXOPOLYPHOSPHATASE; 1. PMID:23687963 In addition to Ppk, there is another polyP-modifying enzyme: the so-called exopolyphosphatase (Ppx), that is in charge of polyP hydrolysis to Pi (Figure 1A). |
| GO:0016462 pyrophosphatase activity | IEA GO_REF:0000120 | TOO GENERAL | Summary: True but overly general parent term subsumed by the accepted GO:0004309 (exopolyphosphatase activity) annotation. Reason: GO:0016462 (pyrophosphatase activity) is a broad parent class of acid anhydride hydrolases that is already entailed by the specific accepted term GO:0004309 (exopolyphosphatase activity). Supporting Evidence: UniProtKB:A0A0H2MBK5 DR PANTHER; PTHR30005:SF14; EXOPOLYPHOSPHATASE; 1. |
| GO:0005829 cytosol | IC PMID:23687963 Accumulation of inorganic polyphosphate enables stress endur... | NEW | Summary: Soluble cytoplasmic enzyme lacking signal peptides or transmembrane segments that hydrolyzes cytosolic polyphosphate granules. Reason: Inferred by curator (IC) because Ppx (VPARA_10490) is a 506-aa soluble protein whose UniProtKB:A0A0H2MBK5 feature table lacks any FT SIGNAL or FT TRANSMEM features and whose substrate β intracellular inorganic polyphosphate polymers synthesized by cytosolic Ppk (VPARA_28600) β resides in the bacterial cytosol. Supporting Evidence: UniProtKB:A0A0H2MBK5 FT DOMAIN 23..307 FT /note="Ppx/GppA phosphatase N-terminal" FT /evidence="ECO:0000259|Pfam:PF02541" FT DOMAIN 315..480 FT /note="Ppx/GppA phosphatase C-terminal" FT /evidence="ECO:0000259|Pfam:PF21447" PMID:23687963 In addition to Ppk, there is another polyP-modifying enzyme: the so-called exopolyphosphatase (Ppx), that is in charge of polyP hydrolysis to Pi (Figure 1A). |
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Download this section (compressed HTML)Q: Is Variovorax paradoxus Ppx (VPARA_10490) allosterically inhibited by the alarmone (p)ppGpp during nutrient or heavy-metal stress to favor Ppk-mediated polyphosphate accumulation, and does it also hydrolyze pppGpp to ppGpp in vivo?
Suggested experts: Nikel PI, de Lorenzo V, Gray MJ
Experiment: Assay recombinant Variovorax paradoxus TBEA6 Ppx (VPARA_10490) in vitro on long-chain inorganic polyphosphate and pppGpp substrates in the presence of Mg2+/K+ and physiological concentrations of ppGpp, and quantify intracellular polyP levels in a Ξppx mutant.
Hypothesis: VPARA_10490 processively hydrolyzes inorganic polyphosphate to Pi in a Mg2+-dependent manner and is competitively inhibited by (p)ppGpp during stringent stress.
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