nucleotide-binding domain (NBD) of Mycobacterium tuberculosis exopolyphosphatase 2 (MtPPX2) and similar proteins
The PPX/GppA family proteins play essential roles in bacterial survival and metabolism. Guanosine pentaphosphate (pppGpp) phosphohydrolase (GppA; EC 3.6.1.40) plays a key role in (p)ppGpp homeostasis. It specifically catalyzes the conversion of pppGpp to ppGpp (guanosine tetraphosphate). Sharing a similar domain structure, GppA is indistinguishable from exopolyphosphatase (PPX; EC 3.6.1.11), which mediates the metabolism of cellular inorganic polyphosphate. Especially, it is responsible for the maintenance of appropriate levels of cellular inorganic polyphosphate (PolyP). Mycobacterium tuberculosis encodes two PPX/GppA homologues, Rv0496 (MtPPX1) and Rv1026 (MtPPX2), which are analogous to the Escherichia coli PPX and GppA enzymes. MtPPX1 functions as an exopolyphosphatase, showing a distinct preference for relatively short-chain poly-P substrates. The exopolyphosphatase activities of MtPPX1 are inhibited by pppGpp. In contrast, MtPPX2 has no detectable exopolyphosphatase activities. Neither MtPPX1 nor MtPPX2 can hydrolyze pppGpp to ppGpp, which is a reaction catalyzed by E. coli PPX and GppA enzymes. Both the MtPPX1 and MtPPX2 proteins have modest ATPase and to a lesser extent ADPase activities. The family corresponds a group of proteins similar to MtPPX2.
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