The two PPX-GppA homologues from Mycobacterium tuberculosis have distinct biochemical activities

PLoS One. 2012;7(8):e42561. doi: 10.1371/journal.pone.0042561. Epub 2012 Aug 3.


Inorganic polyphosphate (poly-P), guanosine pentaphosphate (pppGpp) and guanosine tetraphosphate (ppGpp) are ubiquitous in bacteria. These molecules play a variety of important physiological roles associated with stress resistance, persistence, and virulence. In the bacterial pathogen Mycobacterium tuberculosis, the identities of the proteins responsible for the metabolism of polyphosphate and (p)ppGpp remain to be fully established. M. tuberculosis encodes two PPX-GppA homologues, Rv0496 (MTB-PPX1) and Rv1026, which share significant sequence similarity with bacterial exopolyphosphatase (PPX) and guanosine pentaphosphate 5'-phosphohydrolase (GPP) proteins. Here we delineate the respective biochemical activities of the Rv0496 and Rv1026 proteins and benchmark these against the activities of the PPX and GPP proteins from Escherichia coli. We demonstrate that Rv0496 functions as an exopolyphosphatase, showing a distinct preference for relatively short-chain poly-P substrates. In contrast, Rv1026 has no detectable exopolyphosphatase activities. Analogous to the E. coli PPX and GPP enzymes, the exopolyphosphatase activities of Rv0496 are inhibited by pppGpp and, to a lesser extent, by ppGpp alarmones, which are produced during the bacterial stringent response. However, neither Rv0496 nor Rv1026 have the ability to hydrolyze pppGpp to ppGpp; a reaction catalyzed by E. coli PPX and GPP. Both the Rv0496 and Rv1026 proteins have modest ATPase and to a lesser extent ADPase activities. pppGpp alarmones inhibit the ATPase activities of Rv1026 and, to a lesser extent, the ATPase activities of Rv0496. We conclude that PPX-GppA family proteins may not possess all the catalytic activities implied by their name and may play distinct biochemical roles involved in polyphosphate and (p)ppGpp metabolic pathways.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Acid Anhydride Hydrolases / antagonists & inhibitors
  • Acid Anhydride Hydrolases / isolation & purification
  • Acid Anhydride Hydrolases / metabolism*
  • Adenosine Diphosphate / metabolism
  • Adenosine Triphosphatases / antagonists & inhibitors
  • Adenosine Triphosphatases / metabolism
  • Adenosine Triphosphate / metabolism
  • Bacterial Proteins / isolation & purification
  • Bacterial Proteins / metabolism*
  • Cell-Free System / drug effects
  • Escherichia coli / enzymology
  • Escherichia coli Proteins / metabolism
  • GTP Phosphohydrolases / metabolism
  • Guanosine Pentaphosphate / metabolism*
  • Guanosine Tetraphosphate / pharmacology
  • Hydrolysis / drug effects
  • Kinetics
  • Mycobacterium tuberculosis / drug effects
  • Mycobacterium tuberculosis / enzymology*
  • Sequence Homology, Amino Acid*
  • Substrate Specificity / drug effects


  • Bacterial Proteins
  • Escherichia coli Proteins
  • Guanosine Tetraphosphate
  • Guanosine Pentaphosphate
  • Adenosine Diphosphate
  • Adenosine Triphosphate
  • Acid Anhydride Hydrolases
  • Adenosine Triphosphatases
  • GTP Phosphohydrolases
  • exopolyphosphatase

Grant support

RMW acknowledges financial support from the Infection and Immunology Strategic Research Theme of the University of Hong Kong and the Research Grants Council of Hong Kong through a General Research Fund (GRF) award [#705007]. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.