Sammendrag
The genome of the facultative ribulose monophosphate (RuMP) cycle methylotroph Bacillus methanolicus encodes two bisphosphatases (GlpX), one on the chromosome (GlpXC) and one on plasmid pBM19 (GlpXP), which is required for methylotrophy. Both enzymes were purified from recombinant Escherichia coli and were shown to be active as fructose 1,6-bisphosphatases (FBPases). The FBPase-negative Corynebacterium glutamicum Δfbp mutant could be phenotypically complemented with glpXC and glpXP from B. methanolicus. GlpXP and GlpXC share similar functional properties, as they were found here to be active as homotetramers in vitro, activated by Mn2+ ions and inhibited by Li+, but differed in terms of the kinetic parameters. GlpXC showed a much higher catalytic efficiency and a lower Km for fructose 1,6-bisphosphate (86.3 s−1 mM−1 and 14 ± 0.5 μM, respectively) than GlpXP (8.8 s−1 mM−1 and 440 ± 7.6 μM, respectively), indicating that GlpXC is the major FBPase of B. methanolicus. Both enzymes were tested for activity as sedoheptulose 1,7-bisphosphatase (SBPase), since a SBPase variant of the ribulose monophosphate cycle has been proposed for B. methanolicus. The substrate for the SBPase reaction, sedoheptulose 1,7-bisphosphate, could be synthesized in vitro by using both fructose 1,6-bisphosphate aldolase proteins from B. methanolicus. Evidence for activity as an SBPase could be obtained for GlpXP but not for GlpXC. Based on these in vitro data, GlpXP is a promiscuous SBPase/FBPase and might function in the RuMP cycle of B. methanolicus.