Glucose 6-Phosphate Accumulation in Mycobacteria

Abstract

Glucose 6-phosphate (G6P) is a metabolic intermediate with many possible cellular fates. In mycobacteria, G6P is a substrate for an enzyme, F 420 -dependent glucose-6-phosphate dehydrogenase (Fgd), found in few bacterial genera. Intracellular G6P levels in six Mycobacterium sp. were remarkably higher (ϳ17-130-fold) than Escherichia coli and Bacillus megaterium. The high G6P level in Mycobacterium smegmatis may result from 10 -25-fold higher activity of the gluconeogenic enzyme fructose-1,6-bisphosphatase when grown on glucose, glycerol, or acetate compared with B. megaterium and E. coli. In M. smegmatis this coincided with up-regulation of the first gluconeogenic enzyme, phosphoenolpyruvate carboxykinase, when acetate was the carbon source, suggesting a cellular program for maintaining high G6P levels. G6P was depleted in cells under oxidative stress induced by redox cycling agents plumbagin and menadione, whereas an fgd mutant of M. smegmatis used G6P less well under such conditions. The fgd mutant was more sensitive to these agents and, in contrast to wild type, was defective in its ability to reduce extracellular plumbagin and menadione. These data suggest that intracellular G6P in mycobacteria serves as a source of reducing power and, with the mycobacteriaspecific Fgd-F 420 system, plays a protective role against oxidative stress.

Physiology, Biochemistry, and Applications of F420- and Fo-Dependent Redox Reactions

“…However, this hypothesis has yet to be supported with data on phenotypes or energy, and it remains unclear whether purified FDOR-As are capable of reducing menaquinone (30,32). There is also evidence that mycobacteria instead use electrons liberated from G6P by Fgd to directly detoxify exogenous agents (363). Two independent studies have demonstrated that FDOR-As rapidly reduce menadione and plumbagin using F 420 H 2 (30,32), and it is also plausible that these highly promiscuous proteins (28,55) can directly detoxify certain antibiotics too.…”

“…The survival rate of ⌬fbiC strains of M. tuberculosis is 100-to 1,000-fold lower than wild-type cells following challenge with redox cycling agents (i.e., menadione, plumbagin) and antibiotics (i.e., isoniazid, clofazimine) (32). ⌬fgd strains of M. smegmatis are similarly impaired (363). One explanation is that mycobacteria store electrons as glucose-6-phosphate (G6P) and mobilize them using Fgd (F 420 -dependent glucose-6-phosphate dehydrogenase) in response to oxidative stress; G6P levels in M. smegmatis are consistently approximately 100-fold higher than those of E. coli during preferential growth conditions, but the levels become depleted following challenge with redox cycling agents (363).…”

“…⌬fgd strains of M. smegmatis are similarly impaired (363). One explanation is that mycobacteria store electrons as glucose-6-phosphate (G6P) and mobilize them using Fgd (F 420 -dependent glucose-6-phosphate dehydrogenase) in response to oxidative stress; G6P levels in M. smegmatis are consistently approximately 100-fold higher than those of E. coli during preferential growth conditions, but the levels become depleted following challenge with redox cycling agents (363). F 420 H 2 -derived electrons may be used in endogenous redox processes to prevent or reverse damage from reactive oxygen species.…”

“…The observation that F 420 is synthesized even in M. leprae, rendered an unculturable, host-dependent organism through massive genome decay (362), suggests that it has an evolutionarily conserved central role in mycobacterial metabolism. In contrast to methanogens, F 420 is not essential for the viability of mycobacteria under ideal conditions: F 420 biosynthesis (fbiC) and reduction (fgd) genes have been successfully deleted or disrupted in M. smegmatis (28,31,132,363), M. tuberculosis (32,35), and M. bovis (72). However, there is a range of evidence that F 420 contributes to the notorious ability of mycobacteria to persist in deprived and challenging environments (56).…”

“…First identified in the soil bacterium M. smegmatis (148,404), Fgd has since been identified in multiple other environmental actinobacteria and the obligate pathogens M. tuberculosis and M. leprae (145). Fgd is either the sole or main source of F 420 H 2 in mycobacteria; neither ⌬fbiC and ⌬fgd strains are capable of activating exogenous substrates through F 420 H 2 -dependent reactions in M. tuberculosis (33,35) and M. smegmatis (28,363). Fgd therefore appears to have evolved principally as a mechanism to generate F 420 H 2 .…”