Metabolism of Mollicutes: the Embden--Meyerhof--Parnas Pathway and the Hexose Monophosphate Shunt

Abstract

Cell-free extracts of eighteen fermentative and nonfermentative Mollicutes were examined for enzyme activities associated with the hexose monophosphate shunt (HMS) and Embden---Meyerhof---Parnas (EMP) pathway. All Acholeplasma spp. had glucose-6-phosphate (G6P) dehydrogenase (EC 1.1.1.49), 6-phosphogluconate (6PG) dehydrogenase (EC 1.1.1.44) and hexokinase (EC 2.7.1.1) activity. Of these three enzyme activities, hexokinase was also detected in Mycoplasma sp. Let. 1 but in no other fermentative or nonfermentative Mycoplasma spp. The Acholeplasma and fermentative Mycoplasma spp. possessed all other HMS and EMP activities examined. All Acholeplasma spp. possessed a pyrophosphate (PPi)-dependent phosphofructokinase (PFK) (EC 2.7.1.90) while fermentative Mycoplasma spp. possessed an ATP-dependent PFK (EC 2.7.1.11). Transaldolase (EC 2.2.1.2) activity was detected in some, but not all Acholeplasma and fermentative Mycoplasma spp. 2-Deoxyribose-5-phosphate aldolase (EC 4.1.2.4) activity was present in all mollicute extracts tested except for Mycoplasma gallisepticum and Mycoplasma sp. Let. 1. The two nonfermentative Mycoplasma spp. lacked all enzyme activities of the HMS pathway except for ribulose-5-phosphate epimerase activity, and of the EMP pathway only phosphoglucose isomerase and the enzymes converting glyceraldehyde 3-phosphate (G3P) to phosphoenolpyruvate (PEP) were detected. We believe that the three major observations of this study are: (1) all Mycoplasma spp. lack G6P and 6PG dehydrogenase activities, suggesting a reduction in their NADPH pool, which may relate to the lipid growth requirement of this genus; (2) the fermentative Mycoplasma spp. have an ATP-dependent PFK activity, while the fermentative Acholeplasma spp. have a PPi-dependent PFK activity; and (3) the nonfermentative Mycoplasma spp. lack ATP and PPi-dependent PFK and fructose-1,6-bisphosphate aldolase activities but, like the fermentative Mollicutes, can convert three-carbon compounds, G3P to PEP through the three-carbon arm of the EMP pathway.