By: Richard D. Cummings
We think of glycolysis and the fermentation of glucose as being universal. This common glycolytic pathway in animals is termed the Embden-Meyerhof-Parnas pathway, which metabolizes glucose resulting in production of 2 pyruvates, 2 NADH and a net gain of 2 ATP molecules per glucose molecule. Interestingly, for bacteria this pathway is just one of their options. There are “non-fermenting gram-negative bacilli” that are obligate aerobic, non-spore-forming bacilli and unable to utilize carbohydrates by either fermentation or oxidation. They lack the Embden-Meyerhof-Parnas pathway – such organisms include Pseudomonas aeruginosa, Acinetobacter baumannii, and Stenotrophomonas maltophilia (1). They also usually lack phosphofructokinase-1. They are nosocomial pathogens (2,3), and occur in water, hospitals and the gastrointestinal tracts of animals and humans. These organisms use oxidative pathways for glucose, e.g., conversion to gluconate, 6-phosphogluconate, and its conversion to KDPG (2-keto-3-deoxy-6-phosphogluconate), and its metabolism through the alternative Entner-Doudoroff pathway, first discovered in 1952 by Michael Doudoroff and Nathan Entner (4). This pathway produces 1 ATP, 1 NADH and 1 NADPH per glucose molecule consumed. In that pathway, no pentose compounds are formed.
Another pathway is the pentose phosphate pathway (PPP) (also called the hexose monophosphate shunt as the word shunt is used to denote that in the oxidative branch of the PPP, it bypasses glycolysis) (5,6). All cyanobacteria, including Acetobacter suboxydans and A. xylinum, utilize the PPP as their only metabolic pathway to metabolize glucose. That pathway is considered to have two reaction sequences, oxidative and non-oxidative. In the oxidative branch, glucose-6-phosphate is converted to the pentose derivative ribulose 5-phosphate (Ru5P) and CO2, along with the formation of 2 NADPH molecules. NADPH is used for anabolic reactions, such as lipid synthesis. The Ru5P can be converted to ribose-5-phosphate, essential for nucleotide synthesis. Ru5P can also be converted to xylulose-5-phosphate and glycolytic intermediates, depending on the needs of the cell. The PPP does not directly produce ATP. In the non-oxidative branch of the PPP, 3 molecules of Ru5P are metabolized to 2 molecules of fructose-6-phosphate and one molecule of glyceraldehyde-3-phosphate, available for metabolism. The latter can enter glycolysis steps. So, glycolysis with all the steps as we know it, is not ‘universal’!
References
1. Gayretli Aydiotan, Z. G., Tanir, G., Bayhan, G. I., Aydin Teke, T., Metin Akcan, O., Kaman, A., Yasar Durmus, S., and Oz, F. N. (2020) Risk Factors of Stenotrophomonas maltophilia Blood Stream Infections: Comparison With Other Gram-Negative Blood Stream Infections in Children. Pediatr Infect Dis J 39, e406-e409
2. McGowan, J. E., Jr. (2006) Resistance in nonfermenting gram-negative bacteria: multidrug resistance to the maximum. Am J Med 119, S29-36; discussion S62-70
3. Daddaoua, A., Krell, T., and Ramos, J. L. (2009) Regulation of glucose metabolism in Pseudomonas: the phosphorylative branch and entner-doudoroff enzymes are regulated by a repressor containing a sugar isomerase domain. J Biol Chem 284, 21360-21368
4. Conway, T. (1992) The Entner-Doudoroff pathway: history, physiology and molecular biology. FEMS Microbiol Rev 9, 1-27
5. Alfarouk, K. O., Ahmed, S. B. M., Elliott, R. L., Benoit, A., Alqahtani, S. S., Ibrahim, M. E., Bashir, A. H. H., Alhoufie, S. T. S., Elhassan, G. O., Wales, C. C., Schwartz, L. H., Ali, H. S., Ahmed, A., Forde, P. F., Devesa, J., Cardone, R. A., Fais, S., Harguindey, S., and Reshkin, S. J. (2020) The Pentose Phosphate Pathway Dynamics in Cancer and Its Dependency on Intracellular pH. Metabolites 10
6. Sharkey, T. D. (2021) Pentose Phosphate Pathway Reactions in Photosynthesizing Cells. Cells 10