Differences in Energy Metabolism Between Trypanosomatidae
Section snippets
End-products of carbohydrate degradation
Trypanosomatidae degrade carbohydrates via glycolysis, and in all members of this family the first reactions of the classical Embden–Meyerhof pathway occur inside glycosomes, organelles unique to the order Kinetoplastida2, 3. Pyruvate, the end-product of glycolysis, is often degraded further in the single mitochondrion of these organisms. However, the participation of mitochondrial pathways in the degradation of carbohydrates varies widely, although all Trypanosomatidae contain a mitochondrion
Anaerobic capacity
During their life cycle, Trypanosomatidae encounter large variations in the availability of oxygen; for instance, bloodstream forms have ample oxygen available, whereas certain stages in the midgut of insects can be confronted with hypoxic conditions. Surprisingly, there are large differences in the anaerobic capacity of Trypanosomatidae. The long slender bloodstream form of T. brucei is in effect a facultative anaerobic organism and can function very adequately without oxygen[3], whereas
Electron-transport chains
There are essential differences in respiratory chains among Trypanosomatidae and between various developmental stages of at least some of them, but all species and stages have some kind of branched electron-transport chain, with terminal oxidases that all use oxygen as the final electron acceptor. In the first part of the electron-transport chain, electrons are donated to the ubiquinone/ubiquinol pool via various enzyme complexes, and in the second part of the chain, electrons are transferred
Succinate production by trypanosomatidae
In many Trypanosomatidae, succinate is an end-product of energy metabolism, albeit often a minor one. It has been suggested that this succinate is produced by fumarate reductase, an enzyme homologous to the Krebs cycle enzyme succinate dehydrogenase, which catalyses the reverse reaction—oxidation of succinate. Fumarate reductase is well-known in bacteria and parasitic helminths, where fumarate reduction functions as an electron sink in the anaerobic energy metabolism of these organisms40, 41, 42
Concluding remarks
The glycosomal part of energy metabolism is nearly identical in all Trypanosomatidae. However, long slender T. brucei show a slight deviation, as the sole end-product of the glycosome is 3-phosphoglycerate (3-PGA), and phosphoenolpyruvate (PEP) is converted to pyruvate in the cytosol and not to oxaloacetate in the glycosome.
The mitochondria of Trypanosomatidae, however, show a large variation in metabolic pathways, as some stages possess mitochondria that have the usual Krebs cycle activity and
Acknowledgements
We are grateful to Fred Opperdoes and Paul Michels for many stimulating discussions and for their very valuable comments on the manuscript. Research in the authors' laboratory was supported by The Netherlands Life Science Foundation (SLW) with financial aid from The Netherlands Organization for Scientific Research (NWO).
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