Ornidazole of the 5-nitroimidazole drug family is used to treat protozoan and anaerobic bacterial infections via a mechanism that involves pre-activation by reduction of the nitro group, and production of toxic derivatives and radicals. Metronidazole, another drug family member, has been suggested to affect photosynthesis by draining electrons from the electron carrier, ferredoxin, thus inhibiting NADP+ reduction and stimulating radical and peroxide production. Here we show, however, that ornidazole inhibits photosynthesis via a different mechanism. While having a minute effect on the photosynthetic electron transport and oxygen photoreduction, ornidazole hinders the activity of two Calvin cycle enzymes, triose-phosphate isomerase (TPI) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH). Modeling of ornidazole interaction with ferredoxin of the protozoan Trichomonas suggests efficient electron tunneling from the iron-sulfur cluster to the nitro group of the drug. A similar docking site of ornidazole at the plant-type ferredoxin does not exist, and the best simulated alternative does not support such efficient tunneling. Notably, TPI was inhibited by ornidazole in the dark or when electron transport was blocked by dichloro-methyl diphenyl urea (DCMU), indicating that this inhibition was unrelated to the electron transport machinery. Although TPI and GAPDH isoenzymes are involved in glycolysis and gluconeogenesis, ornidazole effect on respiration of photoautotrophs is moderate, thus raising its value as an efficient inhibitor of photosynthesis. The scarcity of Calvin cycle inhibitors able to penetrate through cell membranes emphasizes the value of ornidazole for studying the regulation of this cycle.
- Inhibitor of photosynthesis
- Triose-phosphate isomerase
- Glyceraldehyde-3-phosphate dehydrogenase
- Mechanism of action
- ©2016 The Author(s)
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