title: The drug ornidazole inhibits photosynthesis in a different mechanism described for protozoa and anaerobic bacteria.
abstract: Ornidazole of the 5-nitroimidazole drug family is used to treat protozoan and anaerobic bacterial infections via a mechanism that involves preactivation 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's 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 dichloromethyl diphenylurea, indicating that this inhibition was unrelated to the electron transport machinery. Although TPI and GAPDH isoenzymes are involved in glycolysis and gluconeogenesis, ornidazole's effect on respiration of photoautotrophs is moderate, thus raising its value as an efficient inhibitor of photosynthesis. The scarcity of Calvin cycle inhibitors capable of penetrating cell membranes emphasizes on the value of ornidazole for studying the regulation of this cycle.
authors.affiliation: Department of Plant Molecular Biology & Ecology, George S. Wise Faculty of Life Sciences, Tel Aviv University, Ramat Aviv, Tel Aviv 6997801, Israel.
authors.forename: Yehouda
authors.initials: Y
authors.lastname: Marcus
authors.affiliation: Department of Chemistry, Faculty of Exact Sciences, Tel Aviv University, Ramat Aviv, Tel Aviv 69978, Israel.
authors.forename: Noam
authors.initials: N
authors.lastname: Tal
authors.affiliation: Department of Plant Molecular Biology & Ecology, George S. Wise Faculty of Life Sciences, Tel Aviv University, Ramat Aviv, Tel Aviv 6997801, Israel.
authors.forename: Mordechai
authors.initials: M
authors.lastname: Ronen
authors.affiliation: The EPR Laboratory, Chemical Research Support, Faculty of Chemistry, Weizmann Institute of Science, Rehovot 76100, Israel.
authors.forename: Raanan
authors.initials: R
authors.lastname: Carmieli
authors.affiliation: Department of Plant Molecular Biology & Ecology, George S. Wise Faculty of Life Sciences, Tel Aviv University, Ramat Aviv, Tel Aviv 6997801, Israel.
authors.forename: Michael
authors.initials: M
authors.lastname: Gurevitz
mesh_terms: D001421:Bacteria, Anaerobic / Q000187:drug effects*; D000458:Cyanobacteria / Q000187:drug effects; D005288:Ferredoxins / Q000378:metabolism; D024581:Glyceraldehyde-3-Phosphate Dehydrogenase (Phosphorylating) / Q000378:metabolism; D005987:Glyceraldehyde-3-Phosphate Dehydrogenases / Q000378:metabolism; D006019:Glycolysis; D008795:Metronidazole / Q000494:pharmacology; D008954:Models, Biological; D009950:Ornidazole / Q000494:pharmacology*; D010788:Photosynthesis / Q000187:drug effects*; D046939:Synechocystis / Q000187:drug effects; D014244:Trichomonas / Q000187:drug effects / Q000378:metabolism; D014305:Triose-Phosphate Isomerase / Q000378:metabolism
chemical_list: D005288:Ferredoxins; D008795:Metronidazole; D009950:Ornidazole; D005987:Glyceraldehyde-3-Phosphate Dehydrogenases; D024581:Glyceraldehyde-3-Phosphate Dehydrogenase (Phosphorylating); D014305:Triose-Phosphate Isomerase
keywords: Calvin cycle; glyceraldehyde-3-phosphate dehydrogenase; inhibitor of photosynthesis; mechanism of action; ornidazole; triose-phosphate isomerase