Ahydrogenosome is amembrane-enclosedorganelle found in someanaerobicciliates,flagellates,fungi, and three species ofloriciferans.[1] Hydrogenosomes are highly variable organelles that have presumably evolved from protomitochondria to produce molecularhydrogen andATP in anaerobic conditions.[2]
Hydrogenosomes were discovered in 1973 by D. G. Lindmark and M. Müller. Because hydrogenosomes hold evolutionary lineage significance for organisms living in anaerobic or oxygen-stressed environments, many research institutions have since documented their findings on how the organelle differs in various sources.[3]
Hydrogenosomes were isolated, purified, biochemically characterized and named in the early 1970s by Lindmark and Müller at Rockefeller University. In addition to this seminal study on hydrogenosomes, they also demonstrated for the first time the presence ofpyruvate:ferredoxin oxido-reductase andhydrogenase ineukaryotes.[3] Further studies were subsequently conducted on the biochemicalcytology andsubcellular organization of several anaerobicprotozoan parasites (ex:Trichomonas vaginalis,Tritrichomonas foetus,Giardia lamblia, andEntamoeba sp.).[2]
Using information obtained from hydrogenosomal and biochemical cytology studies these researchers determined the mode of action ofmetronidazole (Flagyl). Today, metronidazole is recognized as a standardchemotherapeutic agent for the treatment of anaerobic infections.[4][5]
Since their discovery, hydrogenosomes have been found in a variety of anaerobic unicellular ciliates, flagellates, and fungi. The most notable amongst these is the parasiticTrichomonas vaginalis.[6]
Hydrogenosomes are organelles that are speculated to have evolved from mitochondria to provide a different mechanism for anaerobic ATP synthesis utilizing pyruvate. The reaction results in the production of molecular hydrogen, from which the organelle receives its name.[3]
Hydrogenosomes range from 0.5-2 micrometers and are bound by a double membrane. They are most often dumbell-shaped and found in large complexes of stacked hydrogenosomes. These stacks range from 4 or 5 (called juvenile complexes) to 20 or more hydrogenosomes.[2]
In most cases, hydrogenosomes aregenomeless, as a majority of the mitochondrial genome was transferred to the nucleus; because of this, all hydrogenosomal proteins are imported to the organelle.[7][8] However, a hydrogenosomal genome has been detected in the cockroachciliateNyctotherus ovalis, and thestramenopileBlastocystis.[9]
Due to the fact that many organisms have evolved to fit their anaerobic environments, a multitude of organisms have independently evolved hydrogenosomes or structures with similar functions. The similarity betweenNyctotherus andBlastocystis, which are only distantly related, is believed to be the result of convergent evolution, and calls into question whether there is a clear-cut distinction between mitochondria, hydrogenosomes, andmitosomes (another kind of degenerate mitochondria).[2][9]
A non-exhaustive list of organisms containing hydrogenosomes includes:
The vast variety of source organisms can be accredited to the theorized convergent evolution of hydrogenosomes from mitochondria to fit an anaerobic environment.[2][7][9]
In 2010, scientists have also reported their discovery of the first known anaerobicmetazoans with hydrogenosome-like organelles. Three multicellular species ofLoricifera —Spinoloricus nov. sp.,Rugiloricus nov. sp. andPliciloricus nov. sp. — have been found deep in Mediterranean sediments, and use hydrogenosomes in their anaerobic metabolism cycle.[10]
The hydrogenosomes of trichomonads (the most studied of the hydrogenosome-containing microorganisms) produce molecularhydrogen,acetate,carbon dioxide andATP by the combined actions ofpyruvate:ferredoxin oxido-reductase,hydrogenase,acetate:succinate CoA transferase andsuccinate thiokinase.Superoxide dismutase,malate dehydrogenase (decarboxylating),ferredoxin,adenylate kinase andNADH:ferredoxin oxido-reductase are also localized in the hydrogenosome.[11]
