Hydrogenosome

thumb|Hydrogensomes (H) of Trichomonas vaginalis under transmission electron microscope

A hydrogenosome is a double membrane-enclosed organelle found in some anaerobic eukaryotes such as ciliates, flagellates and fungi. Hydrogenosome is a type of mitochondrion that produces energy (ATP molecules) but in the absence of oxygen and also lacks membrane foldings (cristae) unlike typical mitochondria. They are considered to have evolved from mitochondria to produce molecular hydrogen and ATP in anaerobic conditions.

Hydrogenosomes were discovered in 1973 by Donald Gustav Lindmark and Miklós 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.

History

Hydrogenosomes were isolated, purified, biochemically characterized and named in 1973 by Donald Gustav Lindmark and Miklós Müller at the Rockefeller University. In addition to this discovery of novel organelle, they also demonstrated for the first time the presence of pyruvate:ferredoxin oxido-reductase and hydrogenase in eukaryotes.

Since their discovery, hydrogenosomes have been found in a variety of anaerobic unicellular ciliates, flagellates, and fungi. The most notable amongst these is the parasitic Trichomonas vaginalis.

In 2010, new species of microscopic animals in the phylum Loricifera, such as Rugiloricus, Pliciloricus, and Spinoloricus were described as animals (metazoans) capable of living exclusively in oxygenless conditions, the first ones ever known. Under the electron microscope they have organelles that look like hydrogenosomes. Doubt remains in whether they actually have hydrogenosomes.

Description

thumb|Hydrogenosome with cap (operculum)

thumb|Trichomonas vaginalis. a) Whole body. b) Posterior region showing hydrogenosome (H)

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. However, a hydrogenosomal genome recognizably similar to mtDNA has been detected in the cockroach ciliate Nyctotherus ovalis and the stramenopile Blastocystis.

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 between Nyctotherus and Blastocystis, 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, and mitosomes (another kind of degenerate mitochondria, one that has lost all ATP production and only retains the ability to make iron-sulfur clusters).

• Blastocystis (functionally transitional, has DNA).

A well-studied example is the hydrogenosomes of trichomonads. Its main pathway contains the following enzymes, all localized in the organelle:

• pyruvate:ferredoxin oxido-reductase, which converts pyruvate into acetyl-CoA and CO₂ while reducing the ferredoxin.
• Hydrogenase, which converts H⁺ into H₂ while oxidizing the ferredoxin.
• acetate:succinate CoA transferase, which converts succinate and acetyl-CoA to acetate and succinyl-CoA.
• Succinyl-CoA synthatase (in reverse), which converts succinyl-CoA and ADP + Pi into succinate, coenzyme A, and ATP.

Parts also in the trichomonad hydrogenosome but not part of the main loop include:

The hydrogenosome of Blastocystis is transitional and retains far more mitochondrial components. It retains complexs I, II, many transporters, amino acid biosynthesis pathways, an incomplete citric acid cycle, even oxidative phosphorylation. It has a hydrogenase, a pyruvate:ferredoxin oxido-reductase, and an alternative oxidase.