A hydrogenosome is a

membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. B ...

-enclosed

organelle In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as organs are to the body, hence ''organelle,'' the ...

found in some anaerobic ciliates, flagellates, and fungi. Hydrogenosomes are highly variable organelles that have presumably evolved from protomitochondria to produce molecular

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...

and ATP in anaerobic conditions. 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.

History

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 of pyruvate:ferredoxin oxido-reductase and

hydrogenase A hydrogenase is an enzyme that catalyses the reversible oxidation of molecular hydrogen (H2), as shown below: Hydrogen uptake () is coupled to the reduction of electron acceptors such as oxygen, nitrate, sulfate, carbon dioxide (), and fumarat ...

eukaryote Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...

s. Further studies were subsequently conducted on the biochemical

cytology Cell biology (also cellular biology or cytology) is a branch of biology that studies the structure, function, and behavior of cells. All living organisms are made of cells. A cell is the basic unit of life that is responsible for the living an ...

and subcellular organization of several anaerobic

protozoan parasite Protozoan infections are parasitic diseases caused by organisms formerly classified in the kingdom Protozoa. They are usually contracted by either an insect vector or by contact with an infected substance or surface and include organisms that are ...

s (ex:''

Trichomonas vaginalis ''Trichomonas vaginalis'' is an anaerobic, flagellated protozoan parasite and the causative agent of a sexually transmitted disease called trichomoniasis. It is the most common pathogenic protozoan that infects humans in industrialized countries ...

Tritrichomonas foetus ''Tritrichomonas foetus'' is a species of single-celled flagellated parasites that is known to be a pathogen of the bovine reproductive tract as well as the intestinal tract of cats. In cattle, the organism is transmitted to the female vagina ...

Giardia lamblia ''Giardia duodenalis'', also known as ''Giardia intestinalis'' and ''Giardia lamblia'', is a flagellated parasitic microorganism of the genus '' Giardia'' that colonizes the small intestine, causing a diarrheal condition known as giardiasis. ...

, and

Entamoeba ''Entamoeba'' is a genus of Amoebozoa found as internal parasites or commensals of animals. In 1875, Fedor Lösch described the first proven case of amoebic dysentery in St. Petersburg, Russia. He referred to the amoeba he observed microscopic ...

sp.)''. Using information obtained from hydrogenosomal and biochemical cytology studies these researchers determined the mode of action of

metronidazole Metronidazole, sold under the brand name Flagyl among others, is an antibiotic and antiprotozoal medication. It is used either alone or with other antibiotics to treat pelvic inflammatory disease, endocarditis, and bacterial vaginosis. It i ...

(Flagyl) . Today, metronidazole is recognized as a standard

chemotherapeutic agent This is a list of chemotherapeutic agents, also known as cytotoxic agents or cytostatic drugs, that are known to be of use in chemotherapy for cancer. This list is organized by type of agent, although the subsections are not necessarily definitive ...

for the treatment of anaerobic infections. 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''.

Description

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. Hydrogenosomes range from 0.5-2 micrometers and are bound by a double membrane. They are most often dumb-bell-shaped and found in large complexes of stacked hydrogenosomes. These stacks range from 4 or 5 (called juvenile complexes) to 20 or more hydrogenosomes. In most cases, hydrogenosomes are genomeless, as a majority of the mitochondrial genome was transferred to the nucleus; because of this, all hydrogenosomal proteins are imported to the organelle. However, a hydrogenosomal genome has been detected in the cockroach

ciliate The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to flagellum, eukaryotic flagella, but are in general shorter and present in much larger numbers, with a ...

''Nyctotherus ovalis'', and the

stramenopile Stramenopile is a clade of organisms distinguished by the presence of stiff tripartite external hairs. In most species, the hairs are attached to flagella, in some they are attached to other areas of the cellular surface, and in some they have be ...

'' 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 A mitosome is an organelle found in some unicellular eukaryotic organisms, like in members of the supergroup Excavata. The mitosome was found and named in 1999, and its function has not yet been well characterized. It was termed a ''crypton'' by o ...

(another kind of degenerate mitochondria).

Source organisms

A non-exhaustive list of organisms containing hydrogenosomes includes: * parabasalid flagellates (e.g. ''

'', ''

Histomonas meleagridis ''Histomonas meleagridis'' is a species of parasitic protozoan that infects a wide range of birds including chickens, turkeys, peafowl, quail and pheasants, causing infectious enterohepatitis, or histomoniasis (blackhead diseases). ''H. meleagri ...

'') * preaxostylid flagellates (e.g. ''

Trimastix pyriformis ''Trimastix'' is a genus of excavates, the sole occupant of the order Trimastigida. ''Trimastix'' are bacterivorous, free living and anaerobic. When first observed in 1881 by William Kent, the morphology of Trimastix was not well describedKent, ...

'') * heterolobosean amoeboflagellates (e.g. ''

Psalteriomonas lanterna ''Psalteriomonas lanterna'' is a species of amoebae in the group of Heterolobosea. The cells of the flagellate stage show four nuclei, four ventral grooves and four mastigont systems, each with four flagellum, flagella. It lacks a Golgi apparatus a ...

'') * anaerobic ciliates (e.g. '' Nyctotherus ovalis'', ''

Metopus palaeformis ''Metopus'' is a genus of anaerobic organisms An anaerobic organism or anaerobe is any organism that does not require molecular oxygen for growth. It may react negatively or even die if free oxygen is present. In contrast, an aerobic organism ...

'', '' Trimyema compressum'', '' Caenomorpha uniserialis'', '' Dasytricha ruminantium'') * anaerobic chytridiomycete fungi (e.g. ''

Neocallimastix ''Neocallimastix'' is a genus of obligately anaerobic rumen fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar ...

'' spp., ''

Piromyces ''Piromyces'' is a genus of fungi in the family Neocallimastigaceae. ''Piromyces sp.'' E2 physiology and genome Piromyces sp. E2 is an eukaryotic species belonging to the phylum Chytridiomycota, which comprises organisms that possess flagellat ...

'' spp.) The vast variety of source organisms can be accredited to the theorized convergent evolution of hydrogenosomes from mitochondria to fit an anaerobic environment. In 2010, scientists have also reported their discovery of the first known anaerobic

metazoans Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...

with hydrogenosome-like organelles. Three multicellular species of

Loricifera Loricifera (from Latin, '' lorica'', corselet (armour) + ''ferre'', to bear) is a phylum of very small to microscopic marine cycloneuralian sediment-dwelling animals that had been determined to be 37 described species, in 9 genera, bu ...

— '' Spinoloricus nov. sp.'', '' Rugiloricus nov. sp.'' and '' Pliciloricus nov. sp.'' — have been found deep in Mediterranean sediments, and use hydrogenosomes in their anaerobic metabolism cycle.

ATP Synthesis

The hydrogenosomes of trichomonads (the most studied of the hydrogenosome-containing microorganisms) produce molecular

, acetate,

carbon dioxide Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...

and ATP by the combined actions of pyruvate:ferredoxin oxido-reductase,

, acetate:succinate CoA transferase and succinate thiokinase. Superoxide dismutase,

malate dehydrogenase Malate dehydrogenase () (MDH) is an enzyme that reversibly catalyzes the oxidation of malate to oxaloacetate using the reduction of NAD+ to NADH. This reaction is part of many metabolic pathways, including the citric acid cycle. Other malate ...

(decarboxylating), ferredoxin,

adenylate kinase Adenylate kinase ( ECbr>2.7.4.3 (also known as ADK or myokinase) is a phosphotransferase enzyme that catalyzes the interconversion of the various adenosine phosphates (ATP, ADP, and AMP). By constantly monitoring phosphate nucleotide levels inside ...

and NADH:ferredoxin oxido-reductase are also localized in the hydrogenosome.

References

{{portal bar, Biology, Science Organelles

History

Description

Source organisms

ATP Synthesis

See also

References