[[File:Hydrogenosom.svg|thumb||right|370px|Abb.1: Model of ATP-synthesis in hydrogenosomes .
abb.: CoA =
Coenzyme A]]A
hydrogenosome is a membrane-enclosed
organelle of some
anaerobic ciliates,
trichomonads and
fungi. The hydrogenosomes of trichomonads (the most studied of the hydrogenosome-containing microorganisms) produce molecular
hydrogen,
acetate,
carbon dioxide and
ATP by the combined actions of
pyruvate:ferredoxin oxido-reductase,
hydrogenase,
acetate:succinate CoA transferase and
succinate thiokinase.
Superoxide dismutase,
malate dehydrogenase (decarboxylating),
ferredoxin,
adenylate kinase and
NADH:ferredoxin oxido-reductase are also localized in the hydrogenosome. This organelle is thought to have
evolved from anaerobic
bacteria or
archaea, though in the case of trichomonad hydrogenosomes the question remains open.
In 2010, scientists reported their discovery of the first known anaerobic metazoans with hydrogenosome-like organelles.
History
Hydrogenosomes were isolated, purified, biochemically characterized and named in the early 1970s by D. G. Lindmark and M. 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 in
eukaryotes. Further studies were subsequentially conducted on the biochemical cytology and subcellular organization of anaerobic protozoan parasites (
Trichomonas vaginalis, Tritrichomonas foetus, Monocercomonas sp., Giardia lamblia, Entamoeba sp., and Hexamita inflata). Using information obtained from hydrogenosomal and biochemical cytology studies these researchers determined the mode of action of
metronidazole (Flagyl) in 1976. Metronidazole is today recognized as the gold standard
chemotherapeutic agent for the treatment of anaerobic infections caused by
prokaryotes (
Clostridium, Bacteroides, Helicobacter) and eukaryotes (
Trichomonas, Tritrichomonas, Giardia, Entamoeba).
Metronidazole is taken up by diffusion. Once taken up by anaerobes, it is non-enzymatically reduced by reduced ferredoxin which is produced by the action of pyruvate:ferredoxin oxido-reductase. This reduction creates products toxic to the anaerobic cell, and allows for selective accumulation of the drug in anaerobes.
Description
Hydrogenosomes are approximately 1
micrometre in diameter but under stress conditions can reach up to 2
micrometre and are so called because they produce molecular hydrogen.
Like
mitochondria, they are bound by distinct double membranes and one has an inner membrane with some
cristae-like projections.
Some hydrogenosomes may have evolved from mitochondria by the concomitant loss of classical mitochondrial features, most notably its
genome. A hydrogenosomal genome could not be detected in
Neocallimastix,
Trichomonas vaginalis and
Tritrichomonas foetus. However, a hydrogenosomal genome has been detected in the cockroach ciliate
Nyctotherus ovalis, and the
stramenopile Blastocystis. 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).
Sources
The best studied hydrogenosomes are those of the sexually transmitted parasites
Trichomonas vaginalis and
Tritrichomonas foetus and those from
rumen chytrids such as
Neocallimastix.
The anaerobic ciliated protozoan Nyctotherus ovalis, found in the hindgut of several species of cockroach, has numerous hydrogenosomes that are intimately associated with endosymbiotic methane-producing archaea, the latter using the hydrogen produced by the hydrogenosomes. The matrix of N. ovalis hydrogenosomes contains ribosome-like particles of the same size as a numerous type of ribosome (70s) of the endosymbiotic methanogenic archaea. This suggested the presence of an organellar genome which was indeed discovered by Akhmanova and later partly sequenced by Boxma.
Three multicellular species of Loricifera — Spinoloricus nov. sp., Rugiloricus nov. sp. and Pliciloricus — have been found deep in Mediterranean sediments, and use hydrogenosomes in their anaerobic metabolism cycle.
References
See also
Mitochondria
Chloroplast
Category:Organelles
