Hydrogenosome
Encyclopedia
A hydrogenosome is a membrane-enclosed organelle
of some anaerobic
ciliate
s, 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.
. 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 eukaryote
s. 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 prokaryote
s (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.
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).
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 methanogen
ic 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 nov. sp. — have been found deep in Mediterranean sediments, and use hydrogenosomes in their anaerobic metabolism cycle.
Organelle
In cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid bilayer....
of some anaerobic
Anaerobic organism
An anaerobic organism or anaerobe is any organism that does not require oxygen for growth. It could possibly react negatively and may even die if oxygen is present...
ciliate
Ciliate
The ciliates are a group of protozoans characterized by the presence of hair-like organelles called cilia, which are identical in structure to flagella but typically shorter and present in much larger numbers with a different undulating pattern than flagella...
s, trichomonads
Trichomonas
Trichomonas is a genus of anaerobic protists that are parasites of vertebrates. They are included with the parabasalids.Species of Trichomonas include:*Trichomonas vaginalis, an organism generally living inside the vagina of humans...
and fungi. The hydrogenosomes of trichomonads (the most studied of the hydrogenosome-containing microorganisms) produce molecular hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
, acetate
Acetate
An acetate is a derivative of acetic acid. This term includes salts and esters, as well as the anion found in solution. Most of the approximately 5 billion kilograms of acetic acid produced annually in industry are used in the production of acetates, which usually take the form of polymers. In...
, carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
and ATP
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
by the combined actions of pyruvate:ferredoxin oxido-reductase
Pyruvate synthase
In enzymology, a pyruvate synthase is an enzyme that catalyzes the chemical reactionThe 3 substrates of this enzyme are pyruvate, CoA, and oxidized ferredoxin, whereas its 4 products are acetyl-CoA, CO2, reduced ferredoxin, and H+....
, hydrogenase
Hydrogenase
A hydrogenase is an enzyme that catalyses the reversible oxidation of molecular hydrogen . Hydrogenases play a vital role in anaerobic metabolism....
, acetate:succinate CoA transferase
Acetate CoA-transferase
In enzymology, an acetate CoA-transferase is an enzyme that catalyzes the chemical reactionThus, the two substrates of this enzyme are acyl-CoA and acetate, whereas its two products are fatty acid anion and acetyl-CoA....
and succinate thiokinase
Succinyl coenzyme A synthetase
Succinyl Coenzyme A Synthetase is an enzyme that catalyzes the reversible reaction of succinyl-CoA to succinate...
. Superoxide dismutase
Superoxide dismutase
Superoxide dismutases are a class of enzymes that catalyze the dismutation of superoxide into oxygen and hydrogen peroxide. As such, they are an important antioxidant defense in nearly all cells exposed to oxygen...
, malate dehydrogenase
Malate dehydrogenase
Malate dehydrogenase is an enzyme in the citric acid cycle that catalyzes the conversion of malate into oxaloacetate and vice versa...
(decarboxylating), ferredoxin
Ferredoxin
Ferredoxins are iron-sulfur proteins that mediate electron transfer in a range of metabolic reactions. The term "ferredoxin" was coined by D.C. Wharton of the DuPont Co...
, adenylate kinase
Adenylate kinase
Adenylate kinase is a phosphotransferase enzyme that catalyzes the interconversion of adenine nucleotides, and plays an important role in cellular energy homeostasis.-Substrate and products:...
and NADH:ferredoxin oxido-reductase
Ferredoxin-NAD+ reductase
In enzymology, a ferredoxin–NAD+ reductase is an enzyme that catalyzes the chemical reaction:Thus, the two substrates of this enzyme are reduced ferredoxin and NAD+, whereas its 3 products are oxidized ferredoxin, NADH, and H+...
are also localized in the hydrogenosome. This organelle is thought to have evolved
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
from anaerobic bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
or archaea
Archaea
The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...
, though in the case of trichomonad hydrogenosomes the question remains open.
In 2010, scientists reported their discovery of the first known anaerobic metazoans
Eumetazoa
Eumetazoa is a clade comprising all major animal groups except sponges, placozoa and several other little known animals. Characteristics of eumetazoans include true tissues organized into germ layers, and an embryo that goes through a gastrula stage...
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 UniversityRockefeller University
The Rockefeller University is a private university offering postgraduate and postdoctoral education. It has a strong concentration in the biological sciences. It is also known for producing numerous Nobel laureates...
. 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 eukaryote
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
s. Further studies were subsequentially conducted on the biochemical cytology and subcellular organization of anaerobic protozoan parasites (Trichomonas vaginalis
Trichomonas vaginalis
Trichomonas vaginalis is an anaerobic, flagellated protozoan, a form of microorganism. The parasitic microorganism is the causative agent of trichomoniasis, and is the most common pathogenic protozoan infection of humans in industrialized countries. Infection rates between men and women are the...
, Tritrichomonas foetus
Tritrichomonas foetus
Tritrichomonas foetus is a single celled flagellated protozoan parasite 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 and uterus from the foreskin of the bull where the parasite...
, Monocercomonas sp., Giardia lamblia
Giardia lamblia
Giardia lamblia is a flagellated protozoan parasite that colonizes and reproduces in the small intestine, causing giardiasis. The giardia parasite attaches to the epithelium by a ventral adhesive disc, and reproduces via binary fission...
, Entamoeba
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 microscopically as 'Amoeba coli'; however it is not clear whether he was...
sp., and Hexamita inflata). Using information obtained from hydrogenosomal and biochemical cytology studies these researchers determined the mode of action of metronidazole
Metronidazole
Metronidazole is a nitroimidazole antibiotic medication used particularly for anaerobic bacteria and protozoa. Metronidazole is an antibiotic, amebicide, and antiprotozoal....
(Flagyl) in 1976. Metronidazole is today recognized as the gold standard chemotherapeutic agent
Chemotherapy
Chemotherapy is the treatment of cancer with an antineoplastic drug or with a combination of such drugs into a standardized treatment regimen....
for the treatment of anaerobic infections caused by prokaryote
Prokaryote
The prokaryotes are a group of organisms that lack a cell nucleus , or any other membrane-bound organelles. The organisms that have a cell nucleus are called eukaryotes. Most prokaryotes are unicellular, but a few such as myxobacteria have multicellular stages in their life cycles...
s (Clostridium
Clostridium
Clostridium is a genus of Gram-positive bacteria, belonging to the Firmicutes. They are obligate anaerobes capable of producing endospores. Individual cells are rod-shaped, which gives them their name, from the Greek kloster or spindle...
, Bacteroides
Bacteroides
Bacteroides is a genus of Gram-negative, bacillus bacteria. Bacteroides species are non-endospore-forming, anaerobes, and may be either motile or non-motile, depending on the species. The DNA base composition is 40-48% GC. Unusual in bacterial organisms, Bacteroides membranes contain sphingolipids...
, Helicobacter
Helicobacter
Helicobacter is a genus of Gram-negative bacteria possessing a characteristic helix shape. They were initially considered to be members of the Campylobacter genus, but since 1989 they have been grouped in their own genus...
) and eukaryotes (Trichomonas, Tritrichomonas, Giardia, Entamoeba). Metronidazole
Metronidazole
Metronidazole is a nitroimidazole antibiotic medication used particularly for anaerobic bacteria and protozoa. Metronidazole is an antibiotic, amebicide, and antiprotozoal....
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 micrometreMicrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
in diameter but under stress conditions can reach up to 2 micrometre
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
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
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
. A hydrogenosomal genome could not be detected in Neocallimastix
Neocallimastigomycota
Neocallimastigomycota is a phylum of anaerobic fungi, found in the digestive tracts of herbivores. It encompasses only one family.-Discovery:...
, Trichomonas vaginalis and Tritrichomonas foetus. However, a hydrogenosomal genome has been detected in the cockroach ciliate Nyctotherus ovalis, and the stramenopile Blastocystis
Blastocystis
Blastocystis is a genus of single-celled protozoan parasites belonging to a group of organisms known as the Stramenopiles that includes algae, diatoms, and water molds...
. 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 rumenRumen
The rumen, also known as a paunch, forms the larger part of the reticulorumen, which is the first chamber in the alimentary canal of ruminant animals. It serves as the primary site for microbial fermentation of ingested feed...
chytrids such as Neocallimastix.
The anaerobic ciliated protozoan Nyctotherus ovalis, found in the hindgut
Hindgut
The hindgut is the posterior part of the alimentary canal. In mammals, it includes the distal third of the transverse colon and the splenic flexure, the descending colon, sigmoid colon and rectum.-Blood flow:...
of several species of cockroach, has numerous hydrogenosomes that are intimately associated with endosymbiotic
Endosymbiont
An endosymbiont is any organism that lives within the body or cells of another organism, i.e. forming an endosymbiosis...
methane-producing archaea
Archaea
The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...
, the latter using the hydrogen produced by the hydrogenosomes. The matrix of N. ovalis hydrogenosomes contains ribosome
Ribosome
A ribosome is a component of cells that assembles the twenty specific amino acid molecules to form the particular protein molecule determined by the nucleotide sequence of an RNA molecule....
-like particles of the same size as a numerous type of ribosome (70s) of the endosymbiotic methanogen
Methanogen
Methanogens are microorganisms that produce methane as a metabolic byproduct in anoxic conditions. They are classified as archaea, a group quite distinct from bacteria...
ic 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
Loricifera
Loricifera is a phylum of very small to microscopic marine sediment-dwelling animals with twenty-two described species, in eight genera. Aside from these described species, there are approximately 100 more that have been collected and not yet described. Their size ranges from 100 µm to ca....
— 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.