Abstract

The hydrogenosome is an organelle described in trichomonads, parasitic protists where most studies were carried out and also in organisms phylogenetically distant such as rumen ciliates, some free-living ciliates and chytrid fungí (Chytridiomycota). The or­ganisms presenting hydrogenosomes live in anaerobic habitats and they all lack mitochondria. The hydrogenosome contains enzymes that par­ticípate in the metabolism of pyruvate and is the site of formation of ATP and molecular hydrogen. The origin of the hydrogenosome brought a great debate, since, like mitochondria, it has a double membrane envelope, divides autonomously by fission, imports proteins post-translationally, and produces ATP. How- ever, it differs from mitochondria in that it seems to lack a genome, a respiratory chain, cytochromes, the F F system, the tricarboxylic acid cycle, oxidative phosphorylation, and cardiolipin. The hydrogenosome uses pyruvate: ferredoxin oxidoreductase, a counterpart to the mitochondrial pyruvate dehydrogenase complex and presents hydrogenase, an enzyme typically restricted to anaerobes. Hypotheses have been proposed for the origin of the trichomonad hydrogenosome: an independent endosymbiosis of an anaerobic eubacterium with a eukaryotic host or the conversión of an established mi- mitochondrion adapted to an anaerobic lifestyle. The common origin hypothesis for mitochondria and hydrogenosome was refined to state that both organelles evolved from a common progenitor structure present in eukaryotes before the advent of true mitochondria or hydrogenosome.


Hydrogenosome is a spherical or slightly elongated structure (when in process of división), 0.5-1.0 pm diameter, usually associated with cytoskeletal structures such as the axostyle and costa in trichomonads. The matrix of the hydrogenosome is homogeneously granular. Crista-like invaginations of inner-membrane were described in the hydrogenosome of some rumen cili­ates. The matrix also prevents calcium deposits. The hydrogenosome of trichomonads contains a flattened, membrane-bounded vesicle completely dissimilar from the hydrogenosomes matrix. This vesicle con­tains high levéis of Mg++, Ca++ and P and possibly functions in intracellular calcium regulation. Its mem­brane presents N-acetyl-glucosamine-containing glycoconjugates. Close proximity and even continuity, between the endoplasmic reticulum and the hydrogenosome were observed. Hydrogenosomal proteins are synthesized on free polyribosomes, protein import occurs post-translationally, is dependent on the presence of a cleavable N-terminal presequence, cytosolic protein (s) and energy in the form of temperature, ATP and a transmembrane electrochemical potential, similar to that of mitochondria and chloroplasts.