The Hydrogenosome

Summary

Some organisms lack conventional mitochondria and instead contain divergent mitochondrial-related organelles, called hydrogenosomes, which are double-membrane bound organelles and produce molecular hydrogen. Phylogenetic and biochemical analyses of hydrogenosomes indicated a common origin with mitochondria. Hydrogenosomes are spherical or slightly elongated organelles found in non-mitochondrial organisms such as some protists and fungi which live in anaerobic or microaerophilic environments. The most-studied hydrogenosomes are those in the human pathogen, Trichomonas vaginalis. Hydrogenosomes are polyphyletic and have arisen independently in several eukaryotic lineages. Like mitochondria hydrogenosomes produce ATP, participate in the metabolism of pyruvate formed during glycolysis, incorporate calcium, import proteins post-translationally and divide in the same way. However, they differ from mitochondria by the absence of genetic material, at least in trichomonas, lack a respiratory chain and cytochromes, absence of the F₀–F₁ ATPase (see Chap. 6), absence of the tricarboxylic acid cycle, lack of oxidative phosphorylation and absence of cristae. ATP is generated by the Trichomonas hydrogenosome by substrate level phosphorylation involving acetyl CoA released by the decarboxylation of pyruvate. Hydrogenosomes are considered an excellent drug target. The sequencing of the T. vaginalis genome allowed bioinformatic identification of putative hydrogenosomal proteins through screening for the conserved N-terminal presequence motif. The Trichomonas vaginalis hydrogenosome proteome is highly reduced relative to mitochondria. Of the 569 proteins in the hydrogenosomes proteome, ~30 % are proteins with important functions such as amino acid and energy metabolism, Fe–S cluster assembly, flavin-mediated catalysis, oxygen stress response, membrane translocation, chaperonin functions, proteolytic processing and ATP hydrolysis.