hydrogenosomes

Hydrogenosomes are double-membrane–bound organelles found in several anaerobic unicellular eukaryotes. They are related to mitochondria by origin and molecular features but function without oxygen and typically lack a resident genome. In many species they are considered mitochondrion-related organelles (MROs), reflecting their evolutionary link to mitochondria.

Metabolically, hydrogenosomes generate ATP under anaerobic or microaerophilic conditions. They oxidize pyruvate to acetyl-CoA and CO2 via pyruvate:ferredoxin oxidoreductase, producing reduced ferredoxin. Acetyl-CoA is converted to acetate by acetyl-CoA synthetase, yielding ATP through substrate-level phosphorylation. A hydrogenase uses the electrons carried by reduced ferredoxin to produce molecular hydrogen as a byproduct. Unlike mitochondria, hydrogenosomes typically lack an electron transport chain and rely on glycolytic or fermentative pathways for energy.

Genetically, hydrogenosomes often do not contain a complete genome and rely on nuclear-encoded, imported proteins. In some organisms a small genome has been found, but the organelles generally depend on cytosolic gene products. Their composition and presence vary among species, and many hydrogenosomes have been studied primarily in parabasalids such as Trichomonas vaginalis, with other occurrences reported in certain ciliates and fungi.

The study of hydrogenosomes informs broader questions about eukaryotic energy metabolism, endosymbiotic evolution, and the diversity of life under anaerobic conditions. Due to their role in pathogenic organisms, hydrogenosomes also contribute to discussions on potential therapeutic targets.