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Hydrogenosomes: convergent adaptations of mitochondria to anaerobic environments (Hackstein et al., 2001)

This article from Johannes Hackstein (who did the genetics course in my first year of university, btw) is based on two assumptions that are taken for granted but with which I disagree. The first is the view that the anaerobic protists have adapted to this lyfestyle, while I think that they are representatives of a pre-aerobic world and could only survive in anaerobic environments. Second, the endosymbiotic theory and the assumption that ‘mitochondrial’ genes has to be derived from mitochondria.

It is actually surprising to see that hydrogenosomes have absolutely nothing in common with any kind of bacteria we have ever seen, as is acknowledged by Hackstein. Everything is eukaryotic about them, except for some proteins that share some himology to bacterial proteins which is expected with common descent. Nonetheless, a paper with many interesting details about hydrogenosomes. It also shows that many intermediate states exist with respect to metabolic functions.

I show several excerpts that could point to mitochondrial intermediates:


[anaerobic mitochondria:] Since the mitochondria of S. cerevisiae did not retain a mitochondrial complex I, ATP synthesis under anaerobic conditions solely relies on the glycolytic fermentation of glucose to ethanol. In the presence of oxygen, the fermentation pathway is inhibited and the yeast’s metabolism is switched to aerobic respiration with its much higher energy yield (“Pasteureffect”).

[anaerobic mitochondria:] Some of these organisms evolved a peculiar variant of anaerobic respiration, “malate dismutation”, by which endogenous fumarate is reduced to succinate by the enzyme fumarate reductase. In these organisms, fumarate serves as an electron sink. This process requires adaptations of the mitochondrial electron transport chain, i.e., rhodoquinone instead of ubiquinone as electron-carrier (Tielens and van Hellemond, 1998). However, the fumarate “respiration” allows functioning of mitochondrial complex I, i.e., the generation of a proton gradient, also under anoxic conditions. Since the generation of a proton-motive force (PMF) by mitochondrial complex I can be used for the generation of additional ATP, one might conclude that this adaptation is one of the major reasons for the maintenance of the mitochondrial compartment in multicellular anaerobic eukaryotes.

These hydrogenosomes are membrane-bound organelles that measure approximately 1–2 micrometer. They compartmentalise the terminal reactions of the anaerobic cellular energy metabolism and produce hydrogen and ATP. Characteristically, hydrogenosomes import pyruvate that is oxidatively decarboxylated to acetyl-CoA by the action of a pyruvate:ferredoxin oxidoreductase (PFO). An acetate: succinylCoA transferase (ASCT) and a succinate thiokinase (STK) mediate the formation of acetate and ATP, similar to the situation in the “primitive” mitochondria of certain trypanosomes (Fig. 2; Müller, 1993, 1998; van Hellmond et al., 1998; Hackstein et al., 1999). The reduction equivalents that are formed in the decarboxylation of pyruvate are used by a hydrogenase to reduce protons under the formation of molecular hydrogen.

Hydrogenosomes do not co-exist with mitochondria, and, notably, they have neither been detected in multicellular organisms nor in facultative anaerobes that face extended periods of aerobiosis during their life cycles (Roger, 1999). They are found exclusively in anaerobic or microaerophilic unicellular eukaryotes. Since hydrogenosomes compartmentalise terminal reactions of the eukaryotic cellular energy metabolism, they can be regarded as a kind of “anaerobic mitochondria” (Embley et al., 1997; Hackstein et al., 1999; Rotte et al., 2000).

Consequently, all of the approximately 200 proteins identified in the hydrogenosomes of T. vaginalis (Heinze, 2001) should be encoded by nuclear genes, synthesised in the cytoplasm and imported into the hydrogenosome posttranslationally (Fig. 3).

Rather the organelles resemble human mitochondria affected by hereditary mitochondrial diseases (Smeitink et al., 1989; Huizing et al., 1997; Frey and Mannella, 2000).

Because of the mitochondria-like morphology, e.g. the presence of mitochondria-like cristae and putative 70S ribosomes (Fig. 11), it was very suggestive to look for a hydrogenosomal genome, although hydrogenosomes were commonly assumed to lack genomes (Palmer, 1997). Notably, we were able to identify a genome in the “mitochondrial” fraction of homogenates of N. ovalis (Fig.12). Comparison with the mitochondrial DNA from Tetrahymena thermophila suggests that the hydrogenosomal genome might encompass some 40 kb.

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