pre-mitochondrial electron transport

Andrew J. Roger aroger at ac.dal.ca
Mon Apr 15 14:41:22 EST 1996


"James O. McInerney Ph.D." <jamm at nhm.ac.uk> wrote:

>So again I put out the question (maybe clearer this time).  Lets say 
>glycolysis is bacterial in origin (devil's advocate maybeee?), the 
>pyruvate dehydrogenase complex is mitochondrial and hydrogenosomes in 
>Trichomonas are modified mitochondria.....what did these early 
>amitochondrial eukaryotes do for energy?  OR did they exist at all?
>

Well I guess most simplistically one could say that their carbon
metabolism was essentially like that of archaebacteria- Entner-
Doudoroff instead of the top part of Embden-Meyerhof and
GAPDH and below the same as Embden-Meyerhof. It could be
that the enzymes unique to the Embden-Meyerhof (TPI and
above) came in with mitochondria. The enzymes like GAPDH
and below were then replaced by the eubacterial versions because
their properties were better suited to working in concert
with the enzymes including and prior to TPI in the Embden-Meyerhof 
pathway....it is just a guess. It could be that they catabolized
amino acids and lipids also for energy. 

If one considers the ecology of a change from prokaryote to
eukaryotic lifestyle, it is clear that the organism is 
changing from a primarily chemoautotrophic or photoautotrophic
organism (like many archaebacteria) to a 
phagotrophic (heterotrophic) organism
(like most non-photosynthetic protists- e.g. free-living
diplomonads like Trepomonas agilis or Hexamita inflata). So
the origin of phagocytosis probably allowed the organism 
access to complex organic nutrients (lipids, sugars and
proteins) which it formerly did not have. It seems likey
to me that this would cause a huge change in the selective
pressure on its metabolism. It would need to have lots
of diverse catabolic enzymes....but this is where my
knowledge of the evolutionary diversity of biochemistry
breaks down (no surprise). Perhaps taking in an endosymbiont
which was extremely efficient at doing catabolic metabolism
(e.g.- a photosynthetic alpha purple bacterium with oxidative
phosphorylation??) was the best solution to making use of
all of the cheap complex biochemicals.

Cheers
Andrew J. Roger





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