"James O. McInerney Ph.D." <jamm at nhm.ac.uk> wrote:
>Hi,
>>The accepted (??) wisdom is that a proteobacterium became an
>endosymbiont of an early eukaryote and this led to the present day
>system. The timing for this event has been suggested to have occurred
>after the splitting of some organisms (ie Giardia) from the rest of the
>eukaryotes (Giardia lacks mitochondria).
>>My question:
>>What did these early eukaryotes do for energy??
>>We have some examples of eukaryotes (eg. T. vaginalis) that do not have
>mitochondria, but clearly their ancestors HAD mitochondria (sequence
>similarities of chromosomally-encoded genes to bacterial counterparts
>etc.).
>>How did pre-mitochondrial eukaryotes break down complex molecules into
>CO2 and H2O?
>Are there eukaryotic mechanisms (not mitochondrially-encoded) that
>facilitate such processes?
If diplomonads (like Giardia) and microsporidia are truly
primitively amitochondrial, then we need only look at their
carbon metabolism to infer the answer to your question. As Ron
Grunwald said, these organisms (we really only know about the
metabolism of Giardia) use glycolysis. The end-product of this
is pyruvate. Instead of the usual conversion of pyruvate to
acetyl-CoA by the pyruvate dehydrogenase complex which occurs
in the mitochondrion-containing eukaryotes, organisms like
Giardia use pyruvate:ferredoxin oxidoreductase. Subsequently,
acetyl-CoA is converted to acetate by an acetate thiokinase...
this phosphorylates ADP to ATP in the process.
So the end products are not just H20 and C02 but include
acetate and ethanol.
However, the difficult question is whether this is the ancestral
pathway or a pathway of secondarily amitochonrial eukaryotes.
Entamoeba, certainly a secondarily amitochondrial protist,
does all of this in a very similar manner to Giardia-- whose
amitochondriate nature may be primitive. It does not matter
too much if Entamoeba has just reverted to the ancestral state....
but if Giardia is secondarily amitochondrial, it is still
possible that parts of glycolysis and the pyruvate:ferridoxin
oxidoreductase and other carbohydrate-metabolism enzymes are derived from
endosymbiosis.
While this may seem far-fetched, one should examine the case
of GAPDH (glyceraldehyde-3-phosphate dehydrogenase). There are
now two bacterial lineages which are known to possess eukaryotic-like
GAPDH molecules-- molecules which are more similar to the eukaryotic
GAPDH than any archaebacterial enzyme (including the newly discovered
"normal" GAPDH from Haloarcula vallismortis). It is possible that typical
eukaryotic GAPDH is derived from the mitochondrial
endosymbiosis. If many of the carbohydrate metabolism enzymes turn
out to be more similar to the alpha-proteobacterial endosymbionts
genes than to the archaebacterial versions, then we will have
evidence that glycolysis is not necessarily ancestral to
eukaryotes. The fact that most archaebacteria, the sister group
of eukaryotic nucleii, use the Entner-Doudoroff pathway (instead of
Embden-Meyerhof) may also be evidence for such a view.
The only way we can find out if doubts like this are true is
to:
1) characterize genes which are clearly derived from mitochondrial
endosymbiosis in Giardia and microsporidia
2) sequence homologs of the carbohydrate-metabolism enzymes
(especially those found in Giardia) from diverse
eubacterial groups including the sisters of mitochondria-- the
alpha-proteobacteria, to see if there is a close affinity to the
nuclear version to the exclusion of archaebacteria.
Cheers
Andrew J. Roger
Dept. of Biochemistry
Dalhousie University
Halifax, N.S.
aroger at ac.dal.ca
Some references might be useful:
1) M. Muller, 1988, Energy metabolism of protozoa without
mitochondria. Ann. Rev. Microbiol. 42:465-88
2) Martin, W., Brinkmann, H., Savonna, C. & Cerff, R. 1993. Evidence for a
chimeric nature of nuclear genomes: eubacterial origin of eukaryotic
glyceraldehyde-3-phosphate dehydrogenase genes. Proc. Natl. Acad. Sci. USA,
90:8692-6.
3) Clark, C. G. & Roger, A. J. 1995. Direct evidence for secondary loss of
mitochondria in Entamoeba histolytica. Proc. Natl Acad. Sci. USA.,
92:6518-6521.
