pre-mitochondrial electron transport

Andrew J. Roger aroger at ac.dal.ca
Tue Apr 16 08:02:14 EST 1996


grun at acpub.duke.edu (Ron Grunwald) wrote:

>I'm afraid that I'm a bit lost. Even if you posit that modern glycolysis
>is eubacterial in origin, there still appears to be common theme in both
>the archeabacteria and eukaryotes of substrate-level oxidative
>phophorylation of glyceraldehyde-phosphate. If they share a common
>ancestor, then you must be suggesting that the proto-eukaryote lost its
>ancestral GAPDH prior to endosymbiosis. Truly an unfortunate fellow, for
>whom your question seems most appropriate. 

Maybe it was a case where the ancestral eukaryotic GAPDH could not
work well in concert with a eubacterial TPI (and the enzymes above
it in the pathway)- so for a while several enzymes existed and
the one which worked best eventually was retained while the
other was lost.

>
>However, is the apparent similarity between the eukaryotic GAPDH and a
>eubacterial GAPDH compelling reason to believe that there was not an
>ancestral GAPDH? 

>Perhaps the ancestral cytosolic GAPDH was lost
>secondarily to endosymbiosis. We would then answer your question not with
>glycolysis, per se, but with some related pathway involving oxidation of
>GAP. 
>
>Hard to show evidence for, I suppose. Andrew's suggestion is pertinent
>here - one would have to consider the GAPDH of a truly primitive
>amitochondriote (if such a thing exists). The relationships of the other
>enzymes of carbohydrate metabolism would be informative as well if
>remnants of the primitive pathway remain. Indeed, are the eukaryotic E-D
>enzymes related to the archeabacterial enzymes?. 

Do eukaryotes have Entner-Doudoroff enzymes?  I would be
intrigued to know.

>
>Alternatively, is it clear that the archeabacterial GAPDH, so dissimilar
>to the eukaryotic isoform, is truly primitve? I'm way out of field here,
>but isn't some caution due when drawing phylogenetic conclusions from
>metabolic enzymes of these environmental extremists? Perhaps the
>relatedness of the eukaryotic and eubacterial enzymes speaks only to
>common ancestry, shared with the archea, and not to chimerism. The
>question then becomes whither the primitive enzyme in archaebacteria, not
>the eukaryotes. Perhaps this is too ad hoc?

The problem is that there are several classes of eubacterial GAPDHs.
There is a huge bacterial assemblage which is quite distant to
the eukaryotes (in fact the new archaebacterial sequence from
Haloarcula vallismortis is a little more similar to the eukaryote
enzymes than this eubacterial group is). Then there are GAPDH
enzymes found in cyanobacteria (Anabaena and Synechocystis) and
enzymes found in gamma-proteobacteria (ranging from E.coli to
Haemophilus) which fall RIGHT in the eukaryote clade (they are
far more similar to euk enzymes than either archaebacterial ones
or the big eubacterial assemblage). Moreover, there are multiple
copies of GAPDH in E.coli and the cyanobacteria- the above mentioned
ones and ones which fall in the eubacterial assemblage. 

So the explanation is likely either:

1) cyanobacteria and E.coli received these eukaryotic-like GAPDH
genes from lateral transfer from eukaryotes
2) eukaryotes received their GAPDH genes from bacteria (this
scenario invokes a minimum of 2 transfers if one pays attention
to the topology of the GAPDH tree)

Scenario 2) predicts that more eubacterial versions which are 
eukaryotic-like will pop-up (there is some unpublished evidence
for this view), while scenario 1) argues that the eukaryotic
like versions in eubacteria will be phylogenetically restricted
to recently evolved clades of eubacteria.

Anyway, its very complicated any way you look at it and I presonally
am preferring 2) right now. 

>
>> OR did they exist at all?
>
>If not, then what?

What I imagine James was suggesting is it is possible that mitochondrial
endosymbiosis occurred during (or even causing) the origin of the
nucleus, cytoskeleton etc. Thus, there were no premitochondrial
eukaryotes, just premitochondrial prokaryotic ancestors of eukaryotes.
(this view is similar to what Margulis has argued in the past).

Cheers
Andrew J. Roger





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