pre-mitochondrial electron transport

Ron Grunwald grun at acpub.duke.edu
Mon Apr 15 19:57:24 EST 1996


In article <4kt5o4$d44 at mserv1.dl.ac.uk>, jamm at nhm.ac.uk wrote:

> Firstly,
> 
> Many thanks to Ron for the entertaining reply. I'm afraid
> though that Andrew's answer is closer to what I was looking
> for.  But it was definitely entertaining.

As is so often the case, levity is the handy substitute for true knowledge
and understanding. I'm afraid that I was completely unware of the
suggestion that the standard glycolitic enzymes may endosymbiotic and
origin. Fascinating! My thanks to both yourself and Dr. Roger for the
fascinating thread.

> 
> Andrew wrote...
> 
> > 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.
> 
> 
> There are a great many inconsistencies between trees derived
> from sequences from organisms close to the base of the tree of life
> (check out the 9404 archive of this newsgroup for an excellent
> debate on this).  This has led to numerous chimeric/endosymbiont/
> lateral transfer hypotheses (each with their own merits).  Now 
> glycolysis is not clearly proven to be strictly vertically transmitted 
> through eukaryotes (correct me if I'm wrong but pyruvate:ferredoxin 
> oxidoreductase in the amitochondrial T. vaginalis is closely related to 
> the bacterial gene pyruvate:flavodoxin oxidoreductase)  so this gene may 
> be endosymbiotic in origin (Hrdy and Muller, 1995. J. mol. Evol.).
> 
> Then there is the GAPDH question!!
> 
> 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? 

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. 

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?. 

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?

> OR did they exist at all?

If not, then what?

-- 
Ron Grunwald 
Instructor, Department of Botany
grun at acpub.duke.edu                            



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