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[Microbiology] Re: Methyl-Citrate-Cycle - Context and Questions

Bob bbx107 at excite.XXXX.com
Sat Mar 4 22:44:48 EST 2006

On Sat, 04 Mar 2006 20:42:16 +0100, Keno März <Dr.K.No at gmx.de> wrote:

>Bob schrieb:

>> I never heard of anything called the methyl citric acid cycle. If that
>> is supposed to be some analogy to the citric acid cycle (TCA cycle),
>> note that the TCA cycle also works by loss of 2 C. So there is still a
>> problem of the extra C somewhere.
>> I wouldn't be too surprised if there are various ways to deal with
>> propionyl CoA.
>In fact, it seems to be a circle similar to the TCA-Cycle, but not the 
>same. In this path, Propionyle-CoA is combined with Oxalate
>to form Methylcitrate. H2O is split off (what's the correct english word 
>therefore anyway?) 

elimination; more specifically, dehydration

>to form Methyl-Cis-Aconitate. One water is added then 
>to form Methyl-Iso-Citrate (hence the name).

A side note... You realize that the water is added back "randomly" --
giving both citrate and isocitrate? The "forward direction" is
determined by the following step consuming the isocitrate, not by the
aconitase actually knowing what it is doing.

>Unfortunatly, my Book doesn't cover that...
>Is it possible that Methyl-Iso-citrat could be split into Pyruvate and 
>Succinate? That would be a nice explanation for what's happening, but I 
>could not verify that.
>But thanks for your help again :o)



This talks about one step in the pathway you suggest, for Salmonella
growing on propionate. This prompted me to quickly look at some other
things I have. My sense is that this is a fairly recent discovery. A
source not much older than this article said that propionate
catabolism was not well understood, but probably went thru succinate
or pyruvate.

If you follow up on the article above, you will probably uncover the
rest of this methyl-TCA cycle. If you have access to Web of Science,
try it, or search PubMed (e.g., Salmonella propionate), or use
whatever other resources are available to you.

So I now admit there does seem to be such a cycle, probably fairly
recently discovered. But I will remain skeptical that this is the main
pathway that most organisms use for the small amount of propionate
they get from long chain odd-C FA. Perhaps what was well known is no
longer true, but I would want to see the evidence. 

There is nothing wrong with there being multiple pathways for such a
process. Different organisms may use different processes -- either
because one is better suited for large fluxes, or simply due to
historical accident. And sometimes multiple pathways are together,
acting redundantly. That confounds analysis, and sometimes incorrect
oversimplified results are reported.

I don't know your specific context: what specific info you have been
given. So feel free to ignore as much of the previous two paragraphs,
about my cautions, as seems appropriate for what you are specifically

If all else fails, go eat some Swiss cheese, and see what happens.

Ah, also see
(My curiosity...)



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