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

Bob bbx107 at excite.XXXX.com
Sat Mar 4 13:51:20 EST 2006


On Sat, 04 Mar 2006 16:41:13 +0100, Keno März <Dr.K.No at gmx.de> wrote:

>Hello everyone!
>
>I'm a student from Germany, and going through my Microbiology lessons, I 
>stumbled over the Methyl-Citrate-Cycle (I hope this is the correct 
>translation). It is connected with the dissembly of Alkanes (German: 
>Beta-Oxidation). This is what I believe to know:
>- In case the Alkane contains an uneven amount of C-Atoms, the Product 
>of the Beta Oxidation is Propionyl-CoA, which is then processed to 
>Methyl-Isocitrate by the Methyl-Citrate-Cycle.
>
>My Questions:
>- Is the above correct? (That's a stupid one)
>- What happens to the Methyl-Isocitrate?
>- During the course of the Betaoxidation of a C15-Fatty-Acid, would the 
>Products be several Acetyl-CoA and exactly 1 Propionyl-Coa, or several 
>Propionyl-CoA?
>


Let's start there. The system does not know in advance that the fatty
acid (FA) has odd or even number of C. It degrades it by 2 C (to make
acetyl CoA) -- except at the last step, where the C3 acid is left if
the starting FA is odd. So there is only one C3 unit made.

What happens to it?

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.

Propionyl CoA is a common metabolite. The pathway commonly given for
propionyl CoA is to convert it to succinyl CoA, which is a normal TCA
cycle intermediate. This is a complex (multi-step) conversion.
Overall, the C3 compound is carboxylated to a C4 compound, so we no
longer have a C-even compound. It is described in any biochemistry
book. (Good to have a biochem book at hand!)

I wouldn't be too surprised if there are various ways to deal with
propionyl CoA.

bob



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