JTW's Evolutionary Origins - Author: Wachtershauser, Gunter
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Reactions Involved in the Chemoautotrophic Metabolism
Rxn a:
Reductive Carboxylation
Postulate 2
[pp. 127]
R-COSH + CO2 + FeS --a--> R-CO-COOH + FeS2
note:
Hydrogen Sulphide has a catalytic and energetic function.
The carboxylate groups (-COOH) of all constituents are here assumed
to be in an equilibrium with small amounts of thiocarboxylate groups (-COSH).
In the prescence of H2S the carbonyl groups will exist in the thioenol form.
This is expected to lower the dG`, especially at increased pressures.
It is the thioenol form of the thiocarboxylate groups
that undergo reductive carboxylation according to Rxn a.
Example #1a
As written:
COOH-CH3 --a--> COOH-C(SH)=CH2
Breakdown:
COOH-CH3 <==H2S==> COSH-CH3
Thioacetate (TAc) --a--> Thioenol-Pyruvate (TPy)
COSH-CH3 --a--> COOH-C(SH)=CH2
Thioenol-Pyruvate (TPy) <==H2S==> Pyruvate (Py)
COOH-C(SH)=CH2 <==H2S==> COOH-CO-CH3
note:
[pp. 127]
Pyruvate in the presence of H2S, under anaerobic conditions, is in equilibrium with it's thioenol-form Thioenol-pyruvate.
Example #2a
As written:
COOH-CH2-CH2-COOH --a--> COOH-CH2-CH=C(SH)-COOH
2-Thioenol-glutarate
Breakdown:
COOH-CH2-CH2-COOH <==H2S==> COOH-CH2-CH2-COSH
--a-> 2-Thioenol-glutarate
COOH-CH2-CH2-COSH --a-> COOH-CH2-CH=C(SH)-COOH
2-Thioenol-glutarate <==H2S==> 2-Ketoglutarate (KG)
COOH-CH2-CH=C(SH)-COOH <==H2S==> COOH-CH2-CH2-CO-COOH
note:
anaerobic conditions, is in equilibrium with it's
thioenol-form
Example #3a
As written:
COOH-CH=CH-COOH --a--> COOH-CH=CH-CO-COOH
Breakdown:
Fumarate (Fu) <==H2S==>
COOH-CH=CH-COOH <==H2S==> COOH-CH=CH-COSH
--a-->
COOH-CH=CH-COSH --a--> COOH-CH=CH-CO-COOH
- Wachtershauser, Gunter
- Groundworks for an Evolutionary Biochemistry: The Iron-Sulphur World
- Progress in Biophysics and Molecular Biology: Vol. 58, No. 2, pp.85-202
- 1992
- [Pubmed]
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