JTW's Evolutionary Origins - Author: Wachtershauser, Gunter

Sulphur and Sulphur Analogs in a Chemoautotrophic Metabolism

The Central Importance of Sulphur

"According to the theory of a pyrite-pulled chemo-auto-origin, sulphur is an essential element of life from the very start. It has a repertory of reactivities which make it uniquely suitable for its many functions in biochemistry. These functions, notably in conjunction with iron, are essential for a wide range of pathways in the anaerobic metabolism. By the present theory, all these functions can be retrodicted into a hypothetical pyrite-pulled surface metabolist. It is for this reason that the world of life is truly an 'Iron-Sulphur World'."
[Wachtershauser, 1992, pp. 132]

Sulfur's Unique Properties

"Let us now consider the unique chemical properties of sulphur. Its d-orbitals allow for transargononicbonding with ferrous ions which lends high stability to the pyrite crystal structure, the ultimate cause for the pyrite-forming energy source. The capacity of sulphur to form stable S-S bonds gives rise to the disulphide anions in pyrite and to organic disulphides between Cys-units in enzymes, in lipoate, and in heterodisulphides. The high nucleophilicity of H₂S and notably HS^- as compared to water and OH^- means a high rate of substitution and addition reactions. The electron withdrawing inductive effect of sulphur tends to stabilize carbanions, a fact which is important in C-C bond formation..."
[Wachtershauser, 1992, pp. 133]

Other Important Properties of Sulphur

• The higher thermodynamic stability of C-S bonds compared to C-O bonds under anaerobic conditions.
• The lower tendency of sulphur as compared to oxygen to form double bonds to carbon.
• The more facile production of sulphur radicals as compared to oxogen radicals.
• The formation of iron-sulphur clusters.

Sulfur Analogs

"The tendency of sulphur not to form double bonds with carbon has important consequence. The ketoacids, notably 2-ketoglutarate (KG) and pyruvate (Py), exist predominantly in the keto form since oxogen forms stable double bonds to carbon. In the prescence of H₂S, the keto group is in equilibrium with the thioketo group which, however, exists predominantly in the thioenol form.

R-CO-COOH + H₂S <==> R-CO-COSH + H₂O

R-CO-COSH <==> R=C(SH)-COOH

Since the enol structure is the reactive group in condensation reactions (Peliska and O'Leary, 1991; Seeholzer et. al. 1991), the archaic thioenol forms of KG and Py are expected to be more reactive than KG and Py... In the changeover to an environment low in H₂S, thiopyruvate is replaced functionally by phosphoenolpyruvate (PEP) in accordance with HR 10, but with mechanistic continuity."
[Wachtershauser, 1992, pp. 133]

"Lorimer has suggested that a similar equilibrium to that between carboxylate groups and thiocarboxylate groups may exist between carbon dioxide and carbonyl sulphide

CO₂ + H₂S <==> COS + H₂O

so that carbonyl sulphide could enter the reaction in competition with carbon dioxide and form the thioacid directly... In this respect, it is of great interest that volcanic gases have been found to contain 0.003 vol.% COS (Corazza, 1986). This means that COS could be involved in the induction and maintenance of the early RCC-based metabolism."
[Wachtershauser, 1992, pp. 131-132]

"The thermodynamic stability of C-S bonds, in conjuction with the high reactivity of thio or mercapto groups, has important consequences for a number of pathway evolutions, notably for the the origin of the phosphorylated sugar pathways, which are retrodicted by HR10 into thiosugar pathways."
[Wachtershauser, 1992, pp. 133]

Historic Relation #10

Thiol to Alcohol or Phosphorylation

-C-SH ==> -C-OH or -C-O-PO₃^2-
[Wachtershauser, 1992, pp. 133]

"Since a pyrite-pulled chemo-autotrophic origin of life produces surface-bonding carboxylate groups directly by CO₂-fixation, it has been concluded that surface metabolists with only thiolate and carboxylate surface bonding groups (Carboxypeds) are the first organsims and that surface metabolists with additional phosphate surface bonding (Phosphorypeds) are of a later vintage (Wachtershauser 1988d). The fact that most extant branch pathways of the RCC have phosphate groups entering late (E1) seems to bear this out:"
[Wachtershauser, 1992, pp. 136]

"All extant substrate level phosphorylation occurs by converting thioesters into acylphosphates. This is the reason for de Duve's (1988b) proposal for an origin of phosphate acquisition by the phosphorolytic attack of phosphate ions on thioesters, giving rise to the first acylphosphates (R-CO-O-PO₃^2-) and subsequently to polyphosphates (e.g. R-O-P₂O₆^3-). These activated phosphate groups are seen by de Duve as invading the metabolism by phosphoryl transfer, first based mainly on inorganic polyphosphates, and later, on organic polyphosphates such as ATP. This proposal, made in the context of a prebiotic broth theory, suffered however from a lack of a plausible process of thioester formation. Having shown here how thioesters could arise by a pyrite-pulled process (see section III.3.c), we can now adopt and incorporate de Duve's proposal into the theory of a pyrite-pulled origin."
[Wachtershauser, 1992, pp. 137]

"It has recently been shown (Yamagata et. al. 1991) that volcanic gases contain P₄O₁₀ which hydrolyzes in liquid water to produce polyphosphates. This finding is of interest for the theory of a pyrite-pulled chemoautotrophic origin of life which has H₂S-rich volcanic exhalations as its primary geochemical setting. It opens up the possibility that phosphoanhydride activation arises first, giving rise to thioester activation by thiolysis. Alternatively, it makes room for the separate appearance of phosphoanhydride activation and of thioester activation in different chemical species, followed by symbiosis."
[Wachtershauser, 1992, pp. 137]

"With the abandonment of pyrite formation in the course of evolution,
the metabolism becomes wholly dependent on group activation,
first by thioesterification and/or phosphorylation by volcanic polyphosphates;
and later by a variety of transactivation processes:

• Transthiolation
• Transthioesterification
• Thioester phosphorolysis
• Transphosphorylation.

This entire repertoire is still in use in extant metabolisms."
[Wachtershauser, 1992, pp. 137]

• 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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