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RTCA cycle

G. Wachtershauser formulated his suggestions on the initial metabolic processes on the primeval Earth (as described above) in the form of six postulates and eleven theses (Wachtershauser, 1990b). Laboratory experiments planned to check, and perhaps confirm, these theories have already been attempted. The reductive Krebs cycle (rTCA cycle) has recently been the subject of much discussion. Smith and Morowitz consider that the rTCA cycle is a universal, possibly primordial, core process which could have provided substances for the synthesis of biomolecules on the young Earth (Smith and Morowitz, 2004). [Pg.198]

There are new ideas and experiments on the rTCA cycle. A group from Harvard University studied some reaction steps in the rTCA cycle which were kept going by mineral photochemistry. The authors assumed that solar UV radiation can excite electrons in minerals, and that this energy is sufficient to initiate the corresponding reaction steps. In this photocatalytic process, semiconductor particles were suspended in water in the presence of a zinc sulphide colloid (sphalerite) the experiments were carried out in a 500 mL reaction vessel at 288 K. Irradiation involved a UV immersion lamp (200-410 nm) in the photoreactor. Five reactions out of a total of 11 in the rTCA cycle were chosen to check the hypothesis ... [Pg.198]

If the rTCA cycle were to have functioned on the primeval Earth, all the reaction steps (both redox and non-redox) must have proceeded in high yield. One single metal surface can certainly not drive the whole rTCA cycle more complex mixtures of active mineral surfaces are required (Zang and Martin, 2006). [Pg.198]

There are also voices critical of the rTCA cycle Davis S. Ross has studied kinetic and thermodynamic data and concludes that the reductive, enzyme-free Krebs cycle (in this case the sequence acetate-pyruvate-oxalacetate-malate) was not suitable as an important, basic reaction in the life evolution process. Data on the Pt-catalysed reduction of carbonyl groups by phosphinate show that the rate of the reaction from pyruvate to malate is much too low to be of importance for the rTCA cycle. In addition, the energy barrier for the formation of pyruvate from acetate is much too high (Ross, 2007). [Pg.198]

Another model is based on the fact that the genetic code shows a number of regularities, some of which have already been mentioned above. It is suspected that codons beginning with C, A or U code for amino acids which were formed from a-ketoacids (or a-ketoglutarate, 1-KG), oxalacetate (OAA) and pyruvate. This new model, which is quite different from the previous models, assumes that a covalent complex formed from two nucleotides acted as a catalyst for chemical reactions such as the reductive amination of a-ketoacids, pyruvate and OAA. More recent analyses suggest that the rTCA cycle (see Sect. 7.3) could have served as a source of simple a-ketoacids, including glyoxylate, pyruvate, OAA and a-KG. a-Ketoacids could, however, also have been formed via a reductive acetyl-CoA reaction pathway. The bases of the two nucleotides specify the amino acid synthesized and were retained until the modern three-letter codes were established (Copley et al., 2005). [Pg.221]

The acids of the rTCA cycle are low-energy forms at their respective hydrogen reduction stoichiometries. For instance, formaldehyde has higher free energy of formation per carbon than acetate, which has the same chemical composition as two formaldehyde molecules. Thus, in any relaxation pathway for full reduction of ( Ot. likely intermediate states are the rTCA compounds. [Pg.398]

When the regenerative pathway from acetate to oxaloacetate is included, the rTCA cycle develops the possibility of self-production, increasing the density of catalytic oxaloacetate molecules and hence the bulk rate of acetate synthesis from CO2. The gain in this cycle enables an exponentially growing relaxation process to... [Pg.398]

Figure 18.3. The rTCA cycle reactions. Synthesis from acetate -> fumarate is repeated from succinate —> cis-aconitate, with CH2COOH replacing H as end group. AfG values (kJ/mol, bold italic) are for reactions from CO2(aq) and H2(aq) in equilibrium with gases at 1 atm partial pressure, and H2O(1). Computed from AfG values in [66] using AfG° = —386 kJ/mol for CO2(aq), +11.1 kJ/mol for HaCaq), and - EM.7. kJ/mol for H2O(1). A G = -34kJ/mol for ATP hydrolysis corresponds to a local stationary region around pH 6 and pMg 1.5 in [68]. Figure 18.3. The rTCA cycle reactions. Synthesis from acetate -> fumarate is repeated from succinate —> cis-aconitate, with CH2COOH replacing H as end group. AfG values (kJ/mol, bold italic) are for reactions from CO2(aq) and H2(aq) in equilibrium with gases at 1 atm partial pressure, and H2O(1). Computed from AfG values in [66] using AfG° = —386 kJ/mol for CO2(aq), +11.1 kJ/mol for HaCaq), and - EM.7. kJ/mol for H2O(1). A G = -34kJ/mol for ATP hydrolysis corresponds to a local stationary region around pH 6 and pMg 1.5 in [68].
A final observation about the compounds of the rTCA cycle is that they are rich in carbonyl groups from incompletely reacted CO2. Such structures are both... [Pg.400]

All of these observations combined lead us to believe that the development of modem life as a steady-state relaxation process in fact took place through the sequential emergence of two separate charmels. The first in time, and the simple, was the emergence of reductive metabolism through autocatalytic networks either identical or similar to the rTCA cycle. All its reagents are small molecules that are selected by simple kinetic and physical properties from the complete set of CHO molecules of comparable size [65], and the reaction networks involving them are relatively densely sampled, either within the cycle or in the side-reactions that generate biomass from it. [Pg.402]


See other pages where RTCA cycle is mentioned: [Pg.200]    [Pg.201]    [Pg.390]    [Pg.398]    [Pg.398]    [Pg.400]    [Pg.403]    [Pg.403]    [Pg.403]    [Pg.404]   
See also in sourсe #XX -- [ Pg.398 , Pg.399 , Pg.402 , Pg.403 , Pg.404 , Pg.405 ]




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