The Thioester World

The basic aim of this theory is an attempt to avoid the uncertainties of explaining prebiotic mechanism to account for the production of RNA from primordial soup. This is connected with a suggestion that catalytic polypeptides (short sequences of 

Phosphorolytic attack of thioester by inorganic compounds provides a mechanism for introducing high energetic phosphate esters into metabolism leading to the synthesis of pyrophosphate as the first unit of energy. The synthesis of AMP and ATF followed this step. 

The mixture of various catalytic multimers, like thioesters and phosphorylated organic molecules, presents the background for a protometabolism in the thioester world theory. Protometabolitic reaction pathways rapidly form networks. The latter would have included cyclic mass flows that are stabilized by interactions among metabolites. These pathways could have occurred in associations with pyrite, clays or iron dioxide flocks produced by UV photooxidation. 

It looks reasonable to hypothesize that globally ubiquitous iron oxides were incorporated into life development from the early evolutionary stage, but subsequent to the synthesis of simple amines and organic acids. The diffusion of sulfide across primitive membranes is also meaningful for the maintenance of the thioester world . An intracellular iron redox cycle, driven possibly by light and sulfide, could have supported chemosynthesis. Iron respiration and photometabolism could have processed on the external surface of vesicles. This would have contributed to a vectorial transport of thioesters and protons into vesicles and accordingly to the development of the early protonmotive transport system. 

We can share the view of some authors (Fenchel et al., 1998) that this theory may serve a plausible explanation of thioester world as precursor for the present RNA world. Furthermore, it is attractive in a number of respects too. One has to mention the development of very early, global scale geochemical cycling of carbon, sulfur, and iron. It would present the early interactions between atmosphere and biochemical processes, like thioester-bounded hydrogen production contribution to the formation of reducing species that were critical for any atmospheric organic synthesis. 

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However, there are also biogenesis models which do not require phosphate, such as the inorganic hypothesis of the origin of life proposed by Cairns-Smith (see Sect. 7.1), the thioester world proposed by de Duve (see Sect. 7.4) or the sulphur-iron world suggested by Wachtershauser (see Sect. 7.3). The RNA world (see Chap. 6), however, cannot exist without phosphate. 

The thioester world postulated by de Duve should in fact be called the sulphur-iron world , since iron ions are essential for the redox processes occurring in such a thioester world, de Duve (1991, 1996) asks a question which is vital for the whole of prebiotic chemistry where did the redox equivalents necessary for the construction of biomolecules on the primeval Earth come from This question becomes largely irrelevant if the strongly reducing atmosphere postulated by Miller/Urey and... 

Fig. 7.8 Two examples of important reactions in the thioester world a the phosphorolysis of thioesters leads to acylphosphates and b a-ketoadds are formed in the reductive cleavage of thioesters with accompanying carbonylation (de Duve, 1991)...

The thioester hypothesis can be summed up as follows the formation of thiols was possible, for example, in volcanic environments (either above ground or submarine). Carboxylic acids and their derivatives were either formed in abiotic syntheses or arrived on Earth from outer space. The carboxylic acids reacted under favourable conditions with thiols (i.e., Fe redox processes due to the sun s influence, at optimal temperatures and pH values) to give energy-rich thioesters, from which polymers were formed these in turn (in part) formed membranes. Some of the thioesters then reacted with inorganic phosphate (Pi) to give diphosphate (PPi). Transphosphorylations led to various phosphate esters. AMP and other nucleoside monophosphates reacted with diphosphate to give the nucleoside triphosphates, and thus the RNA world (de Duve, 1998). In contrast to Gilbert s RNA world, the de Duve model represents an RNA world which was either supported by the thioester world, or even only made possible by it. 

Figure 18. Relationships among various theoretical or conceptual models of the origin of life. Dashed and solid lines of the figure connect mechanisms with putative metabolic type of first organisms. The thioester world is propo.sed as an intermediate leading to an RNA world Fenchel et ai. 1998).

In the early 1990s C. de Duve suggested the sulfur-based theory of life origin. The author has described the thioester world as a precursor of an RNA world (Figure 25). 

De Duve C (1992) The thioester world. In Tran Thanh Van J, Tran Thanh Van K, Mounolou JC, Schneider J, McKay C (eds) Frontiers of life. Frontieres, Gif-sur-Yvette, pp 1-20... 

Compounds of crucial importance for the RNA world or a precursor phase could have been formed in this or similar ways. Thus, a thioester world seems to support the RNA world hypothesis, although de Duve can be considered as a careful critic of the latter hypothesis. His opinions were expressed in a short article in Nature under the provoking title Did God make RNA (de Duve, 1988). 

When the Fe2+/Fe3+ system is included, the thioester hypothesis, with its roots in sulphur chemistry, shows clear links with the iron-sulphur world of Wachtershauser s chemoautotrophic biogenesis model (see Sect. 7.3). 

Prebiological energy conversion at the prenucleotide level was suggested to involve a thioester world (De Duve, 1987), an iron-sulfur world , in which pyrite (FeS2) is... 

Highlight the main biochemical reactions and reaction pathways in the theory of thioester world . Why could this theory be considered as the most successful Give your understanding of applicability of various theories for life s origin process. 

Thioesters are another group of substances that could have played a key role in primitive energy transactions. We have seen how these substances are uniquely involved in both group and electron transfers and in phosphorylation processes that are associated with some of the most primordial metabolic reactions and do not, unlike those dependent on protonmotive force, require complex membrane-embedded systems. We have also seen how my hypothetical multimers could have arisen from the thioesters of amino acids (see above). These facts and others have led to my proposal of a thioester world (1998). 

Carbonyl condensations are among the most widely used reactions in the biological world for the assembly of new carbon-carbon bonds in such important biomolecules as fatty acids, cholesterol, and steroid hormones. One source of carbon atoms for the synthesis of these biomolecules is acetji-CoA, a thioester of acetic acid and the thiol group of coenzyme A. The function of the coenzyme A group of acetyl-CoA is to anchor the acetyl group on the surface of the enzyme systems that catalyze the reactions we examine in this section. In the discussions that follow, we will not be concerned with the mechanism by which each enzyme-catalyzed reaction occurs. Rather, our concern is with recognizing the type of reaction that takes place in each step. 

It has been proposed by de Duve (see Further Reading) that primordial acetyl phosphate originates by reaction of acetyl-thioester with phosphate ions. According to the Iron-Sulfur World theory, phosphorylation energy may also result from a transfer of the redox energy of CO/H2S in the presence of amino acids (5), which received its first support by phosphate catalysis of peptide formation (16). Subsequently it was supported by the discovery of a formation of aminoacyl phosphate in reaction of phosphate with aminoacyl N-carboxyanhydride (21) ... 

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