Prebiotic metabolism

The synthesis in many extant organisms of these two amide residues from their respective precursors glutamate and aspartate esterified to tRNA (the indirect aminoacylation pathways described in Sections 5.14.3 and 5.14.4) and that of other amino acid residues, such as selenocysteine (which is also synthesized from a precursor esterified on a tRNA °) support the model of prebiotic metabolism taking place at the surface of solid particles, " analogous to ancestral RNAs. 

In a way, the missing link appears to be the aim of a few distinct research groups, who are inquiring into the possibility of prebiotic metabolic pathways prior to enzymes. There is a rather complex kaleidoscope of authors and views, and it is useful to distinguish between the following main trains of thought. 

The selective utilization of prebiotics by some, but not all, of the resident species alters the assemblages, densities and metabolic activities of the GIT bacteria. Of importance is the ability of prebiotics to increase the proportion of the resident bacteria represented by the lactic acid producing bacteria (LAB), resulting in changes of GIT and systemic functions (Swanson et al.. 

The authors chose pyruvic acid as their model compound this C3 molecule plays a central role in the metabolism of living cells. It was recently synthesized for the first time under hydrothermal conditions (Cody et al., 2000). Hazen and Deamer carried out their experiments at pressures and temperatures similar to those in hydrothermal systems (but not chosen to simulate such systems). The non-enzymatic reactions, which took place in relatively concentrated aqueous solutions, were intended to identify the subsequent self-selection and self-organisation potential of prebiotic molecular species. A considerable series of complex organic molecules was tentatively identified, such as methoxy- or methyl-substituted methyl benzoates or 2, 3, 4-trimethyl-2-cyclopenten-l-one, to name only a few. In particular, polymerisation products of pyruvic acid, and products of consecutive reactions such as decarboxylation and cycloaddition, were observed the expected tar fraction was not found, but water-soluble components were found as well as a chloroform-soluble fraction. The latter showed similarities to chloroform-soluble compounds from the Murchison carbonaceous chondrite (Hazen and Deamer, 2007). 

During prebiotic times, water-soluble ferrous iron was present and was used in the first stage of life, while copper was in the water-insoluble Cu(I) state, as highly insoluble sulphides. About 109 years ago the metabolism of a primitive prokaryote (cyanobacteria) led to the evolution of dioxygen into the Earth s atmosphere. A... 

Delzenne N.M. and Kok N. (2001). Effects of fractans-type prebiotics on lipid metabolism . Am J Clin Nutr, 73(2 Suppl), 456S-458S. 

Effects of prebiotics on mineral metabolism . Am J Clin Nut, 73, 459-464. Schrezenmeir J. anddeVreseM. (2001). Probiotics, prebiotics, and synbiotics - approaching a definition . Am J Clini Nutr, 73, 361-364. 

A short look through prebiotic chemistry presents many possibilities that have to be constrained by the second law of thermodynamics and favourable kinetics, but could conceivably have produced all of the molecules required for both metabolism and information propagation. All that remains now is to build a cell and finally an organism. 

Calculation of the internal cell potential is a very complicated matter because the electrochemistry of all of the species within the protocell would have to be balanced subject to their composition quotient Q, after which the standard free energy would have to be established from tabulations. The transport of Na+ would also change this balance, along with the ionic strength of the solution and the stability of the proteins or prebiotic molecules within the protocell. Such non-equilibrium thermodynamics forms the basis of the protocell metabolism. The construction... 

Mountzouris, K. C., Balaskas, C., Fava, F., Tuohy, K. M., Gibson, G. R., and Fegeros, K. (2006). Profiling of composition and metabolic activities of the colonic microflora of growing pigs fed diets supplemented with prebiotic oligosaccharides. Anaerobe 12,178-185. Mulvey, M. A. (2002). Adhesion and entry of uropathogenic Escherichia coli. Cell. Microbiol. 4, 257-271. 

The ability to catalyse the evolution or oxidation of H2 may have been exploited by the earliest life forms as H2 would have been present in the early prebiotic environments. The origins of the proton-dependent chemiosmotic mechanism for ATP synthesis may also reflect the formation of proton gradients created by hydrogenases on either side of the cytoplasmic membrane. In addition, it has been speculated that the coupling of H2 and S metabolisms was also of fundamental importance in the origin of life. These two processes seem intimately coupled in the bifunctional sulfhydrogenase found in Pyrococcus furiosus (a combination of subunits for hydrogenase and sulfite reductase) which can dispose of excess reductant either by the reduction of protons to H2 or S° to H2S (Ma et al. 1993 Pedroni et al. 1995). 

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