Prebiotic organic synthesis amino acids

The variety of prebiotic organic reactions seems to be almost unlimited. Strasdeit et al. (2002) from the University of Hohenheim (Germany) reported the synthesis of zinc and calcium complexes of the amino acids valine and isovaline. They assume that these could have had a certain significance on the mineral-rich primeval Earth on heating to 593 K under nitrogen, valine was converted to the corresponding cyclic dipeptide. 

Endogenous organic synthesis Urey-Miller experiments as a source of prebiotic molecules via the Strecker synthesis for amino acids, HCN polymerisation for purines and pyrimidines and the formose reaction for sugars... 

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. 

It is of course surprising that amino acids can be obtained via the Strecker synthesis, purines from the condensation of HCN, pyrimidines from the reaction of cyanoacetilene with urea, and sugars from the autocatalytic condensation of formaldehyde. The synthesis of chemical constiments of contemporary organisms by non-enzymatic processes under laboratory conditions does not necessarily imply that they were either essential for the origin of life or available in the primitive environment. However, the significance of prebiotic simulation experiments is... 

We can also get some information by comparing the modern biosynthetic pathways to the capabilities of prebiotic chemistry. Amino acids are usually activated in living organisms by reaction with ATP both through the ribosomal and non-ribosomal peptide synthesis pathways. Amino acyl fRNA synthetases bind ATP and free amino acids to cause the highly unfavorable adenylate anhydride formation to be close to equilibrium in the active site. 

An aqueous solution containing ammonia, hydrogen cyanide, and simple organic compounds such as formaldehyde and cyanoethine, in contact with rocks, constituted substrates and the environment for the prebiotic synthesis of more complex organic molecules purine and pyrimidine bases, amino acids, and sugars -the building blocks of all the organisms. 

Schoonen MAA, Xu Y, Bebie J. Energetics and kinetics of the prebiotic synthesis of simple organic acids and amino acids with the FeS-H2S/FeS2 redox couple as reductant. Orig. Life Evol. Biosph. 1999 29 5-32. 

Particularly important here is the role of transition metal sulfides. In 1988 Wachtershauser proposed that pyrite, abundant in hydrothermal vent systems, provided an energy source for the first life. He suggested that pyrite provided the catalyst necessary to drive a number of essential chemical reactions which are important precursors to life. More recent studies have confirmed this view and have shown that the sulfides of Fe, Ni, Co, and Zn can play an important role in the fixation of carbon in a prebiotic world (Cody et al., 2004). Transition metal sulfides also play a role in more advance organic synthesis, and Huber and Wachtershauser (1998) showed how amino acids were converted into their peptides using a (NiFe)S catalyst. 

The general arguments about the antiquity of cofactors apply to PLP. The nonenzymatic synthesis of pyridoxal under prebiotic conditions is considered possible, whereas the presence of a 5 phosphate group could hint to an ancestral attachment of the cofactor to RNA molecules. " Furthermore, there are specific grounds to assume that PLP arrived on the evolutionary scene before the emergence of proteins. In fact, in current metabolism, PLP-dependent enzymes play a central role in the synthesis and interconversion of amino acids, and thus they are closely related to protein biosynthesis. In an early phase of biotic evolution, free PLP could have played many of the roles now fulfilled by PLP-dependent enzymes, since the cofactor by itself can catalyze (albeit at a low rate) reactions such as amino acid transaminations, racemizations, decarboxylations, and eliminations. " This suggests that the appearance of PLP may have preceded (and somehow eased) the transition from primitive RNA-based life forms to more modern organisms dependent on proteins. 

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