Two sulfur containing amino acids are included in the list of the 20 classical protein amino acids. A methionine residue is introduced at the start of the synthesis of all current proteins. Cysteine, thanks to its thiol function, plays an essential role in a very large number of catalytic sites. Here we present what is known about the prebiotic synthesis of these two amino acids and homocysteine, and we discuss their introduction into primitive peptides and more elaborate proteins.
Unlike its shorter analog, cysteine, and its methylated derivative, methionine, homocysteine is not today a proteinogenic amino acid. However, this thiol containing amino acid is capable of forming an activated species intramolecularly. Its thiolactone could have made it an interesting molecular building block at the origin of life on Earth. Here we study the cyclization of homocysteine in water and show theoretically and experimentally that in an acidic medium the proportion of thiolactone is significant. This thiolactone easily reacts with amino acids to form dipeptides. We envision that these reactions may help interpret why a methionine residue is introduced at the start of all protein synthesis.
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In this review, we examined the possibility that some halogenated organic derivatives
were used in the primitive ocean at the beginning of life on Earth. Firstly, we described
the existence of extraterrestrial halogenated molecules, then we studied their nonbiological
syntheses on the present Earth, especially in volcanic environments. In order to
demonstrate the diversity of today’s halogenated biomolecules, representative examples
are given and the biosynthesis of some of them is summarized. Finally, we proposed two
aspects of the chemistry of halogenated compounds that may have been useful en route to
biomolecules, firstly the use of methyl chloride as the first methylation reagent, secondly
the synthesis and use of α-chloro-carbonyl derivatives.
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