Common origins of RNA, protein and lipid precursors in a cyanosulfidic protometabolism : Nature Chemistry : Nature Publishing Group.
Life is built on chemistry, but the chemistry required to jump from the basic amino acids and small organic molecules expected on the Early Earth to the components of Life-As-We-Know-It – lipids, proteins and nucleic acids – has been obscure, until now. The scenario outlined in
Nature Chemistry [doi:10.1038/nchem.2202] also explains why the various chemical components of our kind of life are so very similar. Even though they perform quite different roles, the building blocks are similar, produced originally by very similar chemical processes.
Original caption: The degree to which the syntheses of ribonucleotides, amino acids and lipid precursors are interconnected is apparent in this ‘big picture’. The network does not produce a plethora of other compounds, however, which suggests that biology did not select all of its building blocks, but was simply presented with a specific set as a consequence of the (photo)chemistry of (11) and hydrogen cyanide (12), and that set turned out to work. To facilitate the description of the chemistry in the text, the picture is divided into four parts:
(a) Reductive homologation of
hydrogen cyanide (11) (bold green arrows) provides the C2 and C3 sugars — (1) and glycolaldehyde (4)—needed for subsequent ribonucleotide assembly (bold blue arrows), but also leads to precursors of glyceraldehyde , Glycine , Alanine and Serine . Threonine
(b) Reduction of
(17) (the more stable isomer of dihydroxyacetone glyceraldehyde (4)) gives two major products, (18) and acetone glycerol (19). Reductive homologation of acetone (18) leads to precursors of and Valine , whereas phosphorylation of Leucine glycerol (19) leads to the lipid precursor (21). glycerol-1-phosphate
(c) Copper(I)-catalysed cross-coupling of
hydrogen cyanide (11) and (32) gives acetylene (33), reductive homologation of which gives precursors of acrylonitrile and Proline . Arginine
(d) Copper(II)-driven oxidative cross-coupling of hydrogen cyanide (11) and acetylene (32) gives
(6), which serves as a precursor to cyanoacetylene , Asparagine , Aspartic acid and Glutamine . Pi, inorganic phosphate. Glutamic acid
The key-point is the relatedness and the step-by-step creation of one component or another, by short chemical processes, from the basic materials. To have produced such serial chemistry would have required means of isolating the raw materials and products, then mixing them. The next image from the paper provides a hint of what would’ve been required, on some sun-drenched landscape, swept by occasional rains, in an atmosphere of (probably) H2, N2, CO2 and H2O…
Original caption: A series of post-impact environmental events are shown along with the chemistry (boxed) proposed to occur as a consequence of these events.
(a) Dissolution of atmospherically produced
hydrogen cyanide results in the conversion of (the anoxic corrosion product of the meteoritic inclusion vivianite ) into mixed schreibersite and ferrocyanide salts , with counter cations being provided through neutralization and ion-exchange reactions with bedrock and other meteoritic oxides and salts. phosphate salts
(b) Partial evaporation results in the deposition of the least-soluble salts over a wide area, and further evaporation deposits the most-soluble salts in smaller, lower-lying areas.
(c) After complete evaporation, impact or geothermal heating results in thermal metamorphosis of the evaporite layer, and the generation of feedstock precursor salts (in bold).
(d) Rainfall on higher ground (left) leads to rivulets or streams that flow downhill, sequentially leaching feedstocks from the thermally metamorphosed evaporite layer.
Solar irradiation drives photoredox chemistry in the streams. Convergent synthesis can result when streams with different reaction histories merge (right), as illustrated here for the potential synthesis of
(5) at the confluence of two streams that contained arabinose aminooxazoline glycolaldehyde (1), and leached different feedstocks before merging.