Elements of Metabolic Evolution

Abstract

Research into the origin of evolution is polarized between a genetics-first approach, with its focus on polymer replication, and a metabolism-first approach that takes aim at chemical reaction cycles. Taking the latter approach, we explored reductive carbon fixation in a volcanic hydrothermal setting, driven by the chemical potential of quenched volcanic fluids for converting volcanic C1 compounds into organic products by transition-metal catalysts. These catalysts are assumed to evolve by accepting ever-new organic products as ligands for enhancing their catalytic power, which in turn enhances the rates of synthetic pathways that give rise to ever-new organic products, with the overall effect of a self-expanding metabolism. We established HCN, CO, and CH(3)SH as carbon nutrients, CO and H(2) as reductants, and iron-group transition metals as catalysts. In one case, we employed the "cyano-system" [Ni(OH)(CN)] with [Ni(CN)(4)](2-) as the dominant nickel-cyano species. This reaction mainly produced α-amino acids and α-hydroxy acids as well as various intermediates and derivatives. An organo-metal-catalyzed mechanism is suggested that mainly builds carbon skeletons by repeated cyano insertions, with minor CO insertions in the presence of CO. The formation of elemental nickel (Ni(0)) points to an active reduced-nickel species. In another case, we employed the mercapto-carbonyl system [Co(2)(CO)(8)]/Ca(OH)(2)/CO for the double-carbonylation of mercaptans. In a "hybrid system", we combined benzyl mercaptan with the cyano system, in which [Ni(OH)(CN)] was the most productive for the double-carbon-fixation reaction. Finally, we demonstrated that the addition of products of the cyano system (Gly, Ala) to the hybrid system increased productivity. These results demonstrate the chemical possibility of metabolic evolution through rate-promotion of one synthetic reaction by the products of another.

Massive Thermal Acceleration of the Emergence of Primordial Chemistry, the Incidence of Spontaneous Mutation, and the Evolution of Enzymes

“…Even now, numerous organisms thrive at high temperatures in hot springs and undersea thermal vents, and evidence from ribosomal RNA sequences led Woese to his well known proposal (12) that the earliest branches of the tree of life were largely populated by thermophilic organisms. The constitution of the genetic code suggests that the last common ancestor was a thermophile (13), and the illuminating iron-sulfur theory of Wächtershäuser et al (14) is predicated on the likelihood that life began near thermal vents in the sea. These arguments for a hot origin are necessarily based on retrospective evidence, and it will probably never be possible to repeat the "experiment."…”

From Chemical Invariance to Genetic Variability

“…Retrodiction of an inorganic ancestral cluster (Figure 1.2) is based on chemical synthesis (Rauchfuss reaction) [34]. Incidentally, dehydroglycine is related to the formation of glycine from a CN/CO ligand system [27] CO pool [37]). This leads us to the ancestral structure [Ni-S]Fe(CN) 2 (CO) (Figure 1.2).…”

“…Iterative carbon fixation (up to C5) was demonstrated for CO/H 2 S/NiS: C n -SH → C n -COSH → C n -CHO → C n+1 -SH [55]. Aqueous double carbonylation was demonstrated in the presence of Ca(OH) 2 (H + scavenger) for RSH (R = CH 3 , PhCH 2 ) and CO/Co 2 (CO) 8 /NiS to form RCO-COOH and RCHOH-COOH [27]. Double carbonylation of PhCH 2 -SH was also shown for CO/(Co,Ni)(OH,S), but dramatically higher productivity was found for CO/(Co,Ni)(CN) 2 or CO/(Co,Ni)(OH)(CN) along two pathways: (i) double CO fixation and (ii) CO fixation for CHOH and cyanidation for COOH [27].…”

“…The reaction system comprises layered Ni(OH) 2 and [Ni(CN) 4 ] 2− . When glycine or alanine as products of this reaction were added to the chemical system (CO/Ni(OH)(CN)) for double carbonylation of PhCH 2 -SH, the yield of PhCH 2 -CHOH-COOH was significantly increased -a model for the postulated feed-forward effect [27].…”

“…Common origin of hydrogenases in a set of carbonylated precursors ([Fe1] = Fe(CO) 5 ) in pseudorotation; [Fe2S2CO] forms from [Fe2] under primordial conditions[27]. Notional mechanism of pyrite formation from FeS/H 2 S (X = substrate).…”