The iron–sulfur world hypothesis is a set of proposals for the origin of life and the early evolution of life advanced in a series of articles between 1988 and 1992 by Günter Wächtershäuser, a Munich patent lawyer with a degree in chemistry, who had been encouraged and supported by philosopher Karl R. Popper to publish his ideas. The hypothesis proposes that early life may have formed on the surface of iron sulfide minerals, hence the name. It was developed by retrodiction (making a "prediction" about the past) from extant biochemistry (non-extinct, surviving biochemistry) in conjunction with chemical experiments.
Origin of life
Pioneer organism Wächtershäuser proposes that the earliest form of life, termed the "pioneer organism", originated in a volcanic hydrothermal flow at high pressure and high (100 °C) temperature. It had a composite structure of a mineral base with catalytic transition metal centers (predominantly iron and nickel, but also perhaps cobalt, manganese, tungsten and zinc). The catalytic centers catalyzed autotrophic carbon fixation pathways generating small molecule (non-polymer) organic compounds from inorganic gases (e.g. carbon monoxide, carbon dioxide, hydrogen cyanide and hydrogen sulfide). These organic compounds were retained on or in the mineral base as organic ligands of the transition metal centers with a flow retention time in correspondence with their mineral bonding strength thereby defining an autocatalytic "surface metabolism". The catalytic transition metal centers became autocatalytic by being accelerated by their organic products turned ligands. The carbon fixation metabolism became autocatalytic by forming a metabolic cycle in the form of a primitive sulfur-dependent version of the reductive citric acid cycle. Accelerated catalysts expanded the metabolism and new metabolic products further accelerated the catalysts. The idea is that once such a primitive autocatalytic metabolism was established, its intrinsically synthetic chemistry began to produce ever more complex organic compounds, ever more complex pathways and ever more complex catalytic centers.
Nutrient conversions The water–gas shift reaction (CO + H2O → CO2 + H2) occurs in volcanic fluids with diverse catalysts or without catalysts. The combination of ferrous sulfide (FeS, troilite) and hydrogen sulfide (H2S) as reducing agents (both reagents are simultaneously oxidized in the reaction here under creating the disulfide bond, S–S) in conjunction with pyrite (FeS2) formation:
FeS + H2S → FeS2 + 2 H+ + 2 e− or with H2 directly produced instead of 2 H+ + 2 e− FeS + H2S → FeS2 + H2 has been demonstrated under mild volcanic conditions. This key result has been disputed. Nitrogen fixation has been demonstrated for the isotope 15N2 in conjunction with pyrite formation. Ammonia forms from nitrate with FeS/H2S as reductant. Methylmercaptan [CH3-SH] and carbon oxysulfide [COS] form from CO2 and FeS/H2S, or from CO and H2 in the presence of NiS.
Synthetic reactions Reaction of carbon monoxide (CO), hydrogen sulfide (H2S) and methanethiol CH3SH in the presence of nickel sulfide and iron sulfide generates the methyl thioester of acetic acid [CH3-CO-SCH3] and presumably thioacetic acid (CH3-CO-SH) as the simplest activated acetic acid analogues of acetyl-CoA. These activated acetic acid derivatives serve as starting materials for subsequent exergonic synthetic steps. They also serve for energy coupling with endergonic reactions, notably the formation of (phospho)anhydride compounds. However, Huber and Wächtershäuser reported low 0.5% acetate yields based on the input of CH3SH (methanethiol) (8 mM) in the presence of 350 mM CO. This is about 500 times and 3700 times the highest CH3SH and CO concentrations respectively measured to date in a natural hydrothermal vent fluid. Reaction of nickel hydroxide with hydrogen cyanide (HCN) (in the presence or absence of ferrous hydroxide, hydrogen sulfide or methyl mercaptan) generates nickel cyanide, which reacts with carbon monoxide (CO) to generate pairs of α-hydroxy and α-amino acids: e.g. glycolate/glycine, lactate/alanine, glycerate/serine; as well as pyruvic acid in significant quantities. Pyruvic acid is also formed at high pressure and high temperature from CO, H2O, FeS in the presence of nonyl mercaptan. Reaction of pyruvic acid or other α-keto acids with ammonia in the presence of ferrous hydroxide or in the presence of ferrous sulfide and hydrogen sulfide generates alanine or other α-amino acids. Reaction of α-amino acids in aqueous solution with COS or with CO and H2S generates a peptide cycle wherein dipeptides, tripeptides etc. are formed and subsequently degraded via N-terminal hydantoin moieties and N-terminal urea moieties and subsequent cleavage of the N-terminal amino acid unit. Proposed reaction mechanism for reduction of CO2 on FeS: Ying et al. (2007) have shown that direct transformation of mackinawite (FeS) to pyrite (FeS2) on reaction with H2S till 300 °C is not possible without the presence of critical amount of oxidant. In the absence of any oxidant, FeS reacts with H2S up to 300 °C to give pyrrhotite. Farid et al. have experimentally shown that mackinawite (FeS) has ability to reduce CO2 to CO at temperature higher than 300 °C. They reported that the surface of FeS is oxidized, which on reaction with H2S gives pyrite (FeS2). It is expected that CO reacts with H2O in the Drobner experiment to give H2.
Early evolution
Early evolution is defined as beginning with the origin of life and ending with the last universal common ancestor (LUCA). According to the iron–sulfur world theory it covers a coevolution of cellular organization (cellularization), the genetic machinery and enzymatization of the metabolism.
Cellularization
… excerpt ends here. Continue reading the full article.
