Life's Origin by J. William Schopf

Author:J. William Schopf [Schopf, J. William]
Language: eng
Format: epub
Published: 2011-02-16T05:00:00+00:00

Pyrite-based chemolithotrophic metabolic

networks

106

Stanley L. Miller and Antonio Lazcano

emergent chemical systems synthesized their own organic compounds instead of incorporating ready-made environmental organics. The most articulate proponent of the autotrophic hypothesis is Günter Wächtershäuser (1988, 1992), who has argued that life began with the appearance of an autocatalytic, two-dimensional, chemolithotrophic (chemically powered) metabolic system based on the formation of the highly insoluble mineral pyrite (FeS2).

Formation of pyrite by the reaction FeS ϩ H2S S FeS2 ϩ H2 is very favorable. This reaction is irreversible and highly exergonic (energy-liberating), producing a standard free-energy change (⌬ GЊ) of 9.23 kcal molϪ1 that corresponds to a reduction in chemical potential ( EЊ) of the reactants of 620 mV. Thus, the FeS/H2S combination is a strong reducing agent; even under mild conditions, it should serve as an efficient source of electrons for the reduction (hydrogenation) of organic compounds. The scenario proposed by Wächtershäuser (1988, 1992) fits well with the environmental conditions found at the deep-sea hydrothermal vents, where H2S, CO2, and CO can be quite abundant.

Interestingly, however, the FeS/H2S system does not reduce CO2 to amino acids, purines, or pyrimidines, although the formal chemistry of the reaction shows that the available free energy is more than adequate (Keefe et al. 1995). But pyrite formation can produce molecular hydrogen (H

Ϫ

2) and promote the reduction of nitrate to ammonia (NO3

S

NH3), acetylene to ethylene (C2H2 S C2H4), and thioacetic acid to acetic acid (C2H4OS S C2H4O2), as summarized by Maden (1995).

Recent experiments have shown that the activation of amino acids with CO and an iron-nickel sulfide mineral ([Fe,Ni]S) can lead to formation of peptide bonds like those in proteins (Huber and Wächtershäuser 1998). (In these experiments, the required reactions take place in an aqueous environment to which the powdered sulfide mineral has been added. Thus, the reacting molecules do not form a dense monolayer bound ionically to the mineral. In contrast, the reactions discussed by Maden take place on the surfaces of pyrite grains.) None of the FeS/H2S system experiments shows that the postulated surface-bound metabolism gives rise to enzymes or nucleic acids. In fact, the results to date do not necessarily point to an autotrophic origin of life; rather, they are consistent with a more general, modified model of the primitive broth in which pyrite formation is an important source of electrons for the reduction of organic compounds. But possibilities remain. Under certain geological conditions (not yet described), the FeS/H2S combination might be capable of reducing not only CO but also CO2 (released from molten magma in deep-sea vents), thereby Formation of the Building Blocks of Life

107

producing biochemically relevant monomers (Orgel 1988). Similarly, peptide polymers might be synthesized in an iron-nickel sulfide system at submarine fumaroles (Huber and Wächtershäuser 1998) if amino acids were formed earlier by Strecker-type syntheses near the ocean surface—although such a scenario would require transport of the participating amino acids from their site of formation to the deep-sea vents (Rode 1999). However, if the compounds synthesized by such processes do not remain bound to

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