A Primitive, Metal-Sulfide-Catalyzed Analogue of the Acetyl-CoA Pathway as the
Origin of Biochemistry
Abstract
Michael J. Russell and William Martin propose that three-dimensional (3D) iron
sulfide (FeS) or nickel sulfide (NiS) structures at hydrothermal vents in the Hadean (a
geological period 4 x 109 years ago) ocean catalyzed and contained a primitive analogue
of the acetyl-coenzyme-A (acetyl-CoA) pathway, effectively initiating biochemistry. The
researchers suggest that marine CO2 from undersea volcanoes and hydrothermally vented
H2 met in metal sulfide-rich hydrothermal reactors to produce acetate via an exergonic
pathway analogous to the modern acetyl-CoA pathway. Additionally, the inorganic
reactors served to enclose and concentrate the resulting reduced organic compounds,
maintaining concentrations necessary for subsequent reactions. This paper examines the
hypothesis proposed by Russell and Martin, contrasting it with two prevailing hypotheses
of the origins of life: the ‘prebiotic soup’ hypothesis and the ‘surface metabolism’
hypothesis. After introducing both hypotheses and their major drawbacks, I describe the
evidence which supports the ‘hydrothermal reactor’ hypothesis and draw conclusions
regarding the relative advantages and disadvantages of the three hypotheses discussed.
Introduction: Prebiotic Soup Hypothesis
Russell and Martin propose the hydrothermal reactor hypothesis as an alternative
to the two prevailing hypotheses of the origin of life – the prebiotic soup and surface
metabolism hypotheses. The prebiotic soup hypothesis – also known as the
‘heterotrophic’ hypothesis – posits that the first life emerged abiotically, as a result of
metabolic processes in which carbon and nitrogen were obtained from complex
molecules, such as amino acids, rather than autotrophically, from simple compounds such
as carbon dioxide.1 Powered by various energy sources, such compounds were free to
combine in a primordial organic soup. In this sense, the heterotrophic hypothesis
corresponds with Miller and Urey’s work in the 1950s, which showed that electrical
discharges in a simulated primordial atmosphere rich in methane and ammonia leads to
the synthesis of several amino acids.2 As further evidence of the plausibility of the
hypothesis, Lazcano asserts that subsequent researchers have confirmed the production of
purines and pyrimidines under similar conditions simulating the primordial atmosphere.1
Regarding the wide range of organic compounds produced under allegedly
prebiotic synthetic conditions, Lazcano proposes an additional argument by addressing
the similarities between the products of these prebiotic simulations and the compounds
contained in some carbon-rich meteorites. In particular, he cites the Murchison
meteorite, which fell in an Australian town of the same name on September 28, 1969.
Among the meteorite’s constituents were nearly eighty amino acids – several of which
occur terrestrially in proteins – as well as purines, pyrimidines, carboxylic acids, and
derivatives of ribose and deoxyribose.1 Based on the similarities between the compounds
contained in the meteorite and the results of Miller, Urey, and others, Lazcano argues that
prebiotic reaction pathways yielding such products may have been common in the early
solar system.
Introduction Continued: Surface Metabolism Hypothesis
In contrast, the surface metabolism hypothesis asserts an autotrophic origin of
life, in which the exergonic formation of pyrite (FeS2) from hydrogen sulfide (H2S) and
ferrous ions (Fe2+) served as an energy source for the first organisms. According to
Günter Wächtershäuser,3 this one-step redox reaction is a more likely candidate for the
original biochemical energy source than other reactions which depend on an energy
coupler to mediate between endergonic initial steps and exergonic final steps. He asserts
that we must view energy couplers as results of evolution; as such they are unsuitable
energy sources for the origin of life.
As supporting evidence, Wächtershäuser cites pyrite’s ubiquitous occurrence in
sediments from even the earliest periods.3 Beyond this geological evidence, he indicates
the biological evidence of iron-sulfur reaction centers in modern electron transport chains
and the production of pyrite during the cultivation of archaebacteria such as Pyrodictium
occultum. He cites a personal communication from O. Kandler, in which the researcher
reports that the cultivation of Pyrodictium occultum results in the formation of a pyrite
coating on the reactor surface.3 K. O. Stetter et al.4 offer a possible explanation for this
coating, proposing that the cells may catalyze the pyrite formation in order to reap the
energy produced. Additionally, Wächtershäuser points out that many sulfur-metabolizing
archaebacteria undergo optimum growth in an acidic pH range, at which iron sulfides are
soluble and pyrite is insoluble.3
According to Wächtershäuser, the earliest organisms developed with the
exergonic formation of pyrite as an energy source, gradually diversifying as they
migrated into areas lacking iron and/or H2S. The organisms would have originally
supplemented the pyrite formation pathway with other energy flows, eventually taking on
these alternative flows as substitutes.3 He indicates the presence of enzymes containing
iron-sulfur centers in photosystem I and their absence from photosystem II as supportive
of this model, suggesting that anoxygenic eubacterial photosynthesis arose in a lightexposed environment lacking iron but containing ample H2S and developed an oxidative
metabolism of sulfur which passed through the formation of pyrite’s disulfide bonds.
The emergence of cyanobacteria merged this ancient photosystem with another
photosynthetic pathway independent of FeS-center-containing enzymes.3
Concentration Problem
In his Blueprint for a Cell: The Nature and Origin of Life, Christian DeDuve5
points out two potential flaws of the prebiotic soup hypothesis: the composition of
Earth’s primordial atmosphere and the problem of concentration. DeDuve challenges
Miller’s assumption of an early atmosphere rich in CH4, writing, “According to the latest
geochemical reconstructions, our planet, as it condensed, must have lost most of its
hydrogen, leaving CO2, rather than CH4, as the main source of carbon. Under such
conditions, the yield of amino acids by electric discharges would be drastically reduced.”5
While this uncertainty about prebiotic atmospheric conditions does not rule out the
prebiotic soup hypothesis, it does call into question the plausibility of the model proposed
by Miller and subsequent researchers.
DeDuve goes on to discuss another persistent problem while arguing for the
presence of catalysts in the development of protometabolic pathways. Using the example
of the abiotic synthesis of RNA, he explains that even optimistic models require multiple
steps plagued by low yields and side reactions, ultimately leading to extremely minute
concentrations of correct polynucleotides overwhelmed by the products of the many
counterproductive side reactions.5 Russell and Martin also stress this point, asserting that
such infinitesimal concentrations dissolved in the primitive ocean would lead to a
prebiotic soup far too dilute to initiate biochemistry.6
Russell and Martin also raise the concentration problem in the context of the
surface metabolism hypothesis, for the hypothesis proposes that polymerized products
would remain coordinated on the catalyst until complete cells arose. Russell and Martin
argue that a two-dimensional (2D) catalytic surface, such as the one proposed by
Wächtershäuser et al.,3 would rapidly become saturated with monomers and lose its
catalytic activity. If the products were to diffuse into the ocean, preventing the blocking
of the catalyst, the end result would again be an extremely dilute organic soup incapable
of supporting biochemistry.6
Encapsulation Problem
Russell and Martin also cite the development of fully encapsulated cells as a
problem inherent in both of the previously discussed hypotheses. Both theories assert
that lipid droplets eventually combined with self-replicating systems to form coacervates
– collections of proteins or protein-like compounds encased within small droplets
suspended in a surrounding liquid. Russell and Martin view this explanation as
problematic, raising the question of how an entire self-replicating system – complete with
enzymes, metabolites, and an underlying genome – could develop in free solution before
being incorporated into coacervates.6 Based on this problem, the researchers assert that
some form of inorganic compartment was necessarily a crucial component of the
development of life on Earth.
Hydrothermal Reactors
Russell and Martin propose that 3D structures of (Fe,Ni)S bubbles (probotryoids)
occurring at primordial hydrothermal vents served as both catalytic surfaces and
concentrating compartments in the initiation of biochemistry.6,7 According to this model,
the ferric/ferrous sulfide membranes utilized Fe2+/Fe3+ transitions to catalyze the
reduction of CO, CO2, and formaldehyde. This model differs from that suggested by
Wächtershäuser et al., in which the exergonic formation of pyrite from H2S and FeS
served as a prebiotic energy source,3 for Russell and Martin argue that the formation of
crystalline pyrite would have greatly decreased the flexibility and chemical reactivity of
the (Fe,Ni)S membranes. In contrast, the researchers suggest a fourfold iron-sulfur
coordination similar to that found in biological systems, since iron in this environment
can undergo redox reactions when also liganded to organic componds.7 They also
propose that, after initially growing primarily through hydrostatic inflation, the (Fe,Ni)S
bubbles eventually grew through osmosis driven by catabolism of abiogenic organic
compounds and cleavage of hydrophobic species into hydrophilic species which were
then transported from the membrane to the alkaline interior of the bubbles by the proton
motive force of the Hadean ocean. Autoinsertion of protolipids composed of fatty acids
with thiolate (HS-) headgroups, analogous to the monomeric diffusion of cholesterol into
biological membranes, would also have contributed to membrane expansion.7
In support of this model of gelatinous (Fe,Ni)S bubbles as catalytic membranes,
the researchers cite earlier work in which they formed semipermeable membranes of FeS
by injecting 0.1 M sodium sulfide (Na2S·9H2O) into 0.1 M ferrous chloride
(FeCl2·4H2O).8 This work produced both hollow FeS spires and bubbles similar to those
found as fossilized pyrite botryoids in ore deposits in Ireland. To examine the
plausibility of autoinsertion of hydrophobic organic moieties into these membranes,
Russell added a 100 ppm mixture of potentially abiogenic organic species: 5 ppm each of
alanine and aspartic acid, 15 ppm each of formaldehyde, formic acid, glycine, and valine,
and 30 ppm of glycine. Citing unpublished research, he reports that incorporation of
these compounds into the mixture resulted in a 20 to 40-fold increase in the durability of
the bubbles. While addition of a mixture of concentrations decreased proportionately to
10 ppm had only minor effects on bubble durability, a proportionate increase to 1000
ppm increased the lifetime of the bubbles by 20-fold.7
Primitive Acetyl-CoA Pathway Analogue
As the most plausible candidate for the primordial biochemistry catalyzed by the
hydrothermal reactors, Russell and Martin propose a primitive analogue of the acetylCoA pathway.6 They suggest that hydrothermal H2 served as an electron donor and
marine CO2 as an electron acceptor, since these two compounds were far from
thermodynamic equilibrium on the ancient Earth. In a reaction catalyzed by (Fe,Ni)S
structures, the hydrothermal reactors produced acetate in the form of thioesters – the
authors do not specify a single product, but rather suggest that the reaction occurred with
various thiols to produce a variety of thioesters. Equation 1 shows the overall reaction of
the modern acetyl-CoA pathway. Since thioesters readily undergo hydrolysis to produce
4H2 + 2CO2 + HSCoA → CH3COSCoA + 3H2O, ΔG0’ ≈ - 59 kJ/mol
(Eqn 1)
free thiols and carboxylic acids – in this case, acetic acid – the compartments of the FeS
membrane would have contained hydrophobic hydrolysis products, building up a store of
organic precursors capable of undergoing subsequent reactions (Fig. 1). The researchers
suggest that this process would have eventually produced reactive sulfur-containing
byproducts, which, assuming the presence of phosphate from the primordial ocean and
ammonia from the reduction of N2 deep in the Earth’s crust, could have fueled the
transition from inorganic chemistry to biochemistry.6
Figure 1. A schematic representation of a hydrothermal vent serving as a catalytic
reactor. According to Russell and Martin, “Gradients in temperature (110 to 20 °C), pH
(10 to 6) and redox (- 600 mV to + 100 mV) are steepest at the mound’s exterior.”6 Inset
depicts an enlargement of one of the vents, with acetate leaving as waste and organic
products of thioester hydrolysis remaining. Originally published by Russell and Martin6
as Figure 3.
To support their hypothesis, Russell and Martin draw on geological, chemical,
and biochemical evidence. For information concerning the composition of the Hadean
ocean, they cite earlier research9 and E. L. Shock,10 reporting that 4 x 109 years (4 Gyr)
ago, the hydrothermal fluid venting into the ocean contained concentrations of H2 higher
than today’s significant levels and that the Hadean ocean contained more dissolved CO2
than the modern ocean. In this environment, H2 from the highly reduced hydrothermal
fluid would have met with CO2 at the temperature, pH, and redox gradients which
spanned the colloidal (Fe,Ni)S exteriors of the hydrothermal vent mounds.6 According to
work by Huber and Wächtershäuser,11 a combination of FeS and NiS is capable of
catalyzing the reaction of CO and CH3SH to produce acetic acid: Equation 2 shows the
overall reaction. Conducting experiments in water at 100 °C at a wide range of pH
values, they found that a bimodal catalyst of 1 mmol NiS and 1 mmol FeS yielded a high
CH3SH + CO + H2O → CH3CO2H + H2S
(Eqn 2)
final concentration of acetic acid near physiological pH (Fig. 2). In order to determine
whether the acetic acid formed via a thioester (CH3-CO-SCH3) intermediate, they carried
out additional experiments under conditions biased to favor the thioester as a product:
they reacted a molar ratio of NiSO4 to Na2S to CH3SH of 2:1.5:1 for 20 hours at pH 1.6.
Two runs yielded 7 and 9 μmol of CH3-CO-SCH3 in addition to ~ 25 μmol of acetic acid.
To ensure that the thioester was not a result of a secondary equilibrium between CH3SH
and acetic acid, they conducted a similar experiment, replacing CO with N2 and adding
30 mL of acetic acid, and detected only 0.2 μmol of CH3-CO-SCH3 at pH 1.7.11 Figure 3
illustrates the mechanism of thioester formation proposed by Huber and Wächtershäuser.
Since (Fe,Ni)S clusters such as the Fe4NiS5 ‘C-cluster’ of carbon monoxide
dehydrogenase (CODH) are capable of reducing CO2 to CO,6 it is plausible that the metal
sulfide structures of the hydrothermal vent carried out the same reduction. Given the
availability of CO within the hydrothermal reactor, the results obtained by Huber and
Wächtershäuser support the hypothesis that a primitive CO2 fixation pathway originated
within the confines of a hydrothermal reactor.
Figure 2. Acetic acid yield vs. reaction pH for the reaction of 100 μmol CH3SH and CO
in the presence of four catalysts: a combination of 1 mmol NiS and 1 mmol FeS
(crosses), 1 mmol NiS (triangles), a combination of 1 mmol NiS and 1 mmol CoS
(squares), and 2 mmol NiSO4 (circles). Originally published by Huber and
Wächtershäuser11 as Figure 1.
Figure 3. Hypothetical mechanism of formation of acetic acid from CO and CH3SH on
NiS-FeS proposed by Huber and Wächtershäuser11. (a) Fe center binds CO and Ni center
binds CH3SH. (b) Methyl-Ni center forms. (c) Methyl group migrates to carbonyl group.
(d) Acetyl group migrates to sulfido (or sulfhydryl) group to form thioacetate ligand of Ni
(or Fe). (e) Hydrolysis of thioacetate ligand forms acetic acid. Originally published by
Huber and Wächtershäuser11 as Figure 2.
As evidence from the realm of biochemistry, Russell and Martin cite both the
relative evolutionary age of the acetyl-CoA pathway of CO2 fixation and the presence of
catalytic FeS and (Fe,Ni)S centers in extant proteins. While Juli Peretó et al.12 argue that
the phylogenetic distributions of the acetyl-CoA pathway and the reductive citric acid
cycle fail to conclusively prove which pathway is the oldest, they do cite the work of
Huber and Wächtershäuser11 as supportive of the hypothesis of an acetyl-CoA pathway
analogue as the first biochemical carbon fixation route.12 As examples of contemporary
proteins containing catalytic FeS or (Fe,Ni)S centers, Russell and Martin use carbon
monoxide dehydrogenase (CODH) and the bifunctional enzyme of acetyl-CoA synthase
(ACS) coupled to CODH (ACS-CODH). Specifically, they mention the C-clusters of
CODH and the A-cluster of ACS-CODH (Fig. 4a) as being partially similar in structure
to the Fe42.5+S4 thiocubane unit of the mineral greigite (Fig. 4b).6 According to the
researchers, greigite is a metastable mineral with the quarter cell formula
(SNiS)(Fe4S4)(SFeS). They list the electron carrier ferredoxin, [FeNi]-hydrogenase, and
[Fe]-hydrogenase (Fig. 4c) as additional enzymes in the modern acetyl-CoA pathway
which contain thiocubane units. Although the authors concede that debate continues over
the presence of copper, zinc, and nickel at the active site of ACS-CODH’s A-cluster, they
assert that these metal sulfide catalytic sites were incorporated into enzymes as
biochemistry developed within the hydrothermal reactor. They write, “[T]he structure
and atomic coordination of catalytically essential (Fe,Ni)S centres of (Fe,Ni)S proteins
are not inventions of the biological world, rather they are mimics of minerals that are
indisputably older and have catalytic activity in the absence of protein.”6
Figure 4. (a) The A-cluster of ACS-CODH and the two C-clusters of CODH. (b) A half
cell unit of the metastable mineral greigite,6 showing the Fe42.5+S4 thiocubane unit. (c)
Selected thiocubane centers from three additional enzymes in the acetyl-CoA pathway:
[FeNi]-hydrogenase, ferredoxin, and [Fe]-hydrogenase. For all enzymes, only selected
metal sulfide centers are shown. The shaded areas roughly represent the shape of the
surrounding protein structure. Adapted from Russell and Martin,6 Figure 2.
Conclusions
Because the metal sulfide surfaces of the hydrothermal reactor serve both to
catalyze the production of thioesters from CO2 and H2 and to concentrate reduced organic
compounds within the 3D structures of the hydrothermal vent,6 the model for the origin
of biochemistry proposed by Russell and Martin enjoys a distinct advantage over the
prebiotic soup hypothesis. While the latter requires concentrations of abiotically
produced organic precursors which could not plausibly occur in the Hadean ocean,5 the
former allows useful precursor concentrations to accumulate in inorganic compartments.
Additionally, these compartments could have served to enclose the earliest selfreplicating systems before they developed into mature cells with organic compartments.6
In relation to the surface metabolism hypothesis, the hydrothermal reactor
hypothesis maintains a similar advantage in coping with the problem of concentration. In
the surface metabolism model, self-replicating systems must either remain on the
catalytic surface until the emergence of free-living cells, or their organic reactants must
diffuse into the ocean.6 In the first case, the crowding of the catalytic surface would
greatly diminish the catalytic activity, while in the second case, the diffusion into the
ocean creates the same hopelessly dilute concentrations of organic precursors which
plagues the prebiotic soup hypothesis.
Finally, the empirical results of Huber and Wächtershäuser demonstrate that
carbon fixation reactions such as those proposed by Russell and Martin can occur under
the conditions of a hydrothermal vent. Additionally, since the combined FeS-NiS
catalyst yielded significant amounts of acetic acid at physiological pH,11 this research
begins to draw the link between catalytic metal sulfide clusters in mineral structures and
the catalytic metal sulfide centers of contemporary enzymes. The acetyl-CoA pathway
contains several enzymes bearing such clusters,6 and the great age of the pathway12
suggests that these enzymes incorporated these catalytic centers at an early stage in the
development of life. This supporting evidence, coupled with the persistent concentration
and encapsulation difficulties of the prebiotic soup and surface metabolism hypotheses,
suggests that the hydrothermal reactor hypothesis is a more plausible model for the origin
of life and that a primitive analogue of the acetyl-CoA pathway is a plausible candidate
for the earliest biochemical route.
References
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(2)
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