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Energetics of Amino Acid Synthesis in Hydrothermal Ecosystems

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Science  11 Sep 1998:
Vol. 281, Issue 5383, pp. 1659-1662
DOI: 10.1126/science.281.5383.1659

Abstract

Thermodynamic calculations showed that the autotrophic synthesis of all 20 protein-forming amino acids was energetically favored in hot (100°C), moderately reduced, submarine hydrothermal solutions relative to the synthesis in cold (18°C), oxidized, surface seawater. The net synthesis reactions of 11 amino acids were exergonic in the hydrothermal solution, but all were endergonic in surface seawater. The synthesis of the requisite amino acids of nine thermophilic and hyperthermophilic proteins in a 100°C hydrothermal solution yielded between 600 and 8000 kilojoules per mole of protein, which is energy that is available to drive the intracellular synthesis of enzymes and other biopolymers in hyperthermophiles thriving in these ecosystems.

Recently, Woese (1) suggested that the ancestor of all life on Earth was not a discrete entity but rather a community of cells with a shared physical history. Over time, as the universal tree of life radiated outward from the root, three primary domains of organisms arose. Although interpretations of the complete genomes of over a dozen microbes have raised questions regarding the classification of various organisms within this phylogenetic tree (2), general properties of members belonging to the deepest branches of the Bacteria and Archaea lineages indicate that the earliest life was autotrophic not heterotrophic, relied on chemosynthesis rather than photosynthesis, and required high temperatures for growth (3). On the basis of these findings, ancient hydrothermal systems have been proposed as likely sites for the origin of life (4–6). This view is consistent with the results of hydrothermal experiments that were aimed at identifying the primordial chemosynthesis reactions for life's origin (7).

In the speculative arena of the origin and the early evolution of life, quantification of the energetics of biosynthesis reactions in microorganisms belonging to the deepest branches in the phylogenetic tree is of interest. However, the determination of these energetics in hydrothermal systems on early Earth (4, 8, 9) is hindered somewhat by poorly constrained chemical and physical properties of the earliest oceans, including the redox state, pH, temperature, and concentrations of CO2, NH4 +, and H2S. On the other hand, ample data are available from active hydrothermal ecosystems, which are hosts to the deepest branches of thermophilic, chemoautotrophic Archaea and Bacteria. Analyses of seawater and vent fluids together with reliable equations of state for aqueous organic and inorganic compounds permit well-constrained calculations of the energetics of biosynthesis reactions in hydrothermal ecosystems. Once such a framework for evaluating the energetics of biosynthesis is in place, analogous calculations can be carried out to account for likely conditions on early Earth.

Chemical disequilibria in hydrothermal ecosystems provide substantial amounts of energy, which can drive anabolic reactions in thermophilic and hyperthermophilic chemoautotrophs (8, 10). Furthermore, the formation of many aqueous organic compounds is favored at high temperatures over low temperatures (11–13). We calculated the overall Gibbs free energies (ΔGr)of net amino acid synthesis reactions (r) for hydrothermal systems and contrasted them with the energy requirements of synthesis reactions in surface seawater. We then used these calculations to explore the ramifications for the synthesis of thermophilic proteins, and we considered the implications for early life.

Amino acid synthesis pathways in extant microorganisms, although highly diverse, share two basic features: (i) the nitrogen of α-amino groups in amino acids originates from NH4 + and (ii) the sources of skeletal carbons are intermediates of the tricarboxylic acid cycle and the other major metabolic pathways that are ultimately linked, in autotrophs, to the assimilation of inorganic carbon in the form of CO2. These features allow us to write net reactions for the autotrophic synthesis of the 20 naturally occurring amino acids (Table 1). Implicit in all the calculations presented here is the assumption that the appropriate enzymes for each step in the amino acid synthesis pathways are present and active.

Table 1

Net amino acid synthesis reactions (23).

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The net energetics of amino acid synthesis can be determined by combining standard-state Gibbs free energies of synthesis reactions(ΔG r ° ) with products of intracellular activity. Although the intracellular concentrations of the essential reactants and products are poorly known, we can place constraints on the amounts of energy released or required during amino acid synthesis from CO2, H2, NH4 +, and H2S by autotrophic microbes in natural ecosystems using extracellular activity products. The diffusion of neutral molecules and the transfer of ions by transport proteins establish chemical links across cell membranes between the host aqueous environment and the intracellular fluid. The energetic consequences of differences between the extracellular and intracellular activities of the reactants and products can be assessed when the energetic demands that organisms place on their geochemical environments are established. Even if the activities of free amino acids, for example, are orders of magnitude higher within the cell than in the natural environment, values of ΔGr for their synthesis can be computed.

We calculated values of ΔGr for all 20 net amino acid synthesis reactions in submarine hydrothermal solutions with a temperature (100°C) between that of cold (2°C) seawater and very hot (350°C) vent fluid; this temperature was achieved by mixing these end members in the subsurface. The amounts of energy associated with the reactions in Table 1 can be calculated from the expressionEmbedded Image(1)where ΔGr andΔG r ° are as defined above; Rand T represent the gas constant and temperature in kelvin, respectively; and Qr denotes the activity product. The values of Qr that are required to evaluate ΔGr with Eq. 1 can be determined fromEmbedded Image(2)where ai stands for the activity of the ith species andνi,r represents the stoichiometric reaction coefficient of the ith species in reaction r,which is negative for reactants and positive for products (14).

The standard-state term (ΔG r ° )can be computed at any temperature and pressure by combining the apparent standard Gibbs free energies of formation(ΔG i ° ) of the ith species in r (15) at these conditions in accord with the expressionEmbedded Image(3)The values ofΔG i ° at the temperature and pressure of interest for the aqueous species in the reactions given in Table 1 are readily calculated using thermodynamic properties and parameters for the revised Helgeson-Kirkham-Flowers equation of state (16) together with the SUPCRT92 software package (17). For all 20 reactions, the values of ΔG r ° at 250 bar and at temperatures from 0° to 150°C are negative and exhibit substantial temperature dependencies, increasing with increasing temperature (Fig. 1).

Figure 1

Values of ΔG r °of the net amino acid synthesis reactions (see Table 1) in aqueous solution as a function of temperature at 250 bar. This pressure was chosen to approximate conditions in deep-sea hydrothermal systems.

Evaluating RT ln Qr (Eq. 1) for the reactions in Table 1 requires the activities of the reactants and products under the environmental conditions of interest. For example, a 100°C hydrothermal solution can be generated if ∼2.55 kg of 2°C seawater are mixed with 1 kg of 350°C vent fluid (10). The calculated activities of the aqueous species CO2, H2, H2S, H+, and NH4 + are given in Table 2 for a 100°C mixed hydrothermal solution and surface seawater at 18°C. The activities of the free amino acids in these two fluids were determined with the same mixing model, assuming, however, that the only contribution was from free amino acids in seawater (Table 3) (18).

Table 2

Activities of aqueous inorganic species in mixed hydrothermal solution and surface seawater.

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Table 3

Activities of amino acids in surface seawater (27) and in mixed hydrothermal solution, values of ΔGr for net amino acid synthesis reactions (Table 1) in these two fluids, and mean nominal oxidation states of the carbon atoms (Z C) in the amino acids.

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The value of ΔGr in the 100°C mixed solution for each net amino acid synthesis reaction can now be calculated (Table 3). As an example, we describe the approach used for the synthesis of alanine, represented by the first reaction in Table 1. At 100°C and 250 bar (see Fig. 1 and text above), ΔG 1 ° is equal to −155.34 kJ mol−1, and Q 1, determined fromEmbedded Image(4)using activities a given inTables 2 and 3, is equal to 1.1 × 1020. Combining these values, as in Eq. 1, gives ΔGr = −12.12 kJ mol−1. Therefore, the net synthesis of 1 mol of alanine in a 100°C mixed hydrothermal solution releases 12.12 kJ of energy, which is available to drive otherwise endergonic anabolic reactions such as peptide and protein synthesis.

The value of ΔGr is negative for 11 of the 20 net amino acid synthesis reactions in a 100°C mixed hydrothermal solution that supports the growth of hyperthermophilic microbes (Table 3). The formation of these 11 amino acids from inorganic precursors lowers the energy state of the system at the conditions that occur in this subsurface ecosystem. The net synthesis reactions for the remaining nine amino acids are endergonic and must be coupled to exergonic reactions in order to proceed. By comparison, the net synthesis reactions of all 20 amino acids are strongly endergonic in cooler (18°C), oxidized, surface seawater. The values ofΔGr (Table 3) for the reactions in Table 1under these conditions were generated with the values ofΔG r ° at 18°C and 1 bar together with activity products for seawater computed with data from Tables 2 and 3. Comparisons of ΔGr for the two distinct environments show differences of 65 to 470 kJ mol−1 of amino acid produced.

If the concentrations of free amino acids were, for example, three orders of magnitude higher, values of ΔGr (Table 3) would increase for each amino acid synthesis reaction by 21.43 and 16.72 kJ mol−1 at 100° and 18°C, respectively. As a result, 9 instead of 11 net amino acid synthesis reactions would be exergonic in the 100°C hydrothermal solution. At millimolar concentrations, an increase in ΔGr of 42.86 and 33.44 kJ mol−1 can be calculated at the two temperatures. In this case, six net amino acid synthesis reactions have negative values of ΔGr at 100°C.

These results support the argument that hydrothermal vent environments are well suited for organic synthesis (5, 11, 19). Even biomolecules such as amino acids can be synthesized exergonically in hydrothermal ecosystems. The environmental conditions that permit the exergonic formation of amino acids are entirely geochemically and geologically controlled. The temperature, pressure, and activities of reactants and products that exist in hydrothermal systems determine the energetics discussed above.

Although values of ΔG r ° for the reactions in Table 1 are strongly temperature dependent (see Fig. 1), it is an inescapable consequence of the reducing potential of mixed hydrothermal solutions that synthesis of the more reduced amino acids is favored over the synthesis of the more oxidized ones. In a 100°C hydrothermal solution, negative values of the mean nominal oxidation state of the carbon atoms (Z C) (20) in the amino acids correspond to negative values ofΔGr (Table 3). In fact, the net synthesis reactions are exergonic for the 10 most reduced (lowest values ofZ C) amino acids (Leu, Ile, Val, Lys+, Phe, Met, Pro, Tyr, Trp, and Ala) and are endergonic for 9 of the 10 most oxidized (highest values ofZ C) amino acids (His, Asn, Asp, Gly, Cys, Ser, Gln, Arg+, and Thr). In contrast to the 100°C mixed hydrothermal solution, all values ofΔGr in 18°C surface seawater are positive (endergonic). These observations support the notion that, under oxidizing near-surface conditions, the autotrophic synthesis of amino acids driven by photosynthesis requires a tremendous external energy source (solar radiation), which is not necessary in ecosystems of high or even moderate reducing potential.

The energy yield from the autotrophic synthesis of amino acids in a 100°C mixed hydrothermal fluid (see Table 3) may ultimately be used to overcome the energy requirements of protein synthesis. To calculate this energy yield, we combined sequence data for thermophilic proteins with the thermodynamic evaluation discussed above. For example, from the sequence of rubredoxin from the hyperthermophilic ArchaeonPyrococcus furiosus (53 residues) and the values ofΔGr for all 20 amino acids (Table 3), we computed that, in a 100°C mixed hydrothermal solution, the net synthesis of the amino acids constituting 1 mol of this protein releases 722 kJ. The values of ΔGr for the net amino acid synthesis of this and eight other thermophilic proteins (Table 4) are all negative (exergonic). Combined with the conclusion that peptide bond formation is energetically favored with increasing temperature (12), an argument can be made that thermophilic chemoautotrophs, such as those occupying the deepest branches in the universal tree of life, expend considerably less energy for the synthesis of macromolecules, such as proteins, than do their mesophilic counterparts. Depending on the amino acid composition of the protein, the synthesis of the monomers from CO2, H2, and other inorganic precursors in hot, reduced aqueous solutions may provide substantial surplus energy that can be harnessed to drive intracellular synthesis of enzymes and other polymers.

Table 4

Values of ΔGr in a 100°C mixed hydrothermal solution for the net synthesis of the amino acids that constitute nine proteins from thermophilic and hyperthermophilic Archaea and Bacteria. This is not the net energy for protein synthesis. Numbers in brackets are the number of amino acid residues. GAPDH, glyceraldehyde phosphate dehydrogenase; OMP, orotidine 5′-phosphate.

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Our results might start to explain the phenomenal rates of biomass production around hydrothermal vents (21) and also how hyperthermophilic Archaea in natural or laboratory high-temperature systems are able to synthesize all required intracellular biomolecules in time periods ranging from minutes to hours as their population doubles. Our calculations can be used as a template in concert with constraints on the flow of energy through early hydrothermal systems to determine the potential of such systems as environments where amino acid and protein synthesis, primitive metabolisms, and even the universal ancestor of all extant life emerged. Calculations representing early Earth hydrothermal systems must reflect the differences in geochemistry and geophysics from active analogs. For example, sensitivity tests show that lower O2concentrations in seawater and ultramafic host rocks enhance the potential for hydrothermal organic synthesis (13), and the same should be expected for amino acid and protein synthesis.

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