Self-generating autocatalytic networks: structural results, algorithms and their relevance to early biochemistry.

The concept of an autocatalytic network of reactions that can form and persist, starting from just an available food source, has been formalized by the notion of a reflexively autocatalytic and food-generated (RAF) set. The theory and algorithmic results concerning RAFs have been applied to a range of settings, from metabolic questions arising at the origin of life, to ecological networks, and cognitive models in cultural evolution. In this article, we present new structural and algorithmic results concerning RAF sets, by studying more complex modes of catalysis that allow certain reactions to require multiple catalysts (or to not require catalysis at all), and discuss the differing ways catalysis has been viewed in the literature. We also focus on the structure and analysis of minimal RAFs and derive structural results and polynomial-time algorithms. We then apply these new methods to a large metabolic network to gain insights into possible biochemical scenarios near the origin of life.

What Wilhelm Ostwald meant by "Autokatalyse" and its significance to origins-of-life research: Facilitating the search for chemical pathways underlying abiogenesis by reviving Ostwald's thought that reactants may also be autocatalysts: Facilitating the search for chemical pathways underlying abiogenesis by reviving Ostwald's thought that reactants may also be autocatalysts.

A closer look at Wilhelm Ostwald's articles that originally proposed the concept of autocatalysis reveals that he accepted reactants, not just products, as potential autocatalysts. Therefore, that a process is catalyzed by some of its products, which is the common definition of autocatalysis, is only a proper subset of what Ostwald meant by "Autokatalyse." As a result, it is necessary to reconsider the definition of autocatalysis, which is especially important for origins-of-life research because autocatalysis provides an abiotic mechanism that yields reproduction-like dynamics. Here, we translate and briefly review the two key publications on autocatalysis by Ostwald to revive his understanding of autocatalysis, and we introduce the concepts of recessive and expansive autocatalysis. Then we discuss the twofold significance of such a revival: first, facilitating the search for candidate processes underlying the origins of life, and second, updating our view of autocatalysis in complex reaction networks and metabolism.

Small-molecule autocatalytic networks are universal metabolic fossils.

Life and the genetic code are self-referential and so are autocatalytic networks made of simpler, small molecules. Several origins of life theories postulate autocatalytic chemical networks preceding the primordial genetic code, yet demonstration with biochemical systems is lacking. Here, small-molecule reflexively autocatalytic food-generated networks (RAFs) ranging in size from 3 to 619 reactions were found in all of 6683 prokaryotic metabolic networks searched. The average maximum RAF size is 275 reactions for a rich organic medium and 93 for a medium with a single organic cofactor, NAD. In the rich medium, all universally essential metabolites are produced with the exception of glycerol-1-p (archaeal lipid precursor), phenylalanine, histidine and arginine. The 300 most common reactions, present in at least 2732 RAFs, are mostly involved in amino acid biosynthesis and the metabolism of carbon, 2-oxocarboxylic acid and purines. ATP and NAD are central in generating network complexity, and because ATP is also one of the monomers of RNA, autocatalytic networks producing redox and energy currencies are a strong candidate niche of the origin of a primordial information-processing system. The wide distribution of small-molecule autocatalytic networks indicates that molecular reproduction may be much more prevalent in the Universe than hitherto predicted. This article is part of the theme issue 'Emergent phenomena in complex physical and socio-technical systems: from cells to societies'.

Energy at Origins: Favorable Thermodynamics of Biosynthetic Reactions in the Last Universal Common Ancestor (LUCA).

Though all theories for the origin of life require a source of energy to promote primordial chemical reactions, the nature of energy that drove the emergence of metabolism at origins is still debated. We reasoned that evidence for the nature of energy at origins should be preserved in the biochemical reactions of life itself, whereby changes in free energy, ΔG, which determine whether a reaction can go forward or not, should help specify the source. By calculating values of ΔG across the conserved and universal core of 402 individual reactions that synthesize amino acids, nucleotides and cofactors from H2, CO2, NH3, H2S and phosphate in modern cells, we find that 95-97% of these reactions are exergonic (ΔG ≤ 0 kJ⋅mol-1) at pH 7-10 and 80-100°C under nonequilibrium conditions with H2 replacing biochemical reductants. While 23% of the core's reactions involve ATP hydrolysis, 77% are ATP-independent, thermodynamically driven by ΔG of reactions involving carbon bonds. We identified 174 reactions that are exergonic by -20 to -300 kJ⋅mol-1 at pH 9 and 80°C and that fall into ten reaction types: six pterin dependent alkyl or acyl transfers, ten S-adenosylmethionine dependent alkyl transfers, four acyl phosphate hydrolyses, 14 thioester hydrolyses, 30 decarboxylations, 35 ring closure reactions, 31 aromatic ring formations, and 44 carbon reductions by reduced nicotinamide, flavins, ferredoxin, or formate. The 402 reactions of the biosynthetic core trace to the last universal common ancestor (LUCA), and reveal that synthesis of LUCA's chemical constituents required no external energy inputs such as electric discharge, UV-light or phosphide minerals. The biosynthetic reactions of LUCA uncover a natural thermodynamic tendency of metabolism to unfold from energy released by reactions of H2, CO2, NH3, H2S, and phosphate.

The metabolic network of the last bacterial common ancestor.

Bacteria are the most abundant cells on Earth. They are generally regarded as ancient, but due to striking diversity in their metabolic capacities and widespread lateral gene transfer, the physiology of the first bacteria is unknown. From 1089 reference genomes of bacterial anaerobes, we identified 146 protein families that trace to the last bacterial common ancestor, LBCA, and form the conserved predicted core of its metabolic network, which requires only nine genes to encompass all universal metabolites. Our results indicate that LBCA performed gluconeogenesis towards cell wall synthesis, and had numerous RNA modifications and multifunctional enzymes that permitted life with low gene content. In accordance with recent findings for LUCA and LACA, analyses of thousands of individual gene trees indicate that LBCA was rod-shaped and the first lineage to diverge from the ancestral bacterial stem was most similar to modern Clostridia, followed by other autotrophs that harbor the acetyl-CoA pathway.

The Autotrophic Core: An Ancient Network of 404 Reactions Converts H2, CO2, and NH3 into Amino Acids, Bases, and Cofactors.

The metabolism of cells contains evidence reflecting the process by which they arose. Here, we have identified the ancient core of autotrophic metabolism encompassing 404 reactions that comprise the reaction network from H2, CO2, and ammonia (NH3) to amino acids, nucleic acid monomers, and the 19 cofactors required for their synthesis. Water is the most common reactant in the autotrophic core, indicating that the core arose in an aqueous environment. Seventy-seven core reactions involve the hydrolysis of high-energy phosphate bonds, furthermore suggesting the presence of a non-enzymatic and highly exergonic chemical reaction capable of continuously synthesizing activated phosphate bonds. CO2 is the most common carbon-containing compound in the core. An abundance of NADH and NADPH-dependent redox reactions in the autotrophic core, the central role of CO2, and the circumstance that the core's main products are far more reduced than CO2 indicate that the core arose in a highly reducing environment. The chemical reactions of the autotrophic core suggest that it arose from H2, inorganic carbon, and NH3 in an aqueous environment marked by highly reducing and continuously far from equilibrium conditions. Such conditions are very similar to those found in serpentinizing hydrothermal systems.

Gene Duplications Trace Mitochondria to the Onset of Eukaryote Complexity.

The last eukaryote common ancestor (LECA) possessed mitochondria and all key traits that make eukaryotic cells more complex than their prokaryotic ancestors, yet the timing of mitochondrial acquisition and the role of mitochondria in the origin of eukaryote complexity remain debated. Here, we report evidence from gene duplications in LECA indicating an early origin of mitochondria. Among 163,545 duplications in 24,571 gene trees spanning 150 sequenced eukaryotic genomes, we identify 713 gene duplication events that occurred in LECA. LECA's bacterial-derived genes include numerous mitochondrial functions and were duplicated significantly more often than archaeal-derived and eukaryote-specific genes. The surplus of bacterial-derived duplications in LECA most likely reflects the serial copying of genes from the mitochondrial endosymbiont to the archaeal host's chromosomes. Clustering, phylogenies and likelihood ratio tests for 22.4 million genes from 5,655 prokaryotic and 150 eukaryotic genomes reveal no evidence for lineage-specific gene acquisitions in eukaryotes, except from the plastid in the plant lineage. That finding, and the functions of bacterial genes duplicated in LECA, suggests that the bacterial genes in eukaryotes are acquisitions from the mitochondrion, followed by vertical gene evolution and differential loss across eukaryotic lineages, flanked by concomitant lateral gene transfer among prokaryotes. Overall, the data indicate that recurrent gene transfer via the copying of genes from a resident mitochondrial endosymbiont to archaeal host chromosomes preceded the onset of eukaryotic cellular complexity, favoring mitochondria-early over mitochondria-late hypotheses for eukaryote origin.

The structure of autocatalytic networks, with application to early biochemistry.

Metabolism across all known living systems combines two key features. First, all of the molecules that are required are either available in the environment or can be built up from available resources via other reactions within the system. Second, the reactions proceed in a fast and synchronized fashion via catalysts that are also produced within the system. Building on early work by Stuart Kauffman, a precise mathematical model for describing such self-sustaining autocatalytic systems (RAF theory) has been developed to explore the origins and organization of living systems within a general formal framework. In this paper, we develop this theory further by establishing new relationships between classes of RAFs and related classes of networks, and developing new algorithms to investigate and visualize RAF structures in detail. We illustrate our results by showing how it reveals further details into the structure of archaeal and bacterial metabolism near the origin of life, and provide techniques to study and visualize the core aspects of primitive biochemistry.

Autocatalytic chemical networks at the origin of metabolism.

Modern cells embody metabolic networks containing thousands of elements and form autocatalytic sets of molecules that produce copies of themselves. How the first self-sustaining metabolic networks arose at life's origin is a major open question. Autocatalytic sets smaller than metabolic networks were proposed as transitory intermediates at the origin of life, but evidence for their role in prebiotic evolution is lacking. Here, we identify reflexively autocatalytic food-generated networks (RAFs)-self-sustaining networks that collectively catalyse all their reactions-embedded within microbial metabolism. RAFs in the metabolism of ancient anaerobic autotrophs that live from H2 and CO2 provided with small-molecule catalysts generate acetyl-CoA as well as amino acids and bases, the monomeric components of protein and RNA, but amino acids and bases without organic catalysts do not generate metabolic RAFs. This suggests that RAFs identify attributes of biochemical origins conserved in metabolic networks. RAFs are consistent with an autotrophic origin of metabolism and furthermore indicate that autocatalytic chemical networks preceded proteins and RNA in evolution. RAFs uncover intermediate stages in the emergence of metabolic networks, narrowing the gaps between early Earth chemistry and life.

The Future of Origin of Life Research: Bridging Decades-Old Divisions.

Research on the origin of life is highly heterogeneous. After a peculiar historical development, it still includes strongly opposed views which potentially hinder progress. In the 1st Interdisciplinary Origin of Life Meeting, early-career researchers gathered to explore the commonalities between theories and approaches, critical divergence points, and expectations for the future. We find that even though classical approaches and theories-e.g. bottom-up and top-down, RNA world vs. metabolism-first-have been prevalent in origin of life research, they are ceasing to be mutually exclusive and they can and should feed integrating approaches. Here we focus on pressing questions and recent developments that bridge the classical disciplines and approaches, and highlight expectations for future endeavours in origin of life research.

Catalysts, autocatalysis and the origin of metabolism.

If life on Earth started out in geochemical environments like hydrothermal vents, then it started out from gasses like CO2, N2 and H2. Anaerobic autotrophs still live from these gasses today, and they still inhabit the Earth's crust. In the search for connections between abiotic processes in ancient geological systems and biotic processes in biological systems, it becomes evident that chemical activation (catalysis) of these gasses and a constant source of energy are key. The H2-CO2 redox reaction provides a constant source of energy and anabolic inputs, because the equilibrium lies on the side of reduced carbon compounds. Identifying geochemical catalysts that activate these gasses en route to nitrogenous organic compounds and small autocatalytic networks will be an important step towards understanding prebiotic chemistry that operates only on the basis of chemical energy, without input from solar radiation. So, if life arose in the dark depths of hydrothermal vents, then understanding reactions and catalysts that operate under such conditions is crucial for understanding origins.

Autocatalytic networks in biology: structural theory and algorithms.

Self-sustaining autocatalytic networks play a central role in living systems, from metabolism at the origin of life, simple RNA networks and the modern cell, to ecology and cognition. A collectively autocatalytic network that can be sustained from an ambient food set is also referred to more formally as a 'reflexively autocatalytic food-generated' (RAF) set. In this paper, we first investigate a simplified setting for studying RAFs, which is nevertheless relevant to real biochemistry and which allows an exact mathematical analysis based on graph-theoretic concepts. This, in turn, allows for the development of efficient (polynomial-time) algorithms for questions that are computationally intractable (NP-hard) in the general RAF setting. We then show how this simplified setting for RAF systems leads naturally to a more general notion of RAFs that are 'generative' (they can be built up from simpler RAFs) and for which efficient algorithms carry over to this more general setting. Finally, we show how classical RAF theory can be extended to deal with ensembles of catalysts as well as the assignment of rates to reactions according to which catalysts (or combinations of catalysts) are available.

Metabolic models and gene essentiality data reveal essential and conserved metabolism in prokaryotes.

Essential metabolic reactions are shaping constituents of metabolic networks, enabling viable and distinct phenotypes across diverse life forms. Here we analyse and compare modelling predictions of essential metabolic functions with experimental data and thereby identify core metabolic pathways in prokaryotes. Simulations of 15 manually curated genome-scale metabolic models were integrated with 36 large-scale gene essentiality datasets encompassing a wide variety of species of bacteria and archaea. Conservation of metabolic genes was estimated by analysing 79 representative genomes from all the branches of the prokaryotic tree of life. We find that essentiality patterns reflect phylogenetic relations both for modelling and experimental data, which correlate highly at the pathway level. Genes that are essential for several species tend to be highly conserved as opposed to non-essential genes which may be conserved or not. The tRNA-charging module is highlighted as ancestral and with high centrality in the networks, followed closely by cofactor metabolism, pointing to an early information processing system supplied by organic cofactors. The results, which point to model improvements and also indicate faults in the experimental data, should be relevant to the study of centrality in metabolic networks and ancient metabolism but also to metabolic engineering with prokaryotes.

Serpentinization: Connecting Geochemistry, Ancient Metabolism and Industrial Hydrogenation.

Rock⁻water⁻carbon interactions germane to serpentinization in hydrothermal vents have occurred for over 4 billion years, ever since there was liquid water on Earth. Serpentinization converts iron(II) containing minerals and water to magnetite (Fe3O4) plus H2. The hydrogen can generate native metals such as awaruite (Ni3Fe), a common serpentinization product. Awaruite catalyzes the synthesis of methane from H2 and CO2 under hydrothermal conditions. Native iron and nickel catalyze the synthesis of formate, methanol, acetate, and pyruvate-intermediates of the acetyl-CoA pathway, the most ancient pathway of CO2 fixation. Carbon monoxide dehydrogenase (CODH) is central to the pathway and employs Ni° in its catalytic mechanism. CODH has been conserved during 4 billion years of evolution as a relic of the natural CO2-reducing catalyst at the onset of biochemistry. The carbide-containing active site of nitrogenase-the only enzyme on Earth that reduces N2-is probably also a relic, a biological reconstruction of the naturally occurring inorganic catalyst that generated primordial organic nitrogen. Serpentinization generates Fe3O4 and H2, the catalyst and reductant for industrial CO2 hydrogenation and for N2 reduction via the Haber⁻Bosch process. In both industrial processes, an Fe3O4 catalyst is matured via H2-dependent reduction to generate Fe5C2 and Fe2N respectively. Whether serpentinization entails similar catalyst maturation is not known. We suggest that at the onset of life, essential reactions leading to reduced carbon and reduced nitrogen occurred with catalysts that were synthesized during the serpentinization process, connecting the chemistry of life and Earth to industrial chemistry in unexpected ways.

Something special about CO-dependent CO2 fixation.

Carbon dioxide enters metabolism via six known CO2 fixation pathways, of which only one is linear, exergonic in the direction of CO2 -assimilation, and present in both bacterial and archaeal anaerobes - the Wood-Ljungdahl (WL) or reductive acetyl-CoA pathway. Carbon monoxide (CO) plays a central role in the WL pathway as an energy rich intermediate. Here, we scan the major biochemical reaction databases for reactions involving CO and CO2 . We identified 415 reactions corresponding to enzyme commission (EC) numbers involving CO2 , which are non-randomly distributed across different biochemical pathways. Their taxonomic distribution, reversibility under physiological conditions, cofactors and prosthetic groups are summarized. In contrast to CO2 , only 15 reaction classes involving CO were detected. Closer inspection reveals that CO interfaces with metabolism and the carbon cycle at only two enzymes: anaerobic carbon monoxide dehydrogenase (CODH), a Ni- and Fe-containing enzyme that generates CO for CO2 fixation in the WL pathway, and aerobic CODH, a Mo- and Cu-containing enzyme that oxidizes environmental CO as an electron source. The CO-dependent reaction of the WL pathway involves carbonyl insertion into a methyl carbon-nickel at the Ni-Fe-S A-cluster of acetyl-CoA synthase (ACS). It appears that no alternative mechanisms to the CO-dependent reaction of ACS have evolved in nearly 4 billion years, indicating an ancient and mechanistically essential role for CO at the onset of metabolism.

The last universal common ancestor between ancient Earth chemistry and the onset of genetics.

All known life forms trace back to a last universal common ancestor (LUCA) that witnessed the onset of Darwinian evolution. One can ask questions about LUCA in various ways, the most common way being to look for traits that are common to all cells, like ribosomes or the genetic code. With the availability of genomes, we can, however, also ask what genes are ancient by virtue of their phylogeny rather than by virtue of being universal. That approach, undertaken recently, leads to a different view of LUCA than we have had in the past, one that fits well with the harsh geochemical setting of early Earth and resembles the biology of prokaryotes that today inhabit the Earth's crust.

Failure to Recover Major Events of Gene Flux in Real Biological Data Due to Method Misapplication.

In prokaryotes, known mechanisms of lateral gene transfer (transformation, transduction, conjugation, and gene transfer agents) generate new combinations of genes among chromosomes during evolution. In eukaryotes, whose host lineage is descended from archaea, lateral gene transfer from organelles to the nucleus occurs at endosymbiotic events. Recent genome analyses studying gene distributions have uncovered evidence for sporadic, discontinuous events of gene transfer from bacteria to archaea during evolution. Other studies have used traditional models designed to investigate gene family size evolution (Count) to support claims that gene transfer to archaea was continuous during evolution, rather than involving occasional periodic mass gene influx events. Here, we show that the methodology used in analyses favoring continuous gene transfers to archaea was misapplied in other studies and does not recover known events of single simultaneous origin for many genes followed by differential loss in real data: plastid genomes. Using the same software and the same settings, we reanalyzed presence/absence pattern data for proteins encoded in plastid genomes and for eukaryotic protein families acquired from plastids. Contrary to expectations under a plastid origin model, we found that the methodology employed inferred that gene acquisitions occurred uniformly across the plant tree. Sometimes as many as nine different acquisitions by plastid DNA were inferred for the same protein family. That is, the methodology that recovered gradual and continuous lateral gene transfer among lineages for archaea obtains the same result for plastids, even though it is known that massive gains followed by gradual differential loss is the true evolutionary process that generated plastid gene distribution data. Our findings caution against the use of models designed to study gene family size evolution for investigating gene transfer processes, especially when transfers involving more than one gene per event are possible.