Central Carbon Metabolism

FUNCTION AND MECHANISM

How this system works

FOLLOW THE FLOW

Carbon supply to cellular building blocks

Carbon comes in → it is turned into a few standard building blocks → those are shared out to the systems that build the rest of the cell

SYSTEM 01 / 16

Central Carbon Metabolism

Making the carbon building blocks of life

Air is one source of carbon. Cells can also obtain carbon in ready-made food molecules.

1TWO POSSIBLE SOURCES

Carbon dioxide in the air

CO₂ is a gas in the atmosphere. It dissolves in water; some becomes bicarbonate (HCO₃⁻), another dissolved form of carbon. CO₂ can also enter water from respiration or geological sources.

Carbon in food molecules

Sugars, organic acids and other carbon-containing molecules are already present in the surroundings.

↓

2TWO DIFFERENT WAYS IN

Capture dissolved CO₂

Carbon-fixing cells use enzymes and energy to turn inorganic carbon into organic molecules. The model offers the Wood–Ljungdahl pathway OR the reverse Krebs cycle.

Take up organic molecules

Cells that feed on organic carbon bring suitable molecules through membrane transport proteins, then process them. They may not fix CO₂ at all.

↓

3THE ROUTES FEED CENTRAL METABOLISM

Make a shared supply of starting materials

Enzyme reactions supply small carbon molecules such as acetyl-CoA, pyruvate and sugar phosphates. Different entry routes feed different parts of this connected network.

↓

4SEVERAL OUTPUT BRANCHES

Amino acids → proteins

System 06 makes the building blocks used to build proteins.

Nucleotides → DNA and RNA

System 05 makes the units used to build genetic material.

Lipids → cell membrane

System 07 makes much of the material in the cell boundary.

Cofactors → enzyme helpers

System 02 supplies small chemical helpers for many reactions.

Read from top to bottom. Side-by-side panels show branches or supplies that come together; their labels explain the relationship. Detailed mechanisms and sources follow below.

LOOK DEEPER · CENTRAL CARBON METABOLISM

A detailed metabolic network

Follow individual carbon compounds through enzyme-catalyzed reactions, then into the branches that supply the rest of the cell. Start at the air-to-water carbon supply at the top. Wood–Ljungdahl and rTCA are shown as alternative fixation routes.

Open the large, zoomable network ↗ · Open the full-resolution diagram ↗

Book basis: Otangelo Grasso, X-ray of Life, Volume III, pp. 498 and 536–550: central carbon entry, CODH/ACS, the Wood–Ljungdahl pathway, rTCA, and their links to biosynthetic systems. The reaction-level expansion shows representative documented pathways; it is not a claim that the book’s nine listed entries constitute the whole network. The THF-dependent Wood–Ljungdahl branch is a bacterial example; archaeal carriers and reactions differ.

Reaction sources and how to read the network

Metabolite boxes name substances. Arrow labels name enzymes and selected energy or electron inputs. Repeated names represent the same metabolite pool. Additional biosynthetic reactions connect precursor pools to the products named in the branch panels. ATP supports reactions; carbon fixation is not itself a universal ATP-generating step.

• The Autotrophic Core (2021): reconstructed network of 404 reactions.
• Experimental evidence for reductive TCA carbon fixation and its key enzyme activities (2005).
• Functional expression of acetyl-CoA synthase and Wood–Ljungdahl pathway context (2018).
• Experimental methyl transfer from methyl-THF to the corrinoid iron–sulfur protein (1999).

Starting inputs: A carbon source (CO₂ or ready-made organic molecules), a supply of electrons and energy, working enzymes, and recycled helper molecules (cofactors).

1. Bring carbon in: A cell gets its carbon in one of two ways. Cells that make their own food (autotrophs) use CO₂ dissolved in water, partly present as bicarbonate; the enzyme carbonic anhydrase speeds up the switch between the two forms. Cells that feed on others (heterotrophs) import ready-made organic molecules through membrane transporters (System 11).
2. Fix the CO₂, using one route: “Fixing” means attaching carbon from CO₂ to other molecules so that it becomes part of living material. The book’s model names two possible routes: the Wood–Ljungdahl pathway, which builds acetyl-CoA from two CO₂ molecules, and the reverse Krebs cycle (rTCA), which runs the familiar Krebs cycle backwards so that it captures CO₂ instead of releasing it. They are alternatives, not two stages of one pathway.
3. Make the carbon hub: Every way in ends in the same small set of central molecules: acetyl-CoA, pyruvate, oxaloacetate, 2-oxoglutarate and sugar phosphates. The most important is acetyl-CoA: a two-carbon unit attached to the carrier molecule coenzyme A (CoA). In the Wood–Ljungdahl route, the CODH/ACS enzyme makes acetyl-CoA by joining a methyl group (one branch) with carbon monoxide made from CO₂ (the other branch).
4. Share the building blocks out: From the hub, carbon flows down several branches at the same time: toward amino acids (mainly from pyruvate, oxaloacetate and 2-oxoglutarate), nucleotides (their ribose sugar comes from sugar phosphates), membrane lipids (fatty chains from acetyl-CoA), and cofactors.
5. Keep it running: These reactions need energy (ATP) and electrons (System 03). They also depend on helpers such as CoA and metal-containing enzyme centers (Systems 02 and 16), which must be recycled or rebuilt. A cyclic route like rTCA must also regenerate its own starting molecule, and the cell adjusts reaction speeds to demand.

Usable outputs: Carbon building blocks for Cofactors (System 02), Nucleotides (System 05), Amino acids (System 06), and Lipids & membrane (System 07).

This is a branching network, not a single line. Fixing CO₂ is one way to obtain carbon: a cell does not need both named routes, and a cell that feeds on organic molecules may not fix CO₂ at all.

Evidence for selected mechanisms

A reconstructed autotrophic biosynthetic network

These papers examine the named mechanisms or networks. The numbered order is an explanatory overview; it is not an experimentally established history of cellular origins.

Turning a carbon source into cellular material

An autotrophic cell obtains much of its carbon from carbon dioxide; a heterotrophic cell imports organic carbon. This hypothetical model explores carbon-fixing options. Carbon dioxide must be reduced and incorporated into molecules that other enzymes can use. That requires a supply of electrons, appropriate catalysts, and reactions whose combined energy balance permits sustained turnover.

Two possible carbon-fixing routes

In the Wood–Ljungdahl pathway, separate branches process two carbon dioxide molecules: one provides a methyl group and another supplies carbon monoxide. Carbon monoxide dehydrogenase/acetyl-CoA synthase, abbreviated CODH/ACS, brings carbon units together with coenzyme A to form acetyl-CoA. The reductive tricarboxylic acid cycle, or rTCA cycle, uses a different reaction network to incorporate carbon dioxide into organic intermediates. These are distinct metabolic options; a short list of shared or distinctive enzymes is not a complete reconstruction of either pathway.

Distributing the products

Acetyl-CoA carries an acetyl group that can enter lipid synthesis and other reactions. Other carbon intermediates provide starting materials for amino acids and sugars. Carbonic anhydrase accelerates the reversible conversion between dissolved carbon dioxide and bicarbonate. It helps make the appropriate inorganic carbon form available; it does not by itself fix carbon into organic matter.

Keeping carbon, electrons, and energy coordinated

A carbon pathway can stop when one required cofactor is missing even if the other enzymes are present. CODH/ACS depends on metal centers, while one-carbon chemistry depends on molecular carriers. The cell must regenerate those carriers and supply reducing power. The useful output is a continuing flow of carbon into biomass, not simply the presence of a detectable product.

FUNCTIONALLY INTEGRATED

What it depends on, and why

A functional dependency means that one process supplies material, energy, machinery, or suitable conditions to another. It does not necessarily mean that two proteins physically touch.

SOURCE-BASED SYNTHESIS

The origin problem in brief

How could carbon fixation, catalyst supply, and usable energy become a self-sustaining network?

• Establish a continuing supply of usable carbon building blocks.
• Connect carbon reactions to usable energy, catalysts, and their cofactors.
• Keep the network supplied and balanced as its products feed other pathways.

ORIGINS

What needs to be explained

An origins explanation must connect available chemistry to a system that can persist, reproduce useful functions, and pass them to descendants. Explaining how a modern system works and explaining how its predecessors arose are different tasks. Simpler catalysts, environmental assistance, and alternative chemistries must be evaluated rather than assuming that the complete modern machinery appeared at once.

Major hurdles to investigate

These are requirements to test in proposed scenarios. Their importance depends on the environment and on which intermediate system is proposed.

1. Carbon supply

   Concentrate a usable carbon source and maintain its delivery without relying on modern transport proteins.

2. Catalysis and selectivity

   Promote productive reactions faster than competing reactions, with catalysts that are available and stable in the same setting.

3. Energy coupling

   Connect an energy source to carbon assimilation rather than assuming that favorable individual reactions automatically form a working pathway.

4. Network closure

   Regenerate carriers and intermediates so that a reaction sequence can continue instead of consuming a finite starting stock.

5. Integration

   Supply carbon products at compatible rates to a compartment with hereditary chemistry and an emerging capacity to replace catalysts.

Condensed origin challenges

• Establish a continuing supply of usable carbon building blocks.
• Connect carbon reactions to usable energy, catalysts, and their cofactors.
• Keep the network supplied and balanced as its products feed other pathways.

Three condensed themes from the source passages above; they are not the complete numbered catalogue.

The integration test

Experiments that connect precursor chemistry across molecular classes help test compatibility.³ A strong next test for this system would combine its relevant chemical steps with continued material supply, energy use, retention, and reproduction under an explicitly specified environmental sequence. The open questions above are research questions, not claims that every intermediate is impossible.

Engineered minimal cells help reveal present cellular dependencies, but their construction starts from existing biological machinery and growth conditions. They therefore answer a different question from a prebiotic synthesis experiment.⁴

TERMS IN PLAIN LANGUAGE

Glossary

carbon fixation

Attaching carbon from CO₂ to other molecules so that it becomes part of living material. Plants do this in photosynthesis; many microbes do it without light.

CO₂ and bicarbonate

Carbon dioxide dissolves in water, and part of it turns into bicarbonate (HCO₃⁻). Different enzymes use one form or the other.

autotroph / heterotroph

An autotroph makes its own organic molecules from CO₂. A heterotroph has to take in organic molecules made by other organisms.

organic molecule

A carbon-based molecule of the kind living things are built from, such as sugars, amino acids, and fats.

metabolite

Any small molecule that is made or used in the cell’s chemical reactions.

central metabolites

The small set of hub molecules, such as acetyl-CoA and pyruvate, from which most other building blocks are made.

precursor

A starting molecule that later reactions turn into something else.

acetyl-CoA

A two-carbon unit (acetyl) attached to coenzyme A. It is the main carbon hub of this system and the starting point for fats and many other molecules.

coenzyme A (CoA)

A helper molecule that carries small carbon groups, such as acetyl, from one enzyme to another. It is made by System 02.

Wood–Ljungdahl pathway

A CO₂-fixing route that builds acetyl-CoA from two CO₂ molecules, using electrons from hydrogen or another donor. It occurs in some bacteria and archaea.

reverse Krebs cycle (rTCA)

The Krebs (citric acid) cycle running backwards, so that it captures CO₂ instead of releasing it. It occurs in some bacteria.

CODH/ACS

Carbon monoxide dehydrogenase / acetyl-CoA synthase: a metal-containing enzyme pair at the heart of the Wood–Ljungdahl pathway. It turns CO₂ into carbon monoxide, then joins it with a methyl group and CoA to make acetyl-CoA.

carbonic anhydrase

An enzyme that speeds up the conversion between CO₂ and bicarbonate.

electron donor

A substance, such as hydrogen gas, that gives up electrons. Fixing CO₂ needs electrons as well as energy.

reducing equivalents

Electrons held by carrier molecules such as NADH or ferredoxin, ready for reactions that need them.

enzyme

A molecule, usually a protein, that speeds up a chemical reaction without being consumed by it.

cofactor

A non-protein chemical helper required by some enzymes, such as an organic molecule or metal ion.

SCIENTIFIC LITERATURE

Scientific papers

Linked superscript numbers in the text lead to the papers below, numbered in order of first citation on this page. Each entry links to the publisher and bibliographic record; open full text is included where available. These studies support the specified mechanisms or experiments, not every possible origin scenario.

1. Wimmer JLE, Vieira ADN, Xavier JC, Kleinermanns K, Martin WF, Preiner M. (2021). The Autotrophic Core: An Ancient Network of 404 Reactions Converts H2, CO2, and NH3 into Amino Acids, Bases, and Cofactors. Microorganisms, 9, 458.

   Read paper / article record ↗

   ↩ Back to text

2. Preiner M, Igarashi K, Muchowska KB, Yu M, Varma SJ, Kleinermanns K, Nobu MK, Kamagata Y, Tüysüz H, Moran J, Martin WF. (2020). A hydrogen-dependent geochemical analogue of primordial carbon and energy metabolism. Nat Ecol Evol, 4, 534-542.

   Read paper / article record ↗

   ↩ Back to text

3. Patel BH, Percivalle C, Ritson DJ, Duffy CD, Sutherland JD. (2015). Common origins of RNA, protein and lipid precursors in a cyanosulfidic protometabolism. Nat Chem, 7, 301-307.

   Read paper / article record ↗

   ↩ Back to text

4. Hutchison CA, Chuang RY, Noskov VN, Assad-Garcia N, Deerinck TJ, Ellisman MH, Gill J, Kannan K, Karas BJ, Ma L, Pelletier JF, Qi ZQ, Richter RA, Strychalski EA, Sun L, Suzuki Y, Tsvetanova B, Wise KS, Smith HO, Glass JI, Merryman C, Gibson DG, Venter JC. (2016). Design and synthesis of a minimal bacterial genome. Science, 351, aad6253.

   Read paper / article record ↗

   ↩ Back to text