Carbon fixation

Short description: Conversion of carbon from CO2 to organic compounds

Cyanobacteria such as these carry out photosynthesis. Their emergence foreshadowed the evolution of many photosynthetic plants, which oxygenated Earth's atmosphere.

Carbon fixation or сarbon assimilation is the process by which inorganic carbon (particularly in the form of carbon dioxide) is converted to organic compounds by living organisms.[1] The compounds are then used to store energy and as structure for other biomolecules. Carbon is primarily fixed through photosynthesis, but some organisms use a process called chemosynthesis in the absence of sunlight.

Organisms that grow by fixing carbon are called autotrophs, which include photoautotrophs (which use sunlight), and lithoautotrophs (which use inorganic oxidation). Heterotrophs are not themselves capable of carbon fixation but are able to grow by consuming the carbon fixed by autotrophs or other heterotrophs. "Fixed carbon", "reduced carbon", and "organic carbon" may all be used interchangeably to refer to various organic compounds.[2]

## Contents

* 1 Net vs. gross CO2 fixation
* 2 Overview of pathways
* 3 Oxygenic photosynthesis
* 3.1 Evolutionary considerations
* 3.2 CO2 concentrating mechanisms
* 3.2.1 CAM plants
* 3.2.2 C4 plants
* 3.2.3 C3 plants
* 3.2.4 Bacteria and cyanobacteria
* 3.2.5 Eukaryotic algae
* 4 Other autotrophic pathways
* 4.1 Reverse Krebs cycle
* 4.2 Reductive acetyl CoA pathway
* 4.3 3-Hydroxypropionate bicycle
* 4.4 Two other cycles related to the 3-hydroxypropionate cycle
* 5 Chemosynthesis
* 6 Non-autotrophic pathways
* 7 Carbon isotope discrimination
* 8 See also
* 9 References
* 10 Further reading

## Net vs. gross CO2 fixation

Graphic showing net annual amounts of CO2 fixation by land and sea-based organisms.

The primary form of inorganic carbon that is fixed is carbon dioxide (CO2). It is estimated that approximately 258 billion tons of carbon dioxide are converted by photosynthesis annually. The majority of the fixation occurs in terrestrial environments, especially the tropics. The gross amount of carbon dioxide fixed is much larger since approximately 40% is consumed by respiration following photosynthesis.[2]

## Overview of pathways

While carbon is primarily fixed through the 6 autotrophic pathways, there are non-autotrophic pathways as well.

Six autotrophic carbon fixation pathways are known as of 2011. The Calvin cycle fixes carbon in the chloroplasts of plants and algae, and in the cyanobacteria. It also fixes carbon in the anoxygenic photosynthesis in one type of proteobacteria called purple bacteria, and in some non-phototrophic proteobacteria.[3]

Of the five other autotrophic pathways, two are known only in bacteria (the reductive citric acid cycle and the 3-hydroxypropionate cycle), two only in archaea (two variants of the 3-hydroxypropionate cycle), and one in both bacteria and archaea (the reductive acetyl CoA pathway).

## Oxygenic photosynthesis

In photosynthesis, energy from sunlight drives the carbon fixation pathway. Oxygenic photosynthesis is used by the primary producers—plants, algae, and cyanobacteria. They contain the pigment chlorophyll, and use the Calvin cycle to fix carbon autotrophically. The process works like this:

2H2O → 4e− \+ 4H+ \+ O2

CO2 \+ 4e− \+ 4H+ → CH2O + H2O

In the first step, water is dissociated into electrons, protons, and free oxygen. This allows the use of water, one of the most abundant substances on Earth, as an electron donor—as a source of reducing power. The release of free oxygen is a side-effect of enormous consequence. The first step uses the energy of sunlight to oxidize water to O2, and, ultimately, to produce ATP

ADP + Pi ⇌ ATP + H2O

and the reductant, NADPH

NADP+ \+ 2e− \+ 2H+ ⇌ NADPH + H+

In the second step, called the Calvin cycle, the actual fixation of carbon dioxide is carried out. This process consumes ATP and NADPH. The Calvin cycle in plants accounts for the preponderance of carbon fixation on land. In algae and cyanobacteria, it accounts for the preponderance of carbon fixation in the oceans. The Calvin cycle converts carbon dioxide into sugar, as triose phosphate (TP), which is glyceraldehyde 3-phosphate (GAP) together with dihydroxyacetone phosphate (DHAP):

3 CO2 \+ 12 e− \+ 12 H+ \+ Pi → TP + 4 H2O

An alternative perspective accounts for NADPH (source of e−) and ATP:

3 CO2 \+ 6 NADPH + 6 H+ \+ 9 ATP + 5 H2O → TP + 6 NADP+ \+ 9 ADP + 8 Pi

The formula for inorganic phosphate (Pi) is HOPO32− \+ 2H+. Formulas for triose and TP are C2H3O2-CH2OH and C2H3O2-CH2OPO32− \+ 2H+

### Evolutionary considerations

Somewhere between 3.8 and 2.3 billion years ago, the ancestors of cyanobacteria evolved oxygenic photosynthesis,[4][5] enabling the use of the abundant yet relatively oxidized molecule H2O as an electron donor to the electron transport chain of light-catalyzed proton-pumping responsible for efficient ATP synthesis.[6][7] When this evolutionary breakthrough occurred, autotrophy (growth using inorganic carbon as the sole carbon source) is believed to have already been developed. However, the proliferation of cyanobacteria, due to their novel ability to exploit water as a source of electrons, radically altered the global environment by oxygenating the atmosphere and by achieving large fluxes of CO2 consumption.[8]

### CO2 concentrating mechanisms

Many photosynthetic organisms have not acquired CO2 concentrating mechanisms (CCMs), which increase the concentration of CO2 available to the initial carboxylase of the Calvin cycle, the enzyme RuBisCO. The benefits of a CCM include increased tolerance to low external concentrations of inorganic carbon, and reduced losses to photorespiration. CCMs can make plants more tolerant of heat and water stress.

CO2 concentrating mechanisms use the enzyme carbonic anhydrase (CA), which catalyze both the dehydration of bicarbonate to CO2 and the hydration of CO2 to bicarbonate

HCO3− \+ H+ ⇌ CO2 \+ H2O

Lipid membranes are much less permeable to bicarbonate than to CO2. To capture inorganic carbon more effectively, some plants have adapted the anaplerotic reactions

HCO3− \+ H+ \+ PEP → OAA + Pi

catalyzed by PEP carboxylase (PEPC), to carboxylate phosphoenolpyruvate (PEP) to oxaloacetate (OAA) which is a C4 dicarboxylic acid.

#### CAM plants

CAM plants that use Crassulacean acid metabolism as an adaptation for arid conditions. CO2 enters through the stomata during the night and is converted into the 4-carbon compound, malic acid, which releases CO2 for use in the Calvin cycle during the day, when the stomata are closed. The dung jade plant (Crassula ovata) and cacti are typical of CAM plants. Sixteen thousand species of plants use CAM.[9] These plants have a carbon isotope signature of −20 to −10 ‰.[10]

#### C4 plants

C4 plants preface the Calvin cycle with reactions that incorporate CO2 into one of the 4-carbon compounds, malic acid or aspartic acid. C4 plants have a distinctive internal leaf anatomy. Tropical grasses, such as sugar cane and maize are C4 plants, but there are many broadleaf plants that are C4. Overall, 7600 species of terrestrial plants use C4 carbon fixation, representing around 3% of all species.[11] These plants have a carbon isotope signature of −16 to −10 ‰.[10]

#### C3 plants

The large majority of plants are C3 plants. They are so-called to distinguish them from the CAM and C4 plants, and because the carboxylation products of the Calvin cycle are 3-carbon compounds. They lack C4 dicarboxylic acid cycles, and therefore have higher CO2 compensation points than CAM or C4 plants. C3 plants have a carbon isotope signature of −24 to −33‰.[10]

#### Bacteria and cyanobacteria

Almost all cyanobacteria and some bacteria utilize carboxysomes to concentrate carbon dioxide.[12] Carboxysomes are protein shells filled with the enzyme RuBisCO and a carbonic anhydrase. The carbonic anhydrase produces CO2 from the bicarbonate that diffuses into the carboxysome. The surrounding shell provides a barrier to carbon dioxide loss, helping to increase its concentration around RuBisCO.

#### Eukaryotic algae

In eukaryotic algae, various bicarbonate transporters and carbonic anhydrases [13] serve to increase the CO2 flux balance toward the pyrenoid, a low CO2-permeable subcellular compartment in the chloroplast containing most of the RuBisCO.[14][15]

## Other autotrophic pathways

### Reverse Krebs cycle

The reverse Krebs cycle, also known as reverse TCA cycle (rTCA) or reductive citric acid cycle, is an alternative to the standard Calvin-Benson cycle for carbon fixation. It has been found in strict anaerobic or microaerobic bacteria (as Aquificales)[16] and anaerobic archea. It was discovered by Evans, Buchanan and Arnon in 1966 working with the photosynthetic green sulfur bacterium Chlorobium limicola.[17] The cycle involves the biosynthesis of acetyl-CoA from two molecules of CO2.[18] The key steps of the reverse Krebs cycle are:

* Oxaloacetate to malate, using NADH \+ H+

Oxaloacetate+NADH/HA+⟶Malate+NADA+

* Fumarate to succinate, catalyzed by an oxidoreductase, Fumarate reductase

Fumarate+FADHA2↽−−⇀Succinate+FAD

* Succinate to succinyl-CoA, an ATP dependent step

Succinate+ATP+CoA⟶Succinyl−CoA+ADP+Pi

* Succinyl-CoA to alpha-ketoglutarate, using one molecule of CO2

Succinyl−CoA+COA2+Fd(red)⟶alpha−ketoglutarate+Fd(ox)

* Alpha-ketoglutarate to isocitrate, using NADPH \+ H+ and another molecule of CO2

Alpha−ketoglutarate+COA2+NAD(P)H/HA+⟶Isocitrate+NAD(P)A+

* Citrate converted into oxaloacetate and acetyl-CoA, this is an ATP dependent step and the key enzyme is the ATP citrate lyase

Citrate+ATP+CoA⟶Oxaloacetate+Acetyl−CoA+ADP+Pi

This pathway is cyclic due to the regeneration of the oxaloacetate.[19]

The reverse Krebs cycle is used by microorganisms in anaerobic environments. In particular, it is one of the most used pathways in hydrothermal vents by the Epsilonproteobacteria.[20] This feature is very important in oceans. Without it, there would be no primary production in aphotic environments, which would lead to habitats without life. So this kind of primary production is called "dark primary production".[21]

One other important aspect is the symbiosis between Gammaproteobacteria and Riftia pachyptila. These bacteria can switch from the Calvin-Benson cycle to the rTCA cycle and vice versa in response to different concentrations of H2S in the environment.[22]

### Reductive acetyl CoA pathway

The reductive acetyl CoA pathway (CoA) pathway, also known as the Wood-Ljungdahl pathway, was discovered by Harland G. Wood and Lars G. Ljungdahl in 1965, thanks to their studies on Clostridium thermoaceticum, a Gram positive bacterium now named Moorella thermoacetica.[23] It is an acetogen, an anaerobic bacteria that uses CO2 as electron acceptor and carbon source, and H2 as an electron donor to form acetic acid.[24][25][26][27] This metabolism is wide spread within the phylum Firmicutes, especially in the Clostridia.[24]

The pathway is also used by methanogens, which are mainly Euryarchaeota, and several anaerobic chemolithoautotrophs, such as sulfate-reducing bacteria and archaea. It is probably performed also by the Brocadiales, an order of Planctomycetes that oxidize ammonia in anaerobic condition.[18][28][29][30][31][32][33] Hydrogenotrophic methanogenesis, which is only found in certain archaea and accounts for 80% of global methanogenesis, is also based on the reductive acetyl CoA pathway.

The Carbon Monoxide Dehydrogenase/Acetyl-CoA Synthase is the oxygen-sensitive enzyme that permits the reduction of CO2 to CO and the synthesis of acetyl-CoA in several reactions.[34]

One branch of this pathway, the methyl branch, is similar but non-homologous between bacteria and archaea. In this branch happens the reduction of CO2 to a methyl residue bound to a cofactor. The intermediates are formate for bacteria and formyl-methanofuran for archaea, and also the carriers, tetrahydrofolate and tetrahydropterins respectively in bacteria and archaea, are different, such as the enzymes forming the cofactor-bound methyl group.[18]

Otherwise, the carbonyl branch is homologous between the two domains and consists of the reduction of another molecule of CO2 to a carbonyl residue bound to an enzyme, catalyzed by the CO dehydrogenase/acetyl-CoA synthase. This key enzyme is also the catalyst for the formation of acetyl-CoA starting from the products of the previous reactions, the methyl and the carbonyl residues.[34][35]

This carbon fixation pathway requires only one molecule of ATP for the production of one molecule of pyruvate, which makes this process one of the main choice for chemolithoautotrophs limited in energy and living in anaerobic conditions.[18]

### 3-Hydroxypropionate bicycle

The 3-Hydroxypropionate bicycle, also known as 3-HP/malyl-CoA cycle, was discovered by Helge Holo in 1989. It's a pathway of carbon fixation and is utilized by green non-sulfur phototrophs of Chloroflexaceae family, including the maximum exponent of this family Chloroflexus auranticus by which this way was discovered and demonstrated.[36]

The 3-Hydroxipropionate bicycle is composed of two cycles and the name of this way comes from the 3-Hydroxyporopionate which corresponds to an intermediate characteristic of it.

The first cycle is a way of synthesis of glycoxilate. During this cycle two bicarbonate molecules are fixed thanks to the action of two enzymes: the Acetyl-CoA carboxylase catalyzes the carboxylation of the Acetyl-CoA to Malonyl-CoA and Propionyl-CoA carboxylase catalyses the carboxylation of Propionyl-CoA to Methylamalonyl-CoA. From this point a series of reactions lead to the formation of glycoxylate which will thus become part of the second cycle.[37][38]

In the second cycle, glycoxilate is approximately one molecule of Propionyl-CoA forming Methylamalonyl-CoA. This, in turn, is then converted through a series of reactions into Citramalyl-CoA. The Citramalyl-CoA is split into pyruvate and Acetyl-CoA thanks to the enzyme MMC lyase. At this point the pyruvate is released, while the Acetyl-CoA is reused and carboxylated again at Malonyl-coa thus reconstituting the cycle.[39]

19 are the total reactions involved in 3-Hydroxypropionate bicycle and 13 are the multifunctional enzymes used. The multifunctionality of these enzymes is an important feature of this pathway which thus allows the fixation of 3 bicarbonate molecules.[39]

It is a very expensive way: 7 ATP molecules are used for the synthesis of the new pyruvate and 3 ATP for the phosphate triose.[38]

An important characteristic of this cycle is that it allows the co-assimilation of numerous compounds making it suitable for the mixotrophic organisms.[38]

### Two other cycles related to the 3-hydroxypropionate cycle

A variant of the 3-hydroxypropionate cycle was found to operate in the aerobic extreme thermoacidophile archaeon Metallosphaera sedula. This pathway is called the 3-hydroxypropionate/4-hydroxybutyrate cycle.[40]

Yet another variant of the 3-hydroxypropionate cycle is the dicarboxylate/4-hydroxybutyrate cycle. It was discovered in anaerobic archaea. It was proposed in 2008 for the hyperthermophile archeon Ignicoccus hospitalis.[41]

## Chemosynthesis

Chemosynthesis is carbon fixation driven by energy obtained by oxidating inorganic substances (e.g., hydrogen gas or hydrogen sulfide), rather than from sunlight. Sulfur- and hydrogen-oxidizing bacteria often use the Calvin cycle or the reductive citric acid cycle.[42]

## Non-autotrophic pathways

Although almost all heterotrophs cannot synthesize complete organic molecules from carbon dioxide, some carbon dioxide is incorporated in their metabolism.[43] Notably pyruvate carboxylase consumes carbon dioxide (as bicarbonate ions) as part of gluconeogenesis, and carbon dioxide is consumed in various anaplerotic reactions. Recently, also 6-phosphogluconate dehydrogenase was shown to catalyze the reductive carboxylation of ribulose 5-phosphate to 6-phosphogluconate in E. coli under elevated CO2 concentrations.[44] Considering the CO2 concentration in the habitat of E. coli (e.g. the mammalian gut[45]), this reaction might happen also naturally. In the future, this property could be exploited for the design of synthetic carbon fixation routes.

## Carbon isotope discrimination

Some carboxylases, particularly RuBisCO, preferentially bind the lighter carbon stable isotope carbon-12 over the heavier carbon-13. This is known as carbon isotope discrimination and results in carbon-12 to carbon-13 ratios in the plant that are higher than in the free air. Measurement of this ratio is important in the evaluation of water use efficiency in plants,[46][47][48] and also in assessing the possible or likely sources of carbon in global carbon cycle studies.

## See also

* Nitrogen fixation
* Oxygen cycle

## References

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41. ↑ "A dicarboxylate/4-hydroxybutyrate autotrophic carbon assimilation cycle in the hyperthermophilic Archaeum Ignicoccus hospitalis". Proceedings of the National Academy of Sciences of the United States of America 105 (22): 7851–6. June 2008. doi:10.1073/pnas.0801043105. PMID 18511565. Bibcode: 2008PNAS..105.7851H.
42. ↑ Encyclopedia of Microbiology. Academic Press. 2009. pp. 83–84. ISBN 9780123739445. https://books.google.com/books?id=rLhdW5YzuO4C&q=chemosynthesis+carbon+fixation&pg=RA2-PA83.
43. ↑ Nicole Kresge; Robert D. Simoni; Robert L. Hill (2005). "The Discovery of Heterotrophic Carbon Dioxide Fixation by Harland G. Wood". The Journal of Biological Chemistry 280 (18): e15. http://www.jbc.org/content/280/18/e15.full.
44. ↑ "Awakening a latent carbon fixation cycle in Escherichia coli". Nature Communications 11 (1): 5812. November 2020. doi:10.1038/s41467-020-19564-5. PMID 33199707. Bibcode: 2020NatCo..11.5812S.
45. ↑ "Volume and composition of human intestinal gas determined by means of an intestinal washout technic". The New England Journal of Medicine 284 (25): 1394–8. June 1971. doi:10.1056/nejm197106242842502. PMID 5578321. http://dx.doi.org/10.1056/nejm197106242842502.
46. ↑ "Genetic control of water use efficiency and leaf carbon isotope discrimination in sunflower (Helianthus annuus L.) subjected to two drought scenarios". PLOS ONE 9 (7): e101218. 3 July 2014. doi:10.1371/journal.pone.0101218. PMID 24992022. Bibcode: 2014PLoSO...9j1218A.
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48. ↑ "Carbon isotopes and water use efficiency: sense and sensitivity". Oecologia 155 (3): 441–54. March 2008. doi:10.1007/s00442-007-0932-7. PMID 18224341. Bibcode: 2008Oecol.155..441S.

## Further reading

* "Ecological aspects of the distribution of different autotrophic CO2 fixation pathways". Applied and Environmental Microbiology 77 (6): 1925–36. March 2011. doi:10.1128/AEM.02473-10. PMID 21216907. Bibcode: 2011ApEnM..77.1925B.
* "Diversity and evolutionary history of plastids and their hosts". American Journal of Botany 91 (10): 1481–93. October 2004. doi:10.3732/ajb.91.10.1481. PMID 21652304.
* "Chromalveolates and the evolution of plastids by secondary endosymbiosis". The Journal of Eukaryotic Microbiology 56 (1): 1–8. 2009. doi:10.1111/j.1550-7408.2008.00371.x. PMID 19335769. http://www.botany.ubc.ca/keeling/PDF/09KeelingTCS.pdf.
* "The endosymbiotic origin, diversification and fate of plastids". Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences 365 (1541): 729–48. March 2010. doi:10.1098/rstb.2009.0103. PMID 20124341.
* "Broad phylogenomic sampling and the sister lineage of land plants". PLOS ONE 7 (1): e29696. 2012. doi:10.1371/journal.pone.0029696. PMID 22253761. Bibcode: 2012PLoSO...7E9696T.
* "Evolution. Contemplating the first Plantae". Science 335 (6070): 809–10. February 2012. doi:10.1126/science.1218515. PMID 22344435. Bibcode: 2012Sci...335..809S.
* "Cyanophora paradoxa genome elucidates origin of photosynthesis in algae and plants". Science 335 (6070): 843–7. February 2012. doi:10.1126/science.1213561. PMID 22344442. Bibcode: 2012Sci...335..843P. http://cels.uri.edu/bio/lanelab/docs/Nic docs/PriceEtAl2012.pdf.

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Metabolism, catabolism, anabolism

General|

* Metabolic pathway
* Metabolic network
* Primary nutritional groups

Energy
metabolism|

| Aerobic respiration|

* Glycolysis → Pyruvate decarboxylation → Citric acid cycle → Oxidative phosphorylation (electron transport chain \+ ATP synthase)

|

Anaerobic respiration|

* Electron acceptors are other than oxygen

Fermentation|

* Glycolysis → Substrate-level phosphorylation
* ABE
* Ethanol
* Lactic acid

Specific
paths|

| Protein metabolism|

* Protein synthesis
* Catabolism

|

Carbohydrate metabolism
(carbohydrate catabolism
and anabolism)|

| Human|

* Glycolysis ⇄ Gluconeogenesis

* Glycogenolysis ⇄ Glycogenesis

* Pentose phosphate pathway
* Fructolysis
* Galactolysis

* Glycosylation
* N-linked
* O-linked

Nonhuman|

* Photosynthesis
* Anoxygenic photosynthesis
* Chemosynthesis
* Carbon fixation

* Xylose metabolism
* Radiotrophism

Lipid metabolism
(lipolysis, lipogenesis)|

| Fatty acid metabolism|

* Fatty acid degradation (Beta oxidation)
* Fatty acid synthesis

|

Other|

* Steroid metabolism
* Sphingolipid metabolism
* Eicosanoid metabolism
* Ketosis
* Reverse cholesterol transport

Amino acid|

* Amino acid synthesis
* Urea cycle

Nucleotide
metabolism|

* Purine metabolism
* Nucleotide salvage
* Pyrimidine metabolism

Other|

* Metal metabolism
* Iron metabolism
* Ethanol metabolism

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* t
* e

Metabolism map

#bc00bc;">Carbon
fixation

#5400a9;">Photo-
respiration

#bc00bc;">Pentose
phosphate
pathway

#edb500;">Citric
acid cycle

#edb500;">Glyoxylate
cycle

#89dd00;">Urea
cycle

#00d300;">Fatty
acid
synthesis

#00d300;">Fatty
acid
elongation

#ff66cc;">Beta
oxidation

#ff66cc;">Peroxisomal

#ff66cc;">beta
oxidation

#8a2c2c;">Glyco-
genolysis

#2e427e;">Glyco-
genesis

#ea0000;">Glyco-
lysis

#0033cc;">Gluconeo-
genesis

#ea6000;">Pyruvate
decarb-
oxylation

#d47ff0;">Fermentation

#e06c1f;">Keto-
lysis

#5f8216;">Keto-
genesis

#0033cc;">feeders to
gluconeo-
genesis

#9373d3;">Direct / C4 / CAM
carbon intake

#9373d3;">Light reaction

#c34747;">Oxidative
phosphorylation

#4999c2;">Amino acid
deamination

#00d300;">Citrate
shuttle

#00d300;">Lipogenesis

#ff66cc;">Lipolysis

#0099cc;">Steroidogenesis

#0099cc;">MVA pathway

#af753a;">MEP pathway

#00cc99;">Shikimate
pathway

Transcription &
#79793d;">replication

#79793d;">Translation

#6666cc;">Proteolysis

#79793d;">Glycosyl-
ation

Sugar
acids

Double/multiple
sugars & glycans

Simple
sugars

Inositol-P

Amino sugars
& sialic acids

Nucleotide sugars

Hexose-P

Triose-P

Glycerol

P-glycerates

Pentose-P

Tetrose-P

Propionyl
-CoA

Succinate

Acetyl
-CoA

Pentose-P

P-glycerates

Glyoxylate

Photosystems

Pyruvate

Lactate

Acetyl
-CoA

Citrate

Oxalo-
acetate

Malate

Succinyl
-CoA

α-Keto-
glutarate

Ketone
bodies

Respiratory
chain

Serine group

Alanine

Branched-chain
amino acids

Aspartate
group

Homoserine
group
& lysine

Glutamate
group
& proline

Arginine

Creatine
& polyamines

Ketogenic &
glucogenic
amino acids

Amino acids

Shikimate

Aromatic amino
acids & histidine

Ascorbate
(vitamin C)

δ-ALA

Bile
pigments

Hemes

Cobalamins (vitamin B12)

Various
vitamin Bs

Calciferols
(vitamin D)

Retinoids
(vitamin A)

Quinones (vitamin K)
& carotenoids (vitamin E)

Cofactors

Vitamins
& minerals

Antioxidants

PRPP

Nucleotides

Nucleic
acids

Proteins

Glycoproteins
& proteoglycans

Chlorophylls

MEP

MVA

Acetyl
-CoA

Polyketides

Terpenoid
backbones

Terpenoids
& carotenoids (vitamin A)

Cholesterol

Bile acids

Glycero-
phospholipids

Glycerolipids

Acyl-CoA

Fatty
acids

Glyco-
sphingolipids

Sphingolipids

Waxes

Polyunsaturated
fatty acids

Neurotransmitters
& thyroid hormones

Steroids

Endo-
cannabinoids

Eicosanoids

Major metabolic pathways in metro-style map. Click any text (name of pathway or metabolites) to link to the corresponding article.
Single lines: pathways common to most lifeforms. Double lines: pathways not in humans (occurs in e.g. plants, fungi, prokaryotes).  Orange nodes: carbohydrate metabolism. Violet nodes: photosynthesis. Red nodes: cellular respiration. Pink nodes: cell signaling. Blue nodes: amino acid metabolism. Grey nodes: vitamin and cofactor metabolism. Brown nodes: nucleotide and protein metabolism. Green nodes: lipid metabolism.

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