CENTRAL CARBON → AROMATIC AMINO ACIDS → INDIVIDUAL PATHWAY

Tryptophan biosynthesis

Follow the carbon skeleton from Chorismate to L-Tryptophan.

Editorial reference: Otangelo Grasso, X-Ray of Life, Vol. II, Chapter 6 (Amino Acid Biosynthesis), section 6.13 “Tryptophan”, pp. 273–278.

What it needs

Chorismate, glutamine, PRPP and serine; upstream shikimate metabolism supplies chorismate.

What it supplies

L-Tryptophan. This product can enter downstream cellular reactions; its synthesis depends on the precursor and helper supplies shown.

The pathway

Read from top to bottom. Select an enzyme box to jump to its reaction. Cosubstrates and selected coproducts appear in the account below.

The diagram is the existing published figure for this route and is reproduced unchanged. The sections below add the X-Ray of Life account, the enzyme machinery, the origin-of-life questions the book raises, and a check of every enzyme classification against public databases.

What the source says about this pathway

Section 6.13 opens by noting that tryptophan is one of the twenty standard amino acids, that it is a precursor of serotonin and melatonin, and that the pathway is present across bacteria, archaea and eukaryotes. The author describes tryptophan biosynthesis as “the most complex amino acid synthesis pathway” and then devotes most of the section to one enzyme complex.

The book’s biochemical account follows the same canonical route used on this page: chorismate is converted to anthranilate, anthranilate is attached to PRPP, the resulting ribose derivative is rearranged and closed into the indole ring, and the indole is finally combined with serine. Section 6.13.3 lists five enzymes (counting tryptophan synthase as one enzyme with two subunits) and gives, for each, the amino-acid length of the smallest version the author found, together with its metal or cofactor requirement.

The chapter’s centrepiece is section 6.13.1, which treats the tryptophan synthase complex as a test case for prebiotic explanations. It describes the α2β2 (αββα) hetero-tetramer, the 25 Å hydrophobic tunnel that carries indole from the α active site to the β active site, the allosteric communication between subunits (including the COMM domain of the β subunit), the catalytic efficiency of the complex, and its thermal stability. The author argues that these features are difficult to reconcile with a gradual, stepwise origin.

Section 6.13.2 then sets out eight “key questions” that an origin-of-life account would have to answer, and the pages that follow list fourteen numbered “unresolved dilemmas”, each followed by a “conceptual problem” statement. These are reproduced, summarised and labelled in the origin-of-life section below.

Crystal structure of the tryptophan synthase α2β2 complex: α subunits in green, β subunits in blue, ligands and cofactors as spheres, and the indole channel shown as an orange mesh between the α and β active sites. — Crystal structure of the tryptophan synthase complex, reproduced from *X-Ray of Life*, Vol. II, p. 274. α subunits are green, β subunits blue; subunits are shown as cartoons and ligands/cofactors as spheres. Glycerol 3-phosphate is bound at α and the cofactor PLP at β. The putative indole channel between the α and β active sites was visualised with MOLE as an orange mesh. Scientific content preserved as published.
Source: X-Ray of Life, Vol. II, p. 274 (figure caption as printed in the book).

The five enzymes as listed in the book (section 6.13.3) — checked

Enzyme (as named in the book)	EC: book → verified	Length: book → UniProt	Cofactor (book)	Verdict
Chorismate pyruvate-lyase (anthranilate synthase)	4.2.99.21 → 4.1.3.27	159 aa (E. coli) → 520 aa P00895 · trpE, E. coli K-12	None reported	differs
Anthranilate phosphoribosyltransferase	2.4.2.18 → 2.4.2.18	340 aa (M. tuberculosis) → 370 aa P9WFX5 · trpD, M. tuberculosis H37Rv	Mg²⁺ (book); not listed in the IUBMB entry	differs
Phosphoribosylanthranilate isomerase	5.3.1.24 → 5.3.1.24	198 aa (T. maritima) → 205 aa Q56320 · trpF, T. maritima MSB8	None reported	differs
Indole-3-glycerol-phosphate synthase	4.1.1.48 → 4.1.1.48	248 aa (S. solfataricus) → 248 aa Q06121 · trpC, S. solfataricus	None reported	matches
Tryptophan synthase α chain	4.2.1.20 → 4.2.1.20	248 aa (P. furiosus) → 248 aa Q8U094 · trpA, P. furiosus	—	matches
Tryptophan synthase β chain	4.2.1.20 → 4.2.1.20	397 aa (P. furiosus) → 388 aa Q8U093 · trpB1, P. furiosus	PLP in the β subunit	differs

The book gives, for each enzyme, the amino-acid length of the smallest version it found, and its cofactor. Both were re-checked for this page: EC numbers against KEGG/IUBMB, lengths against the UniProt record for the organism the book names. Three lengths and one EC number do not match. The page shows the book's figure next to the verified one and uses the verified value. With the verified lengths the total is 1,979 aa, not the 1,590 aa stated in the book.

Also discussed in Volume III

Volume III does not describe the pathway, but it uses tryptophan twice in its argument about the genetic code. Both statements are statistics of the code, not of biosynthesis, so they are recorded here and nowhere else on the page.

Tryptophan is the largest of the twenty standard amino acids (204 Da); glycine is the smallest (75 Da). The book uses that pair to describe the optimal size spacing of the standard set (Vol. III, pp. 27–28).
In the information analysis of the code, tryptophan is assigned a single codon (UGG), which gives I(Trp) = 5.93 bits per residue — the highest value of the twenty, alongside methionine (Vol. III, p. 54).
These figures describe codon degeneracy and molecular size. They say nothing about how the pathway is regulated or how it was assembled, and they are not evidence either way for the origin-of-life argument.

Reactions and enzymes

1 · Anthranilate synthase (TrpE/TrpG)

Chorismate → Anthranilate

Anthranilate synthase (TrpE/TrpG) · EC 4.1.3.27

Inputs, helpers or coproducts: Glutamine supplies nitrogen; pyruvate is released

More on this enzyme — sources and databases

The overall reaction is chorismate + L-glutamine → anthranilate + pyruvate + L-glutamate (KEGG R00986). It runs in three parts: glutamine is hydrolysed to glutamate and ammonia; ammonia is added to chorismate to give (2S)-2-amino-4-deoxychorismate; that intermediate is then eliminated to anthranilate and pyruvate.

The enzyme is a two-component amidotransferase. TrpE carries the lyase active site and TrpG the glutaminase site; ammonia is channelled between them. In some organisms the two activities are fused into one polypeptide.

Anthranilate synthase is the regulated step: tryptophan feedback-inhibits it, and in E. coli the enzyme is also inhibited by uncharged tRNA-Trp.

The book names this activity “chorismate pyruvate-lyase” and cites EC 4.2.99.21. That EC number belongs to isochorismate lyase (isochorismate → salicylate + pyruvate); the reaction described is EC 4.1.3.27. See the evidence table.

Naming note: the activity is anthranilate synthase. “Chorismate lyase”, without the “amino-accepting” qualifier, is EC 4.1.3.40 and produces 4-hydroxybenzoate, not anthranilate.

2 · Anthranilate phosphoribosyltransferase (TrpD)

Anthranilate → N-(5-Phosphoribosyl)anthranilate

Anthranilate phosphoribosyltransferase (TrpD) · EC 2.4.2.18

Inputs, helpers or coproducts: PRPP → PPᵢ

More on this enzyme — sources and databases

Phosphoribosyl diphosphate (PRPP) donates the ribose-5-phosphate group and inorganic pyrophosphate is released. IUBMB writes the reaction in the reverse direction: N-(5-phospho-β-D-ribosyl)anthranilate + diphosphate = 5-phospho-α-D-ribose 1-diphosphate + anthranilate.

It is a nucleophilic substitution at C1 of PRPP, so the anomeric configuration is fixed here and constrains everything downstream.

PRPP is a costly metabolite shared with purine, pyrimidine and histidine biosynthesis; this step ties tryptophan synthesis to central carbon and phosphate supply.

The book states that the enzyme requires Mg²⁺; the IUBMB entry does not list a metal, so this is reported as the book’s statement rather than a database fact.

3 · Phosphoribosylanthranilate isomerase (TrpF)

N-(5-Phosphoribosyl)anthranilate → 1-(o-Carboxyphenylamino)-1-deoxyribulose-5-P

Phosphoribosylanthranilate isomerase (TrpF) · EC 5.3.1.24

More on this enzyme — sources and databases

This is an Amadori rearrangement: the ribose ring opens and re-closes as a ribulose (ketose) derivative, moving the carbonyl and producing the substrate for ring closure.

TrpF is a (β/α)8 barrel enzyme and a classic model for protein folding; it shares its fold with HisA of histidine biosynthesis.

In many bacteria and fungi TrpF is fused to TrpC as a bifunctional protein; in others the two are separate.

4 · Indole-3-glycerol phosphate synthase (TrpC)

1-(o-Carboxyphenylamino)-1-deoxyribulose-5-P → Indole-3-glycerol phosphate

Indole-3-glycerol phosphate synthase (TrpC) · EC 4.1.1.48

Inputs, helpers or coproducts: CO₂ and water are released

More on this enzyme — sources and databases

Decarboxylation is followed by ring closure to form the indole ring. IUBMB: 1-(2-carboxyphenylamino)-1-deoxy-D-ribulose 5-phosphate + H⁺ = (1S,2R)-1-C-(indol-3-yl)glycerol 3-phosphate + CO₂ + H₂O.

The chemical difficulty is that the substrate must be de-aromatised and re-aromatised in the right order; the enzyme is a (β/α)8 barrel with conserved catalytic glutamate and lysine residues.

The product, indole-3-glycerol phosphate (IGP), is the intermediate that tryptophan synthase consumes.

5 · Tryptophan synthase α (TrpA)

Indole-3-glycerol phosphate → Indole

Tryptophan synthase α (TrpA) · part of 4.2.1.20

Inputs, helpers or coproducts: Glyceraldehyde-3-phosphate is released

More on this enzyme — sources and databases

The α subunit catalyses a retro-aldol cleavage of IGP to indole and D-glyceraldehyde 3-phosphate. This half-reaction was formerly its own entry, EC 4.1.2.8; IUBMB now folds it into EC 4.2.1.20.

Free indole is hydrophobic and reactive and is lost by diffusion at high temperature. In the complex it does not diffuse: it is channelled through the 25 Å tunnel to the β active site (see the machinery section).

The α active site sits at the end of a (β/α)8 barrel; a conserved glutamate positions the substrate for cleavage.

6 · Tryptophan synthase β (TrpB)

Indole + L-serine → L-Tryptophan

Tryptophan synthase β (TrpB) · EC 4.2.1.20

Inputs, helpers or coproducts: PLP-dependent; water is released

More on this enzyme — sources and databases

The β subunit is a pyridoxal 5′-phosphate (PLP) enzyme. PLP forms an internal aldimine with an active-site lysine (Lys87 in the E. coli enzyme); L-serine displaces it, water is eliminated to give an aminoacrylate, indole attacks that intermediate, and hydrolysis releases L-tryptophan and regenerates PLP.

The overall IUBMB reaction combines both half-reactions: (1S,2R)-1-C-(indol-3-yl)glycerol 3-phosphate + L-serine = D-glyceraldehyde 3-phosphate + L-tryptophan + H₂O.

PLP chemistry is shared with many other enzymes, so the cofactor is not unique to this pathway — but a channel physically joining the two active sites is.

Molecular machinery: the tryptophan synthase complex

Both halves of the last reaction are catalysed by one α2β2 complex. The α subunit makes indole; the β subunit consumes it. Between them lies a hydrophobic tunnel about 25 Å long that the book describes on p. 273 and illustrates on p. 274. Everything in this section is documented structural enzymology; the origin-of-life reading of it is treated separately below.

α subunit (TrpA)

Catalyses the retro-aldol cleavage of indole-3-glycerol phosphate to indole + D-glyceraldehyde 3-phosphate.
Half-reaction formerly EC 4.1.2.8; now part of EC 4.2.1.20.
A (β/α)8 barrel; indole leaves through a hydrophobic tunnel rather than into solvent.

β subunit (TrpB)

PLP-dependent: converts indole + L-serine to L-tryptophan + H₂O.
PLP is bound as an internal aldimine to an active-site lysine.
The indole arrives from the α subunit through the tunnel and is used immediately.

α2β2 quaternary structure

The functional complex is a hetero-tetramer, αββα, with defined interfaces — not a random aggregate.
The interfaces create the tunnel and the communication path between active sites.

The indole channel (~25 Å)

A hydrophobic channel connects the α and β active sites.
Channeling protects reactive indole from solvent and from competing reactions, and prevents its loss at high temperature.
Visualised in the book figure with MOLE (Sehnal et al., 2013) as an orange mesh.

Allostery and the COMM domain

The subunits are allosterically coupled: ligand binding at one site changes the other's activity.
The COMM domain of the β subunit sits at the α/β interface and relays the conformational signal.
This couples indole production to indole consumption.

Why a complex instead of two enzymes

Separate α and β enzymes lose free indole by diffusion; the complex raises throughput.
The reaction is a three-step catalytic sequence (cleavage, channeling, condensation) coordinated in space.

Variation across organisms

In many bacteria the trp operon encodes separate subunits; in some fungi the activities are fused.
In thermophiles a β-like enzyme (EC 4.2.1.122) can work without forming a complex with the α subunit — a documented exception.

What the structure does not show

A crystal structure is a snapshot: the book's image shows bound glycerol 3-phosphate and PLP, not a time-resolved catalytic trajectory.
The channel is inferred from structure and modelling, not observed directly in the figure.

Complex: α2β2 hetero-tetramer (αββα)
Enzyme class: EC 4.2.1.20 · lyase
Cofactor: Pyridoxal 5′-phosphate (PLP), one per β subunit
Tunnel: ≈25 Å hydrophobic channel between the α and β active sites
Overall reaction: IGP + L-serine → D-glyceraldehyde 3-phosphate + L-tryptophan + H₂O
Book treatment: X-Ray of Life Vol. II, §6.13.1–6.13.2, pp. 273–274

Origin-of-life questions raised in the book

The book does not present the pathway as unexplained chemistry. It accepts the chemistry and then asks how an unguided process could have produced the machinery that performs it. Section 6.13.2 (p. 274) lists eight questions; the following pages (pp. 275–278) list fourteen numbered dilemmas, each closing with a “conceptual problem”. They are summarised here with the page where each appears. Nothing in this section is a biochemical finding — it is the author's argument, and it is labelled as such.

Was tryptophan available before life?

The pathway question is preceded by an availability question: even if the enzymes existed, was there tryptophan — or its precursors — in the environment? The other volumes in the collection gather the experimental record on this point, and it is the strongest empirical part of the argument.

X-Ray of Life, Vol. I, p. 81 Open question
Eight of the twenty proteinogenic amino acids have never been synthesised under plausible prebiotic conditions — tryptophan among them, together with arginine, lysine, histidine, methionine, asparagine, glutamine and phenylalanine.
X-Ray of Life, Vol. I, p. 230 Established
Miller–Urey type experiments do yield tryptophan, but at very low concentration: 0.05 mg/L in the 2008 reanalysis of the original experiment.
X-Ray of Life, Vol. I, p. 84; The Cell Unsolved Mystery, p. 69 Established
Tryptophan is unstable in water: at 25 °C and neutral pH its half-life is about 74 days (aspartate, by contrast, about 253 days), and under a day at 100 °C.
The RNA-DNA Nexus, pp. 170, 191, 193 Established
Tryptophan synthesis depends on PRPP — the same precursor that feeds purine and pyrimidine nucleotides and histidine. One molecule, four biosynthetic routes, all interdependent.
The RNA-DNA Nexus, pp. 260–261 Interpretation
The RNA-DNA Nexus states that tryptophan “has never been found as a standalone molecule in nature outside of being synthesised by living cells”, and calls its pathway one of the most complex among the amino acids.

None of this shows that prebiotic tryptophan was impossible. It records that no high-yield, generally accepted prebiotic route is known — which is the state of the field these books describe, and the reason the availability question stays open.

Eight questions the book asks (p. 274)

Bifunctional design Open question
How did a complex of two distinct enzymes (α and β subunits) arise to make tryptophan more efficiently together than apart?
X-Ray of Life, p. 274
Channeling mechanism Open question
How did a precise ~25 Å tunnel arise to guide the unstable indole intermediate between the two active sites?
X-Ray of Life, p. 274
Protection of intermediates Open question
How did the complex come to shield reactive indole from the cellular environment?
X-Ray of Life, p. 274
Allosteric regulation Open question
How did inter-subunit communication for fine control emerge?
X-Ray of Life, p. 274
Conformational changes Open question
How did structural shifts that open and close the tunnel at the right moments arise?
X-Ray of Life, p. 274
Synchronisation Open question
How did the two active sites develop a coordinated workflow, timing indole production to its use?
X-Ray of Life, p. 274
Optimisation Open question
How did the complex reach catalytic efficiency and specificity beyond what separate enzymes provide?
X-Ray of Life, p. 274
Nanoscale precision Open question
How did molecular machinery operating with ångström-level positioning arise?
X-Ray of Life, p. 274

Fourteen numbered dilemmas (pp. 275-278)

1. Enzyme complexity and specificity Author argument
Enzymes must distinguish closely related substrates; the book uses chorismate mutase as its example.
X-Ray of Life, p. 275
2. Multi-step pathway coordination Author argument
Each step needs the product of the previous one, so the whole route must work to deliver tryptophan.
X-Ray of Life, p. 276
3. Cofactor requirements Author argument
PRPP and other complex cofactors must be available and correctly integrated.
X-Ray of Life, p. 276
4. Stereochemical precision Author argument
Strict stereochemical control, illustrated with PRA isomerase (EC 5.3.1.24).
X-Ray of Life, p. 276
5. Regulatory mechanisms Author argument
Feedback inhibition of anthranilate synthase by tryptophan must itself have arisen.
X-Ray of Life, p. 276
6. Thermodynamic considerations Author argument
Steps such as chorismate → anthranilate need energy input; energy coupling is said to be unexplained prebiotically.
X-Ray of Life, pp. 276–277
7. Substrate channeling Author argument
Intermediates pass directly between enzymes without diffusing; the spatial organisation behind this is the question.
X-Ray of Life, p. 277
8. Catalytic efficiency Author argument
Highly optimised catalysis is treated as hard to reach without design.
X-Ray of Life, p. 277
9. Molecular recognition Author argument
Each enzyme must specifically recognise its substrate and cofactors.
X-Ray of Life, p. 277
10. Enzyme subunit coordination Author argument
Tryptophan synthase's two subunits must coordinate; the origin of coordinated multi-subunit enzymes is questioned.
X-Ray of Life, pp. 277–278
11. Precursor availability Author argument
Chorismate and serine must have been available in sufficient quantity at the same time and place.
X-Ray of Life, p. 278
12. Pathway branching Author argument
The route shares early steps with phenylalanine and tyrosine; branched networks are said to be unexplained.
X-Ray of Life, p. 278
13. Enzyme promiscuity and specificity Author argument
The transition from promiscuous to highly specific enzymes is presented as problematic.
X-Ray of Life, p. 278
14. Metabolic network emergence Author argument
The wider question of how branched metabolic networks arise unguided.
X-Ray of Life, p. 278

How to read the list above. The book's conclusion (p. 273) is that the complex's structural integrity, allosteric properties, catalytic efficiency, thermal stability and integration into a wider pathway “seem to defy explanation by undirected, naturalistic processes”. That is an inference drawn from the biochemistry, not a result of it: the structures and reactions summarised above are measured, the inference about their history is not.

Established biochemistry, corrections, and interpretation

Each statement on this page is separated by status. “Established” means it follows from the cited reaction or database entry. “Open question” means the science itself has no settled answer. “Interpretation” means it is the author's argument about origins, not a measurement. Every EC number below was checked against KEGG and the ExPASy/IUBMB ENZYME database.

Statement	Status	Basis
Chorismate is converted to anthranilate by anthranilate synthase (TrpE + TrpG), with glutamine as nitrogen donor.	Established	EC 4.1.3.27; KEGG reaction R00986; overall: chorismate + L-glutamine = anthranilate + pyruvate + L-glutamate.
The book cites EC 4.2.99.21 for this step.	Correction	EC 4.2.99.21 is isochorismate lyase (isochorismate = salicylate + pyruvate) according to the ExPASy/IUBMB entry. The activity described is EC 4.1.3.27. This page uses 4.1.3.27.
The pathway uses five enzymes, with tryptophan synthase counted as one enzyme of two subunits.	Established	Consistent with IUBMB: TrpA and TrpB activities are both within EC 4.2.1.20.
Anthranilate + PRPP → N-(5-phosphoribosyl)anthranilate + PPᵢ.	Established	EC 2.4.2.18. The IUBMB entry does not list a metal cofactor; the book's Mg²⁺ statement is reported as the book's, not as an IUBMB fact.
TrpF performs an Amadori rearrangement of the ribose moiety.	Established	EC 5.3.1.24: N-(5-phospho-β-D-ribosyl)anthranilate = 1-(2-carboxyphenylamino)-1-deoxy-D-ribulose 5-phosphate.
TrpC closes the indole ring and releases CO₂ and water.	Established	EC 4.1.1.48.
The α-subunit half-reaction was formerly a separate entry, EC 4.1.2.8.	Established	Stated in the IUBMB comment for EC 4.2.1.20.
TrpA and TrpB form an α2β2 (αββα) complex with a ~25 Å hydrophobic indole channel and allosteric coupling (COMM domain).	Established	Documented structural enzymology; the book's figure (p. 274) shows the channel as a MOLE mesh.
Tryptophan synthase is the most complex amino-acid biosynthetic pathway.	Interpretation	The author's qualitative assessment. It depends on the chosen measure (number of steps, subunits, cofactors) and is not a database fact.
The complex and its tunnel cannot be reached by undirected processes.	Interpretation	The book's central argument (pp. 273–274). It is an inference about history, not a result of the structural or enzymatic data.
Enzymes must have been fully efficient from the outset, because partial intermediates would not be retained.	Open question	A claim about how the pathway first arose; not directly testable with present-day structures.
Chorismate, PRPP and serine were not reliably available in prebiotic settings.	Open question	Genuine open research area in prebiotic chemistry; the book treats it as a constraint, and the difficulties it lists are real but not settled.
The pathway's presence across bacteria, archaea and eukaryotes shows it was present when life began.	Interpretation	A common inference from phylogenetic distribution; it assumes vertical inheritance without extensive lateral transfer, which is debated.
The smallest versions of the five enzymes total 1,590 amino acids.	Correction	Correction: the figure is the sum of the book's own lengths (159+340+198+248+248+397). With the UniProt lengths for the named organisms it is 1,979 aa (520+370+205+248+248+388). The arithmetic inside the book is correct; the input lengths are not.
The first enzyme is called “chorismate pyruvate-lyase”.	Correction	Ambiguous name: unqualified “chorismate lyase” is EC 4.1.3.40, which makes 4-hydroxybenzoate + pyruvate (ubiquinone biosynthesis). The IUBMB accepted name for the anthranilate-forming activity is anthranilate synthase (EC 4.1.3.27); KEGG lists “chorismate pyruvate-lyase (amino-accepting)” only as a synonym.
Smallest known TrpB in P. furiosus is 397 aa.	Correction	UniProt Q8U093 (trpB1, the subunit that pairs with TrpA) is 388 aa. P. furiosus also carries trpB2 (Q8U0J5, 446 aa), a standalone β-like enzyme (EC 4.2.1.122) that does not form the α2β2 complex — so neither paralogue is 397 aa.
Tryptophan has never been found as a free molecule in nature outside living cells.	Correction	Stated in The RNA-DNA Nexus (p. 260), but it sits in tension with the same author's later wording in The Cell Unsolved Mystery (p. 65): partial routes, meteoritic detections and precursor chemistry have been reported for some of these amino acids. Free tryptophan is at best rare and contested — not established as absent.
Tryptophan is synthesised through completely different enzymatic routes in different lineages.	Correction	Overstated. The chemistry (chorismate → anthranilate → PRPP adduct → indole → tryptophan) is one of the most conserved biosynthetic routes known. What differs between lineages is gene organisation and protein architecture — fused TrpC–TrpF, fused TrpGD, separate versus associated α/β subunits — plus the standalone TrpB2 (EC 4.2.1.122) in thermophiles. Non-homology of the enzymes' packaging is real; a different route is not.
Tryptophan synthesis runs at Km 0.1–0.2 mM with kcat 150–180 s⁻¹.	Open question	Vol. III (p. 621) gives these as system-level design targets for a hypothetical cell. They are not measured constants for a named enzyme: kcat for TrpB varies by organism and substrate, and no accession or assay is cited. Read as a specification, not a measurement.
The tryptophan pathway totals 1,590 amino acids.	Correction	The same figure reappears in the Vol. III enzyme inventory (p. 500, “Tryptophan biosynthesis (5 enzymes): 1,590 amino acids”). With the verified lengths it is 1,979 — the table in Vol. III needs the same correction as §6.13.

Database check performed for this page: KEGG M00023 and R00986; ExPASy/IUBMB ENZYME entries for EC 4.1.3.27, 2.4.2.18, 5.3.1.24, 4.1.1.48, 4.2.1.20, 4.2.99.21 and 4.1.3.40; UniProt P00895, P9WFX5, Q56320, Q06121, Q8U094 and Q8U093 for the lengths. All six reactions match the IUBMB equations. Four factual errors were found in the source chapter and are corrected above: one EC number, one ambiguous enzyme name, and three protein lengths (with the total that depends on them).

Connected pathways

Shikimate pathway Serine biosynthesis Phenylalanine biosynthesis Tyrosine biosynthesis

Evidence and scope

Trp proteins can occur as separate subunits or fused enzymes. This is a representative route, not an accession-resolved Aquifex reconstruction. This page explains selected documented biochemical reactions and summarises the argument of the cited book chapter. It does not establish that every displayed enzyme occurs in Aquifex, that the pathway is universally essential, or that the book image records a complete organism. Gene symbols name common enzyme activities; organism-specific protein records require separate matching. Statements marked as interpretation are the author's argument, not consensus biochemistry.

Original explanatory diagram and reaction account, based on the records below. Arrows show the biosynthetic reading direction; they do not claim that every reaction is irreversible. Selected protons, water and cofactors are omitted from the overview and are not implied absent.

Grasso, O. X-Ray of Life, Vol. II, Ch. 6 (Amino Acid Biosynthesis), §6.13 “Tryptophan”, pp. 273–278 — §6.13.1 p. 273, §6.13.2 p. 274, §6.13.3 p. 275, dilemmas pp. 275–278.
Figure: tryptophan synthase crystal structure, X-Ray of Life Vol. II, p. 274 (α2β2; PLP at β; MOLE channel mesh).
KEGG M00023 · tryptophan biosynthesis, enzyme record
KEGG R00986 · chorismate + L-glutamine = anthranilate + pyruvate + L-glutamate
IUBMB/ExPASy EC 4.1.3.27 · anthranilate synthase
IUBMB/ExPASy EC 2.4.2.18 · anthranilate phosphoribosyltransferase
IUBMB/ExPASy EC 5.3.1.24 · phosphoribosylanthranilate isomerase
IUBMB/ExPASy EC 4.1.1.48 · indole-3-glycerol-phosphate synthase
IUBMB/ExPASy EC 4.2.1.20 · tryptophan synthase
IUBMB/ExPASy EC 4.2.99.21 · isochorismate lyase (the entry the book's number actually points to)
Grasso, O. X-Ray of Life, Vol. I, pp. 81 (amino acids never synthesised prebiotically), 84 (aqueous half-lives), 230–231 (Miller–Urey yields).
Grasso, O. X-Ray of Life, Vol. III, pp. 31 and 273 (co-evolution argument), p. 500 (enzyme inventory total), p. 621 (kinetic targets).
Grasso, O. The Cell Unsolved Mystery, pp. 65 and 69 (prebiotic availability and the stability paradox).
Grasso, O. The RNA-DNA Nexus, pp. 170, 191, 193 (PRPP) and pp. 260–261 (tryptophan).