Editing Tryptophan synthase

{{Short description|Class of enzymes}}
{{Infobox enzyme
| Name       = Tryptophan Synthase
| EC_number  = 4.2.1.20
| CAS_number = 9014-52-2
| GO_code    = 0004834
| image      = Tryptophan Synthase Dimer 3.png
| width      =
| caption    = ''Subunits: <span style="color:blue;">Beta Subunit</span>, <span style="color:lime;">Alpha Subunit</span> with <span style="color:orange;">PLP</span>, <span style="color:magenta;">IGP</span>''
}}

'''Tryptophan synthase''' or '''tryptophan synthetase''' is an [[enzyme]] ({{EnzExplorer|4.2.1.20}}) that catalyzes the final two steps in the biosynthesis of [[tryptophan]].<ref name="Tryptophan synthase">{{cite journal |vauthors=Dunn MF, Niks D, Ngo H, Barends TR, Schlichting I | title = Tryptophan synthase: the workings of a channeling nanomachine | journal = Trends in Biochemical Sciences | volume = 33 | issue = 6 | pages = 254–64 |date=June 2008 | pmid = 18486479 | doi = 10.1016/j.tibs.2008.04.008}}</ref><ref name="pmid2053470">{{cite book | vauthors = Miles EW | title = Advances in Enzymology and Related Areas of Molecular Biology | chapter = Structural basis for catalysis by tryptophan synthase | series = Advances in Enzymology and Related Areas of Molecular Biology | volume = 64 | pages = 93–172 | date = 1991 | pmid = 2053470 | doi = 10.1002/9780470123102.ch3 | isbn = 9780470123102 | author-link = Edith Wilson Miles }}</ref> It is commonly found in [[Eubacteria]],<ref name="Eubacteria">{{cite journal |vauthors=Jablonski P, Jablonski L, Pintado O, Sriranganathan N, Howde C | title = Tryptophan synthase: Identification of Pasteurella multocida tryptophan synthase B-subunit by antisera against strain PI059 | journal = Microbiology | volume = 142 | pages = 115–21 |date=September 1996 | pmid = 8581158 | doi = 10.1099/13500872-142-1-115| doi-access = free }}</ref> [[Archaebacteria]],<ref name="Archaebacteria">{{cite journal |vauthors=Lazcano A, Diaz-Villgomez E, Mills T, Oro J | title = On the levels of enzymatic substrate specificity: Implications for the early evolution of metabolic pathways | journal = Advances in Space Research | volume = 15 | issue = 3 | pages = 345–56 |date=March 1995 | pmid = 11539248 | doi = 10.1016/S0273-1177(99)80106-9}}</ref> [[Protista]],<ref name="Protista">{{cite journal |vauthors=Anderson I, Watkins R, Samuelson J, Spencer D, Majoros W, Grey M, Loftus B | title = Gene Discovery in the Acanthamoeba castellanii Genome | journal = Protist | volume = 156 | issue = 2 | pages = 203–14 |date=August 2005 | pmid = 16171187 | doi = 10.1016/j.protis.2005.04.001}}</ref> [[Fungi]],<ref name="Fungi">{{cite journal |vauthors=Ireland C, Peekhaus N, Lu P, Sangari R, Zhang A, Masurekar P, An Z | title = The tryptophan synthetase gene TRP1 of Nodulisporium sp.: molecular characterization and its relation to nodulisporic acid A production | journal = Appl Microbiol Biotechnol | volume = 79 | issue = 3 | pages = 451–9 |date=April 2008 | pmid = 18389234 | doi = 10.1007/s00253-008-1440-3| s2cid = 7230896 }}</ref> and [[Plantae]].<ref name="Plantae">{{cite journal | author = Sanjaya, Hsiao PY, Su RC, Ko SS, Tong CG, Yang RY, Chan MT | title = Overexpression of Arabidopsis thaliana tryptophan synthase beta 1 (AtTSB1) in Arabidopsis and tomato confers tolerance to cadmium stress | journal = Plant Cell Environ | volume = 31 | issue = 8 | pages = 1074–85 |date=April 2008 | pmid = 18419734 | doi = 10.1111/j.1365-3040.2008.01819.x| doi-access = }}</ref> However, it is absent from [[Animalia]].<ref name="Animalia">{{cite journal |vauthors=Eckert SC, Kubler E, Hoffmann B, Braus GH | title = The tryptophan synthase-encoding trpB gene of Aspergillus nidulans is regulated by the cross-pathway control system | journal = Mol Gen Genet | volume = 263 | issue = 5 | pages = 867–76 |date=June 2000 | pmid = 10905354 | doi = 10.1007/s004380000250| s2cid = 22836208 }}</ref> It is typically found as an α2β2 tetramer.<ref>{{cite journal | vauthors = Ahmed SA, Miles EW, Davies DR | title = Crystallization and preliminary X-ray crystallographic data of the tryptophan synthase alpha 2 beta 2 complex from Salmonella typhimurium | journal = The Journal of Biological Chemistry | volume = 260 | issue = 6 | pages = 3716–3718 | date = March 1985 | pmid = 3882715 | doi = 10.1016/s0021-9258(19)83682-7 | author-link2 = Edith Wilson Miles | doi-access = free }}</ref><ref>{{cite journal | vauthors = Hyde CC, Ahmed SA, Padlan EA, Miles EW, Davies DR | title = Three-dimensional structure of the tryptophan synthase alpha 2 beta 2 multienzyme complex from Salmonella typhimurium | journal = The Journal of Biological Chemistry | volume = 263 | issue = 33 | pages = 17857–17871 | date = November 1988 | pmid = 3053720 | doi = 10.1016/s0021-9258(19)77913-7 | doi-access = free }}</ref> The α subunits catalyze the reversible formation of [[indole]] and [[glyceraldehyde-3-phosphate]] (G3P) from indole-3-glycerol phosphate (IGP). The β subunits catalyze the irreversible condensation of indole and [[serine]] to form tryptophan in a [[pyridoxal phosphate]] (PLP) dependent reaction. Each α active site is connected to a β active site by a 25 Ångstrom long hydrophobic channel contained within the enzyme. This facilitates the diffusion of indole formed at α active sites directly to β active sites in a process known as [[substrate channeling]].<ref name="Overview">{{cite journal |vauthors=Raboni S, Bettati S, Mozzarelli A | title = Tryptophan synthase: a mine for enzymologists | journal = Cell Mol Life Sci | volume = 66 | issue = 14| pages = 2391–403 |date=April 2009 | pmid = 19387555 | doi = 10.1007/s00018-009-0028-0| hdl = 11381/2293687 | s2cid = 30220030 | hdl-access = free }}</ref> The active sites of tryptophan synthase are [[allosterically]] coupled.<ref name="Regulation">{{cite journal |vauthors=Fatmi MQ, Ai R, Chang CA | title = Synergistic regulation and ligand-induced conformational changes of tryptophan synthase | journal = Biochemistry | volume = 48 | issue = 41| pages = 9921–31 |date=September 2009 | pmid = 19764814 | doi = 10.1021/bi901358j}}</ref>

==Enzyme structure==
[[Image:Active_Site_2.png|thumb|alt=caption.|Active sites for α and β subunits showing hypothesized catalytic residues|left]]

=== Subunits ===
Tryptophan synthase typically exists as an α-ββ-α complex. The α and β subunits have molecular masses of 27 and 43 kDa respectively. The α subunit has a [[TIM barrel]] conformation. The β subunit has a fold type II conformation and a binding site adjacent to the active site for monovalent cations.<ref name="Structure">{{cite journal |vauthors=Grishin NV, Phillips MA, Goldsmith EJ | title = Modeling of the spatial structure of ornithine decarboxylases | journal = Protein Sci | volume = 4 | issue = 7 | pages = 1291–304 |date=July 1995 | pmid = 7670372 | doi = 10.1002/pro.5560040705| pmc = 2143167 }}</ref> Their assembly into a complex leads to structural changes in both subunits resulting in reciprocal activation. There are two main mechanisms for intersubunit communication. First, the COMM domain of the β-subunit and the α-loop2 of the α-subunit interact. Additionally, there are interactions between the αGly181 and βSer178 residues.<ref name="Interaction">{{cite journal |vauthors=Schneider TR, Gerhardt E, Lee M, Liang PH, Anderson KS, Schlichting I | title = Loop closure and intersubunit communication in tryptophan synthase | journal = Biochemistry | volume = 37 | issue = 16 | pages = 5394–406 |date=April 1998 | pmid = 9548921 | doi = 10.1021/bi9728957}}</ref> The active sites are regulated allosterically and undergo transitions between open, inactive, and closed, active, states.<ref name="Regulation" />

=== Hydrophobic channel ===
The α and β active sites are separated by a 25 Ångstrom long [[hydrophobic]] channel contained within the enzyme allowing for the diffusion of indole. If the channel did not exist, the indole formed at an α active site would quickly diffuse away and be lost to the cell as it is hydrophobic and can easily cross membranes. As such, the channel is essential for enzyme complex function.<ref name="Channel">{{cite journal |vauthors=Huang X, Holden HM, Raushel FM | title = Channeling of Substrates and Intermediates in Enzyme-Catalyzes Reactions | journal =Annu Rev Biochem | volume = 70 | pages = 149–80 | year = 2001 | pmid = 11395405 | doi = 10.1146/annurev.biochem.70.1.149}}</ref>

==Enzyme mechanism==
[[Image:Tryptophan Synthase Mechanism 5.gif|thumb|left|alt=caption.|Proposed mechanism of tryptophan synthase]]The net reaction of tryptophan synthase turns indole-3-glycerol phosphate and serine into glyceraldehyde-3-phosphate, tryptophan and water. The reaction happens in two steps, each catalyzed by one of the subunits:[[Image:Tryptophan synthetase rn.png|thumb|500px|Reaction catalyzed by tryptophan synthase|center]]

=== α subunit reaction ===
The α subunit catalyzes the formation of indole and G3P from a retro-aldol cleavage of IGP. The αGlu49 and αAsp60 are thought to be directly involved in the catalysis as shown.<ref name="Overview" /> The rate limiting step is the isomerization of IGP.<ref name="a Rate Limiting Step">{{cite journal |vauthors=Anderson KS, Miles EW, Johnson KA | author-link2=Edith Wilson Miles|title = Serine modulates substrate channeling in tryptophan synthase. A novel intersubunit triggering mechanism | journal = J Biol Chem | volume = 266 | issue = 13 | pages = 8020–33 |date=May 1991 | doi=10.1016/S0021-9258(18)92934-0| pmid = 1902468 | doi-access=free}}</ref> See image 2.

=== β subunit reaction ===
The β subunit catalyzes the β-replacement reaction in which indole and serine condense to form tryptophan in a PLP dependent reaction. The βLys87, βGlu109, and βSer377 are thought to be directly involved in the catalysis as shown.<ref name="Overview" /> Again, the exact mechanism has not been conclusively determined. See image 2.

==Biological function==
Tryptophan synthase is commonly found in Eubacteria, Archaebacteria, Protista, Fungi, and Plantae. It is absent from animals such as humans. [[Tryptophan]] is one of the twenty standard [[amino acids]] and one of nine [[essential amino acids]] for humans. As such, tryptophan is a necessary component of the human diet.

== Substrate scope ==
Tryptophan synthetase is also known to accept indole analogues, e.g., fluorinated or methylated indoles, as substrates, generating the corresponding tryptophan analogues.<ref>{{cite journal | vauthors = Wilcox M | title = The enzymatic synthesis of L-tryptophan analogues | journal = Analytical Biochemistry | volume = 59 | issue = 2 | pages = 436–440 | date = June 1974 | pmid = 4600987 | doi = 10.1016/0003-2697(74)90296-6 }}</ref>

==Disease relevance==
As humans do not have tryptophan synthase, this enzyme has been explored as a potential [[drug target]].<ref name="Drug Target">{{cite journal |vauthors=Chaudhary K, Roos DS | title = Protozoan genomics for drug discovery | journal = Nat Biotechnol | volume = 23 | issue = 9 | pages = 1089–91 |date=September 2005 | pmid = 16151400 | doi = 10.1038/nbt0905-1089| pmc = 7096809 }}</ref> However, it is thought that bacteria have alternate mechanisms to produce amino acids which might make this approach less effective. In either case, even if the drug only weakens bacteria, it might still be useful as the bacteria are already vulnerable in the hostile host environment. As such, the inhibition of tryptophan synthase along with other PLP-enzymes in amino acid metabolism has the potential to help solve medical problems.<ref name="Weakness">{{cite journal |vauthors=Becker D, Selbach M, Rollenhagen C, Ballmaier M, Meyer TF, Mann M, Bumann D | title = Robust Salmonella metabolism limits possibilities for new antimicrobials | journal = Nature | volume = 440 | issue = 7082 | pages = 303–7 |date=March 2006 | pmid = 16541065 | doi = 10.1038/nature04616| s2cid = 4426157 }}</ref>

Inhibition of tryptophan synthase and other PLP-enzymes in amino acid metabolism has been suggested for:
* Treatment of [[tuberculosis]]<ref name="Drug Target" />
* Treatment of [[Human eye|ocular]] and [[genital]] infections<ref name="Infections">{{cite journal |vauthors=Caldwell HD, Wood H, Crane D, Baily R | title = Polymorphisms in Chlamydia trachomatis tryptophan synthase genes differentiate between genital and ocular isolates | journal = J Clin Invest | volume = 111 | issue = 11 | pages = 1757–69 |date=June 2003 | pmid = 12782678 | doi = 10.1172/JCI17993| pmc = 156111 }}</ref>
* Treatment of [[cryptosporidiosis]]<ref name="Drug Target"/>
* [[Herbicide]] use<ref name="Herbicide">{{cite journal |vauthors=Kulik V, Hartmann E, Weyand M, Frey M, Gierl A, Niks D, Dunn MF, Schlichting I | title = On the structural basis of the catalytic mechanism and the regulation of the α-subunit of tryptophan synthase from Salmonella typhimurium and BXI from maize, two evolutionarily related enzymes | journal = J Mol Biol | volume = 352 | issue = 3 | pages = 608–20 |date=September 2005 | pmid = 16120446 | doi = 10.1016/j.jmb.2005.07.014}}</ref>

==Evolution==
It is thought that early in evolution the trpB2 gene was duplicated. One copy entered the [[trp operon]] as trpB2i allowing for its expression with trpA. TrpB2i formed transient complexes with TrpA and in the process activated TrpA unidirectionally. The other copy remained outside as trpB2o, and fulfilled an existing role or played a new one such as acting as a salvage protein for indole. TrpB2i evolved into TrpB1, which formed permanent complexes with trpA resulting in bidirectional activation. The advantage of the indole salvage protein declined and the TrpB gene was lost. Finally, the TrpB1 and TrpA genes were fused resulting in the formation the bifunctional enzyme.<ref name="Evolution">{{cite journal |vauthors=Leopoldseder S, Hettwer S, Sterner R | title = Evolution of Multi-Enzyme Complexes: The Case of Tryptophan Synthase | journal = Biochemistry | volume = 45 | issue = 47 | pages = 14111–9 |date=November 2006 | pmid = 17115706 | doi = 10.1021/bi061684b}}</ref>

==Historical significance==
Tryptophan synthase was the first enzyme identified that had two [[catalytic]] capabilities that were extensively studied. It was also the first identified to utilize substrate channeling. As such, this enzyme has been studied extensively and is the subject of great interest.<ref name="Overview"/>

==See also==
*[[Lyase]]
*[[Synthase]]
*[[Tryptophan synthase (indole-salvaging)]]

==References==
{{Reflist}}

{{Multienzyme complexes}}
{{Carbon-oxygen lyases}}
{{Enzymes}}
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{{DEFAULTSORT:Tryptophan Synthase}}
[[Category:EC 4.2.1]]