tryptophan synthase

Image:Active_Site_2.png

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.{{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 }} 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.{{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}} The active sites are regulated allosterically and undergo transitions between open, inactive, and closed, active, states.

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.{{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}}

Image:Tryptophan Synthase Mechanism 5.gifThe 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

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. The rate limiting step is the isomerization of IGP.{{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}} See image 2.

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. Again, the exact mechanism has not been conclusively determined. See image 2.

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.

Tryptophan synthetase is also known to accept indole analogues, e.g., fluorinated or methylated indoles, as substrates, generating the corresponding tryptophan analogues.{{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 }}

As humans do not have tryptophan synthase, this enzyme has been explored as a potential 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 }} 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.{{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 }}

Inhibition of tryptophan synthase and other PLP-enzymes in amino acid metabolism has been suggested for:

• Treatment of tuberculosis
• Treatment of ocular and genital 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 }}
• Treatment of cryptosporidiosis
• Herbicide use{{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}}

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.{{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}}

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.

• Lyase
• Synthase
• Tryptophan synthase (indole-salvaging)

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{{Multienzyme complexes}}

{{Carbon-oxygen lyases}}

{{Enzymes}}

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Category:EC 4.2.1