Tyrocidine: Difference between revisions

| Watchedfields = changed

| verifiedrevid = 443922983

|ImageFile= Tyrocidine.png

| CASNo= 8011-61-8

'''Tyrocidine''' is a mixture of cyclic decapeptides produced by the bacteria ''[[Bacillus brevis]]'' found in soil. It can be composed of 4 different amino acid sequences, giving tyrocidine A–D (See figure 1). Tyrocidine is the major constituent of [[tyrothricin]], which also contains [[gramicidin]].<ref name= "pcid16129635">Pubchem: Tyrocidine and Tyrothricin.{{cite web |url=http://www.ncbi.nlm.nih.gov/sites/entrez?term=16129635%5Buid%5D&cmd=search&db=pccompound |title=PubChem Compound Result |format= |work= |accessdate=}}</ref> Tyrocidine was the first commercially available antibiotic, but has been found to be toxic toward human blood and reproductive cells. The function of tyrocidine within its host ''B. brevis'' is thought to be regulation of [[sporulation]].<ref name="pmid9352938">{{cite journal |author=Mootz HD, Marahiel MA |title=The tyrocidine biosynthesis operon of Bacillus brevis: complete nucleotide sequence and biochemical characterization of functional internal adenylation domains |journal=J. Bacteriol. |volume=179 |issue=21 |pages=6843–50 |year=1997 |month=November |pmid=9352938 |doi= |url=http://jb.asm.org/cgi/pmidlookup?view=long&pmid=9352938 |pmc=179617}}</ref>

| date = 2007

| id =

| accessdate = May 2008}}</ref>

Tyrocidine has a unique mode of action in which it disrupts the cell membrane function, making it a favorable target for engineering derivatives.<ref name="pmid12857101">{{cite journal |author=Qin C, Bu X, Wu X, Guo Z |title=A chemical approach to generate molecular diversity based on the scaffold of cyclic decapeptide antibiotic tyrocidine A |journal=J Comb Chem |volume=5 |issue=4 |pages=353–5 |year=2003 |pmid=12857101 |doi=10.1021/cc0300255 |url=}}</ref> Tyrocidine appears to perturb the lipid bilayer of a microbe’s inner membrane by permeating the lipid phase of the membrane. The exact affinity and location of tyrocidine within the phospholipid bilayer is not yet known.<ref>{{cite journal |author=Prenner EJ, Lewis RN, McElhaney RN |title=The interaction of the antimicrobial peptide gramicidin S with lipid bilayer model and biological membranes |journal=Biochim. Biophys. Acta |volume=1462 |issue=1–2 |pages=201–21 |year=1999 |month=December |pmid=10590309 |doi= 10.1016/S0005-2736(99)00207-2|url=http://linkinghub.elsevier.com/retrieve/pii/S0005-2736(99)00207-2}}</ref>

The biosynthesis of Tyrocidine is similar to Gramicidin S, and is achieved through the use of nonribosomal protein synthetases (NRPSs).<ref name="pmid17653357">{{cite journal |author=Kopp F, Marahiel MA |title=Macrocyclization strategies in polyketide and nonribosomal peptide biosynthesis |journal=Nat Prod Rep |volume=24 |issue=4 |pages=735–49 |year=2007 |month=August |pmid=17653357 |doi=10.1039/b613652b |url=}}</ref> Its biosynthesis is via an enzymatic assembly consisting of 3 peptide synthetase proteins, TycA, TycB, and TycC, which contain 10 modules. The different tyrocidine analogues (A–D) are not produced by different enzymes, but rather by an enzyme system that is capable of incorporating different amino acids of structural similarity at specified sites. The amino acid sequence is determined by the organization of the enzyme and not by any RNA template.<ref>{{cite journal |author=Roskoski R, Gevers W, Kleinkauf H, Lipmann F |title=Tyrocidine biosynthesis by three complementary fractions from Bacillus brevis (ATCC 8185) |journal=Biochemistry |volume=9 |issue=25 |pages=4839–45 |year=1970 |month=December |pmid=4320358 |doi= 10.1021/bi00827a002|url=}}</ref>

The tyrocine synthetases TycA, TycB, and TycC are encoded on the tyrocine operon. This consists of the three genes encoding for the three synthetases as well as three additional [[open reading frames]] (ORFs). These ORFs, labeled as TycD, TycE, and TycF are downstream of the three synthetase genes (see figure 2). TycD &TycE have the highest similarity to members of the [[ATP-binding cassette]] (ABC) transporter family which aid in the transport of substrates across a membrane. It has been suggested that the tandem transporters play a role in conferring resistance in the producer cell through tyrocidine secretion. TycF has been identified as a thioesterase (TE) and is similar to other TEs in bacterial operons used for encoding peptide synthetases. However, the precise function of these TEs remains unknown <ref name="pmid9352938"/>. The size of the peptide synthetases corresponds to the amount of activation they carry out. TycA is the smallest and activates a single amino acid from one module, TycB is intermediate in size and activates 3 amino acids with 3 modules, and TycC is the largest and activates 6 amino acids with 6 modules (See figure 3).<ref name="pmid9352938"/>

[[Image:Tyrocidine domain organization.png|200px|center|frame|Figure 3: Modules and Domains for Tyrocidine biosynthesis]]

Each module performs all the catalytic reactions necessary to incorporate a single amino acid onto the peptide chain. This is accomplished through the subdomains for adenylation (A), peptityl carrier protein (PCP), condensation (C), and depending on the amino acid position, an [[epimerization]] (E). The adenylation subdomain is used in activating the specific amino acid. Each module uses one molecule of the selected substrate amino acid with one molecule of [[Adenosine triphosphate|ATP]] to give an aminoacyl adenylate enzyme complex and pyrophosphate. The activated amino acid can then be transferred to the enzyme bound 4'-[[phosphopantetheine]] of the carrier protein with the expulsion of [[Adenosine monophosphate|AMP]] from the system. The carrier protein uses the 4'-phosphopantetheine prosthetic group for loading of the growing peptide and their monomer precursors.<ref name="pmid12167866">{{cite journal |author=Kohli RM, Walsh CT, Burkart MD |title=Biomimetic synthesis and optimization of cyclic peptide antibiotics |journal=Nature |volume=418 |issue=6898 |pages=658–61 |year=2002 |month=August |pmid=12167866 |doi=10.1038/nature00907 |url=}}</ref> Elongation of the peptide chain is achieved through condensation of the upstream PCP onto an adjacent downstream PCP-bound monomer. In certain domains you will find modification subdomains, such as the E subdomain seen in domains 1 and 4 in tyrocidine, which will generate the D-configured amino acid. On the final module is the TE domain used as a catalyst for cyclization or product release. The release of the product from the carrier protein is achieved through acylation of the active site serine of TE in which the decapeptide is transferred from the thiol ether to the serine residue. Deacylation can then occur through intramolecular cyclization or through hydrolysis to give the cyclic or linear product respectively (See figure 4). [[Image:Tyrocidine cyclization.png|500px|thumb|right|Figure 4: Proposed cyclization reaction catalyzed by thioesterase]]

In the case of tyrocidine, ring closure has been shown to be highly favorable due to 4 H-bonds helping the decapeptide backbone to adopt a stable conformation (See figure 5).<ref name="pmid12857101"/><ref name="pmid12167866"/> This intramolecular cyclization occurs in a head-to-tail fashion involving the N-terminus of the <small>D</small>-Phe1 and the C-terminus of the <small>L</small>-Leu10 (See figure 4).<ref name="pmid17653357"/><ref>{{cite journal |author=Trauger JW, Kohli RM, Mootz HD, Marahiel MA, Walsh CT |title=Peptide cyclization catalysed by the thioesterase domain of tyrocidine synthetase |journal=Nature |volume=407 |issue=6801 |pages=215–8 |year=2000 |month=September |pmid=11001063 |doi=10.1038/35025116 |url=}}</ref>

There is no chemical solution for macrocyclization of a peptide chain. Isolated tyrocidine (Tyc) TE domains can be used to cyclize chemically derived peptidyl-thioester substrates, providing a powerful route to new cyclic compounds. In order for this macrocyclization to occur, the peptide chain must be activated at its C-terminus with an ''N''-acethylcysteamine (SNAC) [[leaving group]].<ref name="pmid17653357"/> An [[alanine scan]] through the 10 positions of tyrocidine shows that only the <small>D</small>-Phe and <small>L</small>-Orn are required for sufficient cyclization.

Tyc TE can also be used biomimetically in which it mimics the environment created by the TE domain with the substrate’s PCP through use of a synthetic tether linked to a [[polyethylene glycol]] (PEG) amide resin.<ref name="pmid12167866"/> Use of this resin bound to a desired substrate with isolated TE can allow for catalytic release of the resin as well as macrocyclization of the substrate (See figure 6 <ref name="pmid12167866"/>). Use of [[solid phase peptide synthesis]] (SPPS) allowed the incorporation of a diverse array of monomers into the peptide chain. Later studies used the high tolerance of Tyc TE in order to modify the peptide backbone post-synthetically. This also allowed for glycosylation of the tyrosine or serine residues to be incorporated.<ref name="pmid17653357"/> Use of these methods has led to many promising new therapeutic agents.{{Citation needed|date=September 2011}}[[Image:Tyrocidine syn.png|600px|thumb|center|Figure 6: Biomimetic macrocyle synthesis.]]

[[Category:Antibiotics]]

[[fr:Tyrocidine]]

[[it:Tirocidina]]