Tyrocidines

Antibiotics. The genes involved in the synthesis of a variety of antibiotics have been isolated (34,35). These include antibiotics such as erythromycin, streptomycin, and also peptide antibiotics such as gramicidin and tyrocidin. Characterization of these gene products facUitates the design of novel antibiotics. In addition, overexpression of some of these gene products is also expected to improve the yield of the antibiotic (34,35). 

In 1939 the isolation of a mixture of microbial products named tyrotbricin from a soil bacillus was described. Further investigation showed this material to be a mixture of gramicidin and tyrocidine. In rapid succession the isolation of actinomycin (1940), streptothricin (1942), streptomycin (1943), and neomycin (1949), produced by Streptomjces were reported and in 1942 the word antibiotic was introduced. Chloramphenicol, the first of the so-called broad spectmm antibiotics having a wide range of antimicrobial activity, was discovered in 1947. Aureomycin, the first member of the commercially important tetracycline antibiotics, was discovered in 1948. 

Although tyrothricia is too toxic for parenteral therapy, it was formerly sold in the United States as oral lo2enges. Modem tyrothricin formulations are composed of 70—80% tyrocidines and 30—20% linear gramicidins. Tyrocidines are not as active as linear gramicidins and are too toxic for any therapeutic use by themselves. The bactericidal linear gramicidins are used in the United States solely as an ophthalmic solution in combination with polymyxin B sulfate and neomycin sulfate. The linear gramicidin is used in this aqueous product as a substitute for bacitracin, which lacks stabiUty under such conditions. 

Tyrocidine [8011-61-8] is a mixture of three closely related components. Tyrocidine studies on mechanism of action (98), biosynthesis on multien2yme complexes (93,99,100), and chemistry (101) are available, and tyrothricin production is discussed (102). Although the mechanism of action of linear gramicidins has been well researched, such work on tyrocidine is more limited it appears that tyrocidine damages membranes (103,104). 

Figure 13.3 NRP synthase biosynthetic architecture. The tyrocidine C is shown, made up of three modules and 32 domains...

Trauger, J.W., Kohli, R.M. and Walsh, C.T. (2001) Cyclization of backbone-substituted peptides catalyzed by the thioesterase domain from the tyrocidine nonribosomal peptide synthetase. Biochemistry, 40 (24), 7092-7098. 

Gramicidin is a peptide antibiotic first isolated by Dubos in 19391 as a crude complex together with a second peptide antibiotic known as tyrocidin. The mixture of the two antibiotics was called tyrothricin. Although the mixture was discovered about ten years after penicillin2, tyrothricin was the first antibiotic utilized in clinical practice. 

Kreuzig has studied the agar diffusion assay for tyrocidine in detail using gel chromatography as an analytical technique146. 

Ivashkiv has used this reaction to assay gramicidin and tyrocidine in fermentation broth155. The antibiotics are... 

Bartley and coworkers separated gramicidin from tyrocidine using Sephadex LH20199. 

There are several biologically important peptides which contain tyrosine but not tryptophan. These include small molecules with molecular weights of about 1000 or less. Molecules such as oxytocin, vasopressin, and tyrocidine A are cyclic, while others such as angiotensin II and enkephalin are linear. Schiller 19) has reviewed the literature up through 1984 on fluorescence of these and several other peptides. One major finding that has been reported recently is that the anisotropy and fluorescence intensity decays of many peptides are complex. This is especially evident in some of the tyrosine-containing peptides, and we expect that there will be considerable effort made over the next few years toward understanding the physical basis for these complex kinetics. 

In linear NRPSs a product consisting of amino acids is biosynthesized in an N- to C-terminal manner by the multidomain assembly line with a domain organization of A-PCP-(C-A-PCP) i-TE. The initiation module of a linear NRPS lacks a C domain, while the following modules may include any required additional domains. After formation of the full-length peptide, the product is released from the assembly line by a termination domain. Thus, the number and order of amino acids in the peptide directly coincides with the number and order of synthetase modules. Many NRPs are biosynthesized in this manner, and characterized examples include the penicillin tripeptide precursor -(L-0 -aminoadipyl)-L-cysteinyl-D-valine (ACV, Figure 4 (a)), complestatin, cyclosporin, fengycin, surfactin, and tyrocidine. "... 

Figure 10 N M R structural analysis of carrier domains. Three conformations of the PCP domain from tyrocidine synthetase (brown box) and the NMR structure of the related AGP domain from a polyketide synthase. The star symbol signifies the position of the conserved phosphopantetheinylated serine residue. The protein ribbon representations are rainbow colored from red (N-terminus) to violet. PDB codes A/H state, 2GDW H-state, 2GDX A-state, 2GDY AGP, 2AF8.

Many natural products are constrained by macrocyclic motifs, which are often essenhal for natural products to possess the desired biological properties. In the biosynthesis of macrocyclic NRPs and PKs, linear peptides or PKs are often mac-rocyclized by a TE domain located at the C-terminal of multi-modular synthases. For example, in the biosynthesis of the antibiotic tyrocidine A (Tyc A), a linear enzyme-bound decapephde, which is transferred from the last carrier protein (or thiolahon) domain of the Tyc A synthase, is cyclized by an intramolecular Sn2 reachon between the N-terminal amine nucleophile and the C-terminal ester, which is covalently linked to serine residual in the TE domain prior to macro-cyclization (Scheme 7.9) ([35] and references therein). 

Lipmann, F., Gevers, W., Kleinkauf, H., and Roskoski, R. Jr. (1971). Polypeptide synthesis on protein templates the enzymatic synthesis of gramicidin S and tyrocidine. Adv. Enzymol. Relat. Areas Mol. Biol. 35, 1-34. 

In 1939, Rene Dubos, Waksman s former postdoctoral student, extracted two chemicals, tyrocidine and gramicidin, from the soil germ Bacillus brevis. These chemicals cured bacterial infections in cattle but were too toxic for humans. This discovery prompted a number of scientists to expand the search for microbes in the soil, microbes capable of making chemicals that could kill disease-causing bacteria in humans. 

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