Tyrocidine

Tyrocidine is a cyclic decapeptide antibiotic that is produced by Bacillus brevis, a Gram-positive aerobic spore-forming bacillus commonly found in soil, air, water, and decaying matter. Tyrocidine is in fact a mixture of four cyclic decapeptides, tyrocidines A, B, C, and D. Tyrocidine is a constituent of tyrothricin, a mixture of polypeptide antibiotics.

Contents:

1. History

2. Structure

3. Biosynthesis

4. Mechanism of action

5. Clinical use

6. References

History

Tyrocidine is a constituent of tyrothricin. In 1939 the American microbiologist René Dubos demonstrated that a soil bacterium was capable of decomposing the starchlike capsule of the pneumococcus bacterium, without which the pneumococcus is harmless and does not cause pneumonia. Dubos then found in the soil a microbe, Bacillus brevis, from which he obtained a product, tyrothricin, that was highly toxic to a wide range of bacteria.

Structure

Biosynthesis

The cyclic decapeptide antibiotic tyrocidine is produced by Bacillus brevis on an enzyme complex comprising three peptide synthetases, TycA, TycB, and TycC (tyrocidine synthetases 1, 2, and 3), via the nonribosomal pathway. Nonribosomal pathways are characterized by substrates that are not restricted to the 20 proteinogenic amino acids; that is to say, the amino acids that are found in proteins and that are coded for in the standard genetic code. Products synthesized using this pathway can incorporate nonproteinogenic amino acids, such as hydroxy-, D-, and N-methylated amino acids.

The diagram above shows the tyrocidine biosynthesis operon containing the tycA, tycB, and tycC genes. TycA consists of one module bearing peptide synthetase. TycB consists of three modules, including an epimerization (interconversion of epimers) domain, and TycC is composed of six modules and contains a putative thioesterase domain. Each module is defined to harbor all catalytic activities to incorporate a single amino acid residue into the peptide product. The modules coincide in number with the number of incorporated amino acids. 

a, The tyrocidine non-ribosomal peptide synthetase from B. brevis . Synthetase subunits TycA, TycB and TycC are represented by a series of boxes. Each box represents a functional domain: A, adenylation (catalyses amino-acid activation); PCP, peptidyl carrier protein; C, condensation (catalyses peptide-bond formation); E, epimerization. The TE domain is shaded. Thiol (SH) and hydroxyl (OH) groups represent phosphopantetheine and the TE active-site serine residue, respectively. b, Left, proposed mechanism of TE-domain catalysed macrocyclization and product release. Decapeptide intermediates are shown with the N-terminal residue d-Phe1 highlighted in red. Right, Experimental system for probing the mechanism and specificity of TE-domain catalysed cyclization.

This peptide synthetase activates the amino acid constituents as aminoacyl adenylate at the expense of ATP and thioesterifies them on the thiol moiety of an enzyme-attached cofactor, 4'phosphopantetheine.

Subsequently, activated amino acids are condensed stepwise in an amino-to carboxy-terminal direction. Nucleophilic attack by the amino group of the neighboring aminoacyl thioester is catalysed by the C domain and results in amide (peptide) bond formation. As well as activating the amino acids and catalyzing the formation of the peptide linkages, the enzyme may possess other domains that are responsible for epimerizing L-amino acids to D-amino acids. A terminal thioesterase domain is responsible for terminating chain extension and releasing the peptide from the enzyme. Cyclization occurs when the amino acids at the two termini of a linear peptide link up to form another peptide bond, but very often, ring formation can be the result of ester or amide linkages.

Figure 1, which breaks down the primary structure of tyrocidine A into its three modules, shows each module and the amino acids that it activates.

Mechanism of action

Tyrocidine kills bacteria by interacting with their cytoplasmic membranes and causing leakage of their intracellular content. It also affects intracellular membranes such as those of mitochondria. Tyrocidine inhibits RNA synthesis in an in-vitro transcriptional system by forming a complex with the DNA. Bacillus brevis, under conditions of severe nitrogen starvation brought about by nutritional shift-down, is unable to induce sporulation (process of becoming a spore) unless supplemented with the peptide antibiotic tyrocidine.

Clinical use

Tyrocidines are too toxic for therapeutic use on their own, but are incorporated into lozenges for relief of throat infections. They are also found to be toxic to red blood and reproductive cells in humans but can be used to good effect when applied as an ointment on body surfaces, and are active against many Gram-positive bacteria. 

References

1. http://www.infoplease.com/ce6/sci/A0856640.html

2. Henning D. Mootz, Mohamed A. Marahiel, “The Tyrocidine Biosynthesis Operon of Bacillus brevis: Complete Nucleotide Sequence and Biochemical Characterization of Functional Internal Adenylation Domains”, Journal of Bacteriology, Nov. 1997, p. 6843-6850

3. Paul M Dewick, “Medicinal Natural Products – A Biosynthetic Approach”, Second Edition, John Wiley & Sons, LTD