Thiostrepton is a natural cyclic oligopeptide

Oligopeptide

An oligopeptide consists of between 2 and 20 amino acids and includes dipeptides, tripeptides, tetrapeptides, pentapeptides, etc.Examples of oligopeptides include:...

 antibiotic

Antibiotic

An antibacterial is a compound or substance that kills or slows down the growth of bacteria.The term is often used synonymously with the term antibiotic; today, however, with increased knowledge of the causative agents of various infectious diseases, antibiotic has come to denote a broader range of...

, derived from several strains of strepromycetes

Streptomyces

Streptomyces is the largest genus of Actinobacteria and the type genus of the family Streptomycetaceae. Over 500 species of Streptomyces bacteria have been described. As with the other Actinobacteria, streptomycetes are gram-positive, and have genomes with high guanine and cytosine content...

, such as Streptomyces azureus and Streptomyces laurentii.

History

Thiostrepton was discovered by Donovick et al. who described its antibacterial properties in 1955. Dorothy Crowfoot Hodgkin

Dorothy Crowfoot Hodgkin

Dorothy Mary Hodgkin OM, FRS , née Crowfoot, was a British chemist, credited with the development of protein crystallography....

 solved the structure of thiostrepton in 1970.
Early in 1978, Bycroft and Gowland proposed the biosynthesis of thiostrepton, which was still unclear until 2009. Several studies of thiopeptide biosynthesis have been contemporarily published in 2009 and two of them (Liao et al. and Kelly et al.) included the similar biosynthesis of thiostrepton: it's ribosomally synthesized from thiostrepton biosynthetic genes (tsr genes) and posttranslational modification is needed.

A total synthesis of Thiostrepton was completed by K.C. Nicolaou, et al. in 2005.

Applications

Thiostrepton has been used in veterinary medicine in mastitis

Mastitis

Mastitis is the inflammation of breast tissue. S. aureus is the most common etiological organism responsible, but S. epidermidis and streptococci are occasionally isolated as well.-Terminology:...

 caused by gram-negative

Gram-negative

Gram-negative bacteria are bacteria that do not retain crystal violet dye in the Gram staining protocol. In a Gram stain test, a counterstain is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color...

 organisms and in dermatologic disorders. It is mostly used in complex ointments containing neomycin

Neomycin

Neomycin is an aminoglycoside antibiotic that is found in many topical medications such as creams, ointments, and eyedrops. The discovery of Neomycin dates back to 1949. It was discovered in the lab of Selman Waksman, who was later awarded the Nobel Prize in Physiology and medicine in 1951...

, nystatin

Nystatin

Nystatin is a polyene antifungal medication to which many molds and yeast infections are sensitive, including Candida. Due to its toxicity profile, there are currently no injectable formulations of this drug on the US market...

, Thiostrepton and topical steroid

Steroid

A steroid is a type of organic compound that contains a characteristic arrangement of four cycloalkane rings that are joined to each other. Examples of steroids include the dietary fat cholesterol, the sex hormones estradiol and testosterone, and the anti-inflammatory drug dexamethasone.The core...

s. It is also active against gram-positive bacteria. It is notable that ointments for human usage contain neomycin

Neomycin

Neomycin is an aminoglycoside antibiotic that is found in many topical medications such as creams, ointments, and eyedrops. The discovery of Neomycin dates back to 1949. It was discovered in the lab of Selman Waksman, who was later awarded the Nobel Prize in Physiology and medicine in 1951...

, nystatin

Nystatin

Nystatin is a polyene antifungal medication to which many molds and yeast infections are sensitive, including Candida. Due to its toxicity profile, there are currently no injectable formulations of this drug on the US market...

, and topical steroid

Steroid

A steroid is a type of organic compound that contains a characteristic arrangement of four cycloalkane rings that are joined to each other. Examples of steroids include the dietary fat cholesterol, the sex hormones estradiol and testosterone, and the anti-inflammatory drug dexamethasone.The core...

s, but no Thiostrepton.

Thiostrepton was reported (in 2008) to exhibit activity against breast cancer cells through targeting the transcription factor forkhead box M1

FOXM1

Forkhead box protein M1 is a protein that in humans is encoded by the FOXM1 gene. The protein encoded by this gene is a member of the FOX family of transcription factors...

 (FOXM1

FOXM1

Forkhead box protein M1 is a protein that in humans is encoded by the FOXM1 gene. The protein encoded by this gene is a member of the FOX family of transcription factors...

)., also in 2011.
It has also been shown to circumvent acquired cisplatin resistance in breast cancer cells under in invitro conditions.

Thiostrepton is used in molecular biology as a reagent for both positive and negative selection of genes involved in nucleotide metabolism.

Biosynthesis

There are total 21 genes (tsrA~tsrU) in the biosynthetic gene cluster. The precursor of thiostrepton contains 58 amino acids in the peptide chain, which includes 41-aa leading peptide (LP) and 17-aa structural peptide (IASASCTTCICTCSCSS). Once the precursor is synthesized, cyclodehydratase tsrO and dehydrogenase tsrM catalyze the formation of thiazole or thiazoline from every cysteine residues in the peptide chain. After thiazole/thiazoline formation, dehydratases tsrJ, K and S then convert all the serine residues into dehydroalanines. A hetero Diels-Alder cyclization of the central dehydropiperidine (at S5, C13, and S14) has been suggested by Bycroft back to 1978 and been employed in the chemical synthesis of this core structure by Nicolaou et al. in 2005. An alternative mechanism of the dehydropiperidine formation has also been suggested by Kelly et al. in 2009. Nevertheless, based on experimental evidence, tsrN and L are suggested to be responsible for the hetero Diels-Alder cyclization. The quinaldic acid moiety is suggested to be synthesized by the nine genes tsrFAEBDUPQI from tryptophan and then results in the closure of quinaldic acid macrocycle. At last, tsrR serves as a candidate for the oxidation of the Ile residue to afford thiostrepton.

Alternative mechanism for the formation of the dehydropiperidine core

Total Synthesis

In 2005, Nicolaou et al. published the total synthesis of thiostrepton. At first, they constructed the key building blocks of thiostrepton (1): dehydropiperidine core (2), thiazoline macrocycle (3), bis-dehydroalanine tail (4), and quinaldic acid macrocycle (5). Then they assembled the building blocks sequentially as shown in the synthetic scheme (compound numbers are from the reference).

Building blocks for Thiostrepton