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Tolaasin F
General info
Chemical properties
| CAS | n.a. |
| Molecular formula | C94H163N21O25 |
| Molecular weight | 1987.42 g/mol |
| Mono-isotopic mass | 1986.2128 Da |
| Solubility | Water, Methanol, Acetonitrile, DMSO, DMF,TFE |
| CMC | n.a. |
| Minimal surface tension | n.a. |
| 3D conformation | n.a. |
| NMR data available in literature | n.a. |
Introduction
Tolaasins are cyclic lipodepsipeptides (CLiP) produced by Pseudomonas species and are the sole causal agents for brown blotch disease in mushrooms (Wong, 1979). The CLiPs features 18 amino acids, whereby the N-terminus is capped by a (3R)-hydroxy fatty acid moiety. The C terminus forms a macrocycle by formation of an ester bond to a side chain hydroxyl function.
Tolaasin F is the main CLiP produced by Pseudomonas costantinii DSM16734 (Scherlach, 2013). It was described as the causal agent of brown blotch disease in mushrooms in Finland (cfr. tolaasin I in Pseudomonas tolaasii).
Tolaasin F belongs to the Tolaasin (18:5) group that consists of tolaasin I-II (Nutkins, 1991), tolaasin A-E (Bassarello, 2004), tolaasin F (Scherlach, 2013) and sessilin (D’Aes, 2014).
Chemical structure
Tolaasins are relatively large CLiP, containing 18 amino acids, 5 of which form a lactone macrocycle. The peptide cyclization involves an ester bond between the C-terminus and the alcohol side-chain function of D-aThr14. Unnatural amino acids, such as homoserine (Hse), 2,4-diaminobutyric acid (Dab) and 2,3-didehydroaminobutyric acid (Dhb) are common in the lipopeptides of the Tolaasin group, while they do not occur in e.g. the Viscosin, Orfamide or Amphisin group, indicating a more complex assembly process. The structures of tolaasin F, as produced by Pseudomonas costantinii DSM16734, was elucidated in 2013 by means of bioinformatic analysis of the NRPS enzyme and mass spectrometry. The stereochemistry of the lipopeptide was characterized by bioinformatic analysis of the dual function epimerization/condensation (E/C) domains of the biosynthetic NRPS. To determine the stereochemistry of Val9 (predicted to be D-configured due to the presence of an E/C domain), tolaasin F was characterized by mass spectrometry after adding alpha-deuterated L-valine to the growth medium. If indeed epimerization occurs to D-Val, the label will be lost. On the other hand, if no epimerization occurs, the label is introduced into the peptide structure. Whereas incorporation of deuterium-labeled L-Val by Pseudomonas tolaasii could be unequivocally shown by mass spectrometry (mass shift +1), no deuterium label was found in the tolaasin derivative produced by Pseudomonas costantinii. Consequently, the structure of tolaasin F was found to be highly similar to that of tolaasin I, with the only differences being the presence of D-Val9/L-Leu15 instead of L-Val9/L-Ile15. Similar as in tolaasin I, the N-terminus of the tolaasin F is capped by a 3-hydroxy octanoic acid (3-OH C8:0). Summarizing, the structure of tolaasin F is 3-OH C8:0 – zDhb – D-Pro – D-Ser – D-Leu – D-Val – D-Ser – D-Leu – D-Val – D-Val – D-Gln – L-Leu – D-Val – zDhb – D-aThr – L-Leu – L-Hse – D-Dab – L-Lys. The structure bears two positive charges (Dab, Lys) at physiologic pH.
Biological activity
More than 100 years ago, the Gram-negative Pseudomonas tolaasii Paine bacterium was revealed as being the organism responsible for causing brown-blotch disease on cultivated mushrooms, effectively making these unappealing and ruining their consumption. Several other Pseudomonas species, such as Pseudomonas costantinii, have also been shown to cause similar disease symptoms. In all cases, a Tolaasin (18:5) group member has been found to be the sole toxic responsible for the disease.
References
Bassarello, et al. “Tolaasins A-E, five new lipodepsipeptides produced by Pseudomonas tolaasii.” Journal of Natural Products67, 5 (2004).
D’Aes, et al. “To settle or to move? The interplay between two classes of cyclic lipopeptides in the biocontrol strain Pseudomonas CMR12a.” Environmental Microbiology16, 7 (2014): https://dx.doi.org/10.1111/1462-2920.12462.
Nutkins, et al. “Structure determination of tolaasin, an extracellular lipodepsipeptide produced by the mushroom pathogen, Pseudomonas tolaasii Paine.” Journal of the American Chemical Society113, 7 (1991).
Scherlach, et al. “Biosynthesis and mass spectrometric imaging of tolaasin, the virulence factor of brown blotch mushroom disease.” ChemBioChem14, 18 (2013): https://dx.doi.org/10.1002/cbic.201300553.
Wong, et al. “Identification of Pseudomonas tolaasii: the white line in agar and mushroom tissue block rapid pitting tests.” Journal of Applied Bacteriology47 (1979).