Anabaenopeptins - versatile in structure and function

To date, more than 600 cyanobacterial peptides have been described, which can be divided in 7 classes according to Welker & Von Döhren (2006). Thus far, microcystins, noduralin, and cyanopeptolins have been presented as “toxin of the week”. Here, we present the class of Anabaenopeptin. Anabaenopeptins were named after the cyanobacterium Anabaena flos-aquae  (Figure 1), from which the first anabaenopeptins were isolated, however they can also be produced by other cyanobacterium genera such as Planktothrix, Nodularia and Microcystis. To date, at least 96 anabaenopeptins have been reported in the literature³ and these toxins have been isolated from a variety of habitats, including terrestrial samples, freshwater, brackish water, as well as marine sponges.

Anabaenopeptin is a class of cyclic peptides characterized by a lysine (Figure 1, red moiety), with two structural roles: (a) Lysine realizes the ring formation by an peptide bond between it is R-group amine and the carboxyl group of the neighboring amino acid (green moiety); (b) the α-N of lysine connects via a ureido bond to the α-N of the side chain amino acid (purple moiety). Many anabaenopeptin variants exist because the other 5 positions in the molecule are variable. Anabaenopeptins range in their molecular weight from 679 Da for anabaenopeptin 679 to 956 Da for Oscillamide C.

Figure 1: Left: Chemical structure of anabaenopeptin A representative of the general chemical structure of anabaenopeptin type peptides with the common lysine moiety (Lys, red). Variation in the other five moieties are listed as the three-letter code of canonical amino acids and other modifications (Cl, chlorination;Br, bromination; M, α-amino-methylation; m, O-methylation, modified from Welker et al., 2006). Right: Microscopic image of  Anabaena flos-aquae (© A. Ballot).

Anabaenopeptins show not only a structural variability but also an impressive diversity regarding their bioactivity. Some anabaenopeptins are known to inhibit protein phosphatase 1 and 2A, similar to microcystins and nodularins, with IC50 values ranging from 16 to 435 ng/mL. Others anabaenopeptins present serine protease inhibition activity. Oscillamide Y, for example, strongly inhibits chymotrypsin activity at 86 ng/mL ². Other variants are also potent inhibitors of exopeptidases like carboxypeptidase A (CPA) and B (CPB), showing activity against CPA with IC50 values ranging from below 3 to 45 μg/mL^1,3. It is hypothesized that substitutions of the amino acid side-chain attached to the ureido group (purple moiety in Figure 1) are closely linked to the inhibitory activity and enzyme specificity. While enzyme inhibition is apparent, the actual motif for cyanobacteria to produce anabaenopeptins is still uncertain. Some theories discuss their role as a defense mechanism against parasites or to control cyanobacterial cell density.

SMILES: CC1C(=O)NC(C(=O)NCCCCC(C(=O)NC(C(=O)NC(C(=O)N1C)CCC2=CC=C(C=C2)O)C(C)C)NC(=O)NC(CC3=CC=C(C= C3)O)C(=O)O)CC4=CC=CC=C4

REFERENCES

1. Harms et al. (2016) Discovery of anabaenopeptin 679 from freshwater algal bloom material : Insights into the structure-activity relationship of anabaenopeptin protease inhibitors. Bioorganic & Medicinal Chemistry Letters 26: 4960–4965.
2. Sano et al. (1995) Oscillamide Y, a chymotrypsin inhibitor from toxic Oscillatoria Agardhii. Tetrahedron Letters, 36, 30:5933-5936.
3. Spoof et al. (2015) Structures and activity of new Anabaenopeptins produced by Baltic Sea cyanobacteria. Mar.drugs,14,8.
4. Welker, M. & Von Döhren, H. (2006) Cyanobactcerial peptides – Nature’s own combinatorial biosynthesis. FEMS Microbiol Rev, 30:530-563.