Aeruginosin – a thrombin and trypsin inhibitor

Cyanobacterial bloom events are becoming more prevalent all over the globe and their massive occurrence poses a severe threat to drinking water supplies. Cyanobacteria can produce a wide range of bioactive compounds, including the infamous Microcystins and other 6 classes of cyanopeptides according to Welker & Von Döhren (2006) that together comprise more than 600 cyanobacterial peptides. Thus far, four different cyanopeptides classes have been presented as “toxin of the week”. Here, we present the class of Aeruginosin. Aeruginosins are linear peptides characterized by a derivative of hydroxyl-phenyl lactic acid (Hpla) at the N-terminus and the amino acid-2-carboxy-6-hydroxyoctahydroindole (Choi) at the C-terminus (Figure 1). At present, 27 variants have been published. Aeruginosins have been isolated from marine sponges and cyanobacterial blooms, being detected in strains isolated from Japan and Australia. Aeruginosins, also known in the literature under the name microcin and spumigin, can be produced by the following cyanobacterial species: Microcystis, Planktothrix, Oscillatoria and Nodularia¹. 

Figure 1: Left: Chemical structure of aeruginosin 98-A representative of the general chemical structure of aeruginosin type peptides with the common Hpla (1) and Choi (3) moieties. Variation in the other two moieties are listed as the three-letter code of canonical amino acids and other modifications (Cl, chlorination;Br, bromination; Su, sulphation; pent, glycosylation, modified from Welker et al., 2006). Right: Microscopic image of Planktothrix rubescens (Source:CCALA).

These peptides exhibited a potent inhibitory activity against trypsin and trypsin-like serine proteases, such as thrombin. These enzymes are involved in a number of important physiological processes, and their relevance in the complex blood coagulation cascade is well established. Aeruginosin 98-A, isolated from the cultured cyanobacteria Microcystis aeruginosa, inhibited trypsin with an IC50 of 0.87 µM and plasmin and thrombin with IC50 of 8.72 µM and 10.17 µM, respectively³. Furthermore, aeruginosin 828A, isolated from a MC-deficient Planktothirx strain, showed not only strong inhibition against thrombin (IC50= 21.8nM) and trypsin (IC50= 112nM) but was also found to be toxic for the crustacean Thamnocephalus platyurus with a LC50 value of 22.4µM, which is only slightly higher than the toxicity reported for MC-LR (LC50= 10.8 µM). It was hypothesized that both, chlorine and sulfate moieties are responsible for the strong inhibition and toxicity of these peptides². Those findings reinforce the need to consider not only microcystins but also other cyanobacterial peptides to forecast the toxicity of cyanobacterial blooms.

SMILES:

• Aeruginosin 98-A: CCC(C)C(C(=O)N1C2CC(CCC2CC1C(=O)NCCCCN=C(N)N)OS(=O)(=O)O)NC(=O)C(CC3=CC(=C(C=C3)O)Cl)O
• Aeruginosin 828A: CC(C)C(C(C(=O)N1C2CC(CCC2CC1C(=O)NCCC3=CCN(C3)C(=N)N)OC4C(C(C(CO4)OS(=O)(=O)O)O)O)NC(=O)C(CC5=CC=CC=C5)O)Cl

References:

1. Ersmark et al. (2008) Chemistry and biology of the aeruginosin family of serine protease inhibitors. Angew.Chem.Int, 47:1202-1223.
2. Kohler et al. (2014) The toxicity and enzyme activity of a chlorine and sulfate containing aeruginosin isolated from a non-microcystin-producing Planktothrix strain. Harmful Algae, 39:154-160.
3. Murakami et al. (1995) Aeruginosins 98-A and B, Trypsin inhibitors from the blue-green alga Microcystis aeruginosa (NIES-98). Tetrahedron Letters, 36:2785-2788.
4. Welker, M. & Von Döhren, H. (2006) Cyanobactcerial peptides – Nature’s own combinatorial biosynthesis. FEMS Microbiol Rev, 30:530-563.