Abstract
The global problem of microbial resistance to antibiotics has resulted in an urgent need to develop new antimicrobial agents. Natural antimicrobial peptides are considered promising candidates for drug development. Echinoderms, which rely on innate immunity factors in the defence against harmful microorganisms, are sources of novel antimicrobial peptides. This study aimed to isolate and characterise antimicrobial peptides from the Edible sea urchin Echinus esculentus. Using bioassay-guided purification and cDNA cloning, three antimicrobial peptides were characterised from the haemocytes of the sea urchin; two heterodimeric peptides and a cysteine-rich peptide. The peptides were named EeCentrocin 1 and 2 and EeStrongylocin 2, respectively, due to their apparent homology to the published centrocins and strongylocins isolated from the green sea urchin Strongylocentrotus droebachiensis. The two centrocin-like peptides EeCentrocin 1 and 2 are intramolecularly connected via a disulphide bond to form a heterodimeric structure, containing a cationic heavy chain of 30 and 32 amino acids and a light chain of 13 amino acids. Additionally, the light chain of EeCentrocin 2 seems to be N-terminally blocked by a pyroglutamic acid residue. The heavy chains of EeCentrocins 1 and 2 were synthesised and shown to be responsible for the antimicrobial activity of the natural peptides. EeStrongylocin 2 contains 6 cysteines engaged in 3 disulphide bonds. A fourth peptide (Ee4635) was also discovered but not fully characterised. Using mass spectrometric and NMR analyses, EeCentrocins 1 and 2, EeStrongylocin 2 and Ee4635 were all shown to contain post-translationally brominated Trp residues in the 6 position of the indole ring.
Highlights
Increasing numbers of pathogenic bacterial strains are becoming resistant to antibiotics
Four different solid phase extraction (SPE) eluates obtained from an aqueous extract and one organic extract of E. esculentus coelomocytes were tested for antibacterial activity
The analysis revealed that the peptide Ee4835 seems to have a heterodimeric structure, containing a heavy chain (HC) composed of 30 amino acids (Seq 5, Table 1) and a light chain (LC) composed of 13 amino acids (Seq 6, Table 1), possibly connected via a single disulphide bond
Summary
Increasing numbers of pathogenic bacterial strains are becoming resistant to antibiotics. In contrast to commercial antibiotics where the development of resistance is a problem, bacterial resistance towards AMPs is much less pronounced [1, 4]. Because of their propensity to be rapidly metabolised in the gastrointestinal tract, peptides have been considered poor drug candidates. This problem has diminished somewhat in recent years with the development of new synthetic strategies to improve bioavailability and reduce metabolism of peptides, and bolstered by the development of alternative routes of administration [5, 6]. Less studied, have proven to be a promising source for discovering AMPs with novel scaffolds [9]
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