Structural features
Penaeidins have been initially characterized from Litopenaeus vannamei by biochemical approach and molecular cloning. The three peptides (Litvan PEN2-2, -2-1 and -3-1; initially named Pen-1,-2,-3) were isolated in their active and mature form (5.48 – 6.62 kDa) from the hemocytes of animals collected from intensive shrimp farms in Ecuador (Destoumieux et al., 1997). The penaeidins are highly cationic molecules composed of a N-terminal proline-rich domain, followed by a C- terminal domain containing 6 cysteine residues organized in two doublets. This overall structure is quite unique among the AMP families and this originality has led to the identification of the new family of penaeidins. Very recently, the solution structure of Litvan PEN3-1 and Litvan PEN4-1 has been determined revealing the overall organization of the two domains and the arrangement of the disulfide bonds (Yang et al., 2003).
The N-terminal domain is unstructured, in contrast to the C-terminal domain that is shown to be a highly constrained and well-structured domain. The structure of the peptide suggests that the two domains of the penaeidin may have different and complementary properties, which may contribute to a multifunctional character of the peptides.
Penaeidins display post-transcriptional modifications. Litvan PEN2-1 and -3-1 are C-terminally amidated. The role of this modification is not yet clearly understood. It would be a common feature of peptides constitutively present and stored in hemocytes. This modification is also observed in peptides forming alpha-helices where the amidation would enhance the helical structure and may consequently have a significant contribution to the electrostatic interaction between the peptide and the microorganism membrane (ref). When penaeidins are not C-terminally amidated, their antifungal activity is not affected whereas the antibacterial activity is reduced two-fold (Destoumieux et al., 1999). In addition, Litvan PEN3-1 is blocked at the N-terminus by a pyroglutamic acid resulting from the cyclization of a glutamine residue.
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