Tachyplesin I (TPI) is an antimicrobial polypeptide originally detected in the leukocytes of Japanese Horse Shoe Crab.
The antimicrobial activity of the peptide is closely related to the composition of the pathogen membrane and ability of the peptide to permeabilize the cell membranes. Bacteria and fungi have negatively charged membranes, and the interaction of is mediated in large part by electrostatic interactions[1] (you can see the Hydrophobic and Polar amino acids).
It shows high affinity for lipopolysaccharides (LPS) of gram-negative bacteria, thus neutralizing its effects. It has also been reported to inhibit the growth of gram positive bacteria, fungui and viruses.
Structural highlights
Tachyplesine I is a 17-residue peptide containing six cationic residues.
It's molecular weight is 2,269 and has pI of 9.93. [2]
The amino acid sequence of the TPI is NH₂-Lys-Trp-Cys-Phe-Arg-Val-Cys-Tyr-Arg-Gly-Ile-Cys-Tyr-Arg-Arg-Cys-Arg-CONH₂.
It adopts antiparallel β-sheet (hairpin) conformation in solution stabilized by two cross-strand between Cys³-Cys¹⁶ and Cys⁷-Cys¹², and its C-terminus is amidated.[1][3]
Tachyplesin is highly stable at low pH and high temperature. This stability seems to be due to the rigid structure imposed by the two disulfid linkage.[4]
Besides, there exists H-bond and aromatic ring stacking interactions which helps stabilizing the hairpin loop structure of the peptide.
Along with TPI, there exists three linear derivatives: , TPF4 and TPA4 as shown below.
Image:Derivatives.jpg
Of those 3 linear derivatives of TPI, TPA4 was inactive which was due to its incapability to form hairpin loop structure. This guided to the conclusion that linear tachyplesin analogues do not show preferential affinity for LPS. Therefore, the hairpin properties of the peptide seems to be important for recognition of lipopolysaccharides and its biological activities.
CDT is a TP I mutant in which all Cys residues are deleted.
TPI undergoes confirmation change in . The backbone of the polypeptide becomes , making it more stable.
Mode of action
TPI can bind to LPS and also has ability to permeabilize the cell membrane of pathogens. Docking model suggests strong affinity to LPS gained by interaction between cationic residues of TPI with phosphate group and sachharides of LPS. Furthermore, interaction between hydrophobic residues of TPI with acyl chains of LPS strengthens the TPI/LPS interaction. The binding of TPI to LPS neutralizes LPS, which is widely considered as endotoxin. In addition to LPS binding, footpriting analysis has revealed the binding of TPI to DNA by interacting specifically in minor groove of DNA duplex. The interaction between TPI and DNA is contributed by secondary structure of the peptide which contains an antiparallel beta-sheet constrained by two disulfide bridges and connected by β-turn.
Importance and relevance
Evidences suggest that TPI has ability to permeabilize the cell membranes of pathogens.[1]. Also, LPS and DNA being the potential biological targets of the peptide, its antimicrobial activity might be exploited. Eyeing the potential of TPI, it has been insetred successfully in genome of Ornithogalum dubium and Ornithogalum thyrsoides. These ornamentals plants were originally sensitive to soft rot erwinias (SREs) and insertion of TPI in the plants has successfully protected them without affecting their normal physiology.
Function
The cationic nature of tachyplesin allows it to interact with anionic phospholipids present in the bacterial membrane and thereby disrupt membrane function.
The structural nature of tachyplesin suggested that it might also posses antitumor properties. Since it can interact with the membrance of prokaryotic cell, it is likely that TP I can also interact with the mitochondrial membrane of eukaryotic cells.
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