Defensin-like Peptides
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C-Type Natriuretic Peptides


       Photo courtesy of
     Besides CNP, there are several other interesting protein components that can be found in platypus venom. A class of four peptides which are “unique in that their primary structures (amino acid sequence) do not share strong resemblance” with any other proteins currently known to man have been isolated in platypus venom. These special peptides are called defensin-like peptides (DLPs) because DLP-1 has a three-dimensional structural fold (overall peptide shape) that bears a strong similarity to b-defensin-12 (Torres et al., 2000). A peptide found in mammalian white blood cells, b-defensin-12 exhibits antimicrobial activity against E.coli and Staphylococcus aureus (Torres et al., 1999). However, DLP-1, unlike b-defensin-12, does not show any antimicrobial activity. Torres et al. (1999) attribute this difference in biological function to key differences in fine structural details, namely the positioning of several cationic (positively charged) and anionic (negatively charged) residues in each protein.
                                                                            Figure 6: Tertiary structure of DLP-1 (left) and DLP-2 (right). 
                                                                                              Photo courtesy of
     Torres et al (1999) also determined that the tertiary protein structure of DLP-1 (Fig. 6) is similar to the protein structure of ShI, a sea anemone toxin. This similarity in peptide shape is due to the common disulphide-connectivity pattern between DLPs and ShI. However, experiments show that DLPs do not function in the same manner as ShI, which is known to interfere with sodium channel activity in ganglion (nerve) cells. This difference in biological function could be due to the fact that the amino acid sequence of DLPs and ShI bear little resemblance (Torres et al., 1999). Although DLPs do not share the same biological function as b-defensin-12 or ShI, the similar tertiary shape of all three proteins suggests that this distinct structural peptide core has evolved as an effective way to yield small compact molecule displaying many pharmological and physiological activities (Torres et al., 2000).

     The biological role of DLPs in platypus venom is yet to be determined. One possible explanation is that the DLPs act synergistically with nerve growth factor (NGF) to produce pain, the prominent symptom of platypus envenomation. NGF has been shown to increase pain sensitivity in humans for three to four weeks. The nature and time course of these symptoms bear a striking resemblance to the reported symptoms of platypus envenomation (de Plater et al., 2001). However, more research must be done on these novel components of platypus venom before a definitive understanding of their pharmacology can be attained. 

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