Membrane Interactions of Novicidin, a Novel Antimicrobial Peptide:
Entry by Angelo Mao, AP 225, Fall 2010
Title: Membrane Interactions of Novicidin, a Novel Antimicrobial Peptide: Phosphatidylglycerol Promotes Bilayer Insertion
Authors: Dorosz J, et al
Journal: J. Phys. Chem. B
Volume: Vol 114(34)
Novicidin is a novel antimicrobial peptide that is an improvement on certain preexisting antimicrobial peptides (such as melittin) in that it more effectively targets bacteria and causes less hemolysis. Through various experiments, the researchers propose that an explanation for this difference is the relative proportion of hydrophobic to hydrophilic elements in novicidin composition, as well as the locations of these elements, which lowers the free energy of novicidin interaction and embedding in negative (bacterial) membranes in comparison to zwitterionic (human) membranes.
soft matter keywords: phosphilipid, membrane, hydrophobic interactions, hydrophilic interactions, free energy
Novicidin is a peptide derived from a previous antimicrobial candidate, ovispirin, which unfortunately was highly cytotoxic. As shown in figure 1, novicidin is a generally positively charged peptide. The helical wheel projection of novicidin indicates where the positively charged amino acids, lysine and arginine (in gray), are localized. The hydrophobic residues (green and yellow) are situated on the other side of the wheel, leading to a hydrophobic moment (arrow) in the novicidin molecule. From this, one would expect that novicidin would be able to embed itself in phospholipid membranes with its positive charges facing outside and its hydrophobic elements facing the hydrophobic core of the membrane.
Novicidin incorporation into membranes of varying charge negativities
The researchers created phospholipid monolayers with varying amounts of a zwitterionic phosopholipid called dimyristoylphosphatidylcholine (DMPG) and negatively charged phospholipid called dimyristoylphosphatidylglycerol(DMPG). The line (iv) in figure 3 indicates the result from the membrane that is entirely composed of DMPG, while the line (i) indicates the result from membrane without any DMPG. Adsorption of novicidin into the membrane increased with DMPG composition, starting from a baseline of near zero adsorption. The researchers performed a similar set of experiments with vesicles of varying amounts of zwitterionic phospholipid/polydiacetylene (PDA) and DMPG. In this other set of experiments, the results of which are shown in figure 4, fluorescent materials leaking from the vesicles were quantified. The results showed positive correlation between DMPG composition and fluorescence intensity. The implication is that novicidin incorporation is not only correlative with the present of negatively charged components in the membrane, but that this incorporation is also correlative to membrane disruption.
Novicidin penetration into membranes of varying charge negativities
Novicidin has a tyrosine residue, a fact which the authors utilized in tyrosine fluorescence titration experiments. As shown in figure 5, there was little fluorescence in zwitterionic DMPC vesicles (x in figure 5), but increased fluorescence in vesicles with mixed composition. This indicated very little incorporation of novicidin in DMPC vesicles and increased incorporation in vesicles with DMPG. This experiment seemed primarily to confirm the results of figures 3 and 4, and seems also to be quantitatively inferior, due to the quick saturation of tyrosine.