Towards a structure-function analysis of bovine lactoferricin and related tryptophan- and arginine-containing peptides

Abstract

The iron-binding protein lactoferrin is a multifunctional protein that has antibacterial, antifungal, antiviral, antitumour, anti-inflammatory, and immunoregulatory properties. All of these additional properties appear to be related to its highly basic N-terminal region. This part of the protein can be released in the stomach by pepsin cleavage at acid pH. The 25-residue antimicrobial peptide that is released is called lactoferricin. In this work, we review our knowledge about the structure of the peptide and attempt to relate this to its many functions. Microcalorimetry and fluorescence spectroscopy data regarding the interaction of the peptide with model membranes show that binding to net negatively charged bacterial and cancer cell membranes is preferred over neutral eukaryotic membranes. Binding of the peptide destabilizes the regular membrane bilayer structure. Residues that are of particular importance for the activity of lactoferricin are tryptophan and arginine. These two amino acids are also prevalent in "penetratins", which are regions of proteins or synthetic peptides that can spontaneously cross membranes and in short hexapeptide antimicrobial peptides derived through combinatorial chemistry. While the antimicrobial, antifungal, antitumour, and antiviral properties of lactoferricin can be related to the Trp/Arg-rich portion of the peptide, we suggest that the anti-inflammatory and immunomodulating properties are more related to a positively charged region of the molecule, which, like the alpha- and beta-defensins, may act as a chemokine. Few small peptides are involved in as wide a range of host defense functions as bovine and human lactoferricin.

β-Sheet Pore-Forming Peptides Selected from a Rational Combinatorial Library: Mechanism of Pore Formation in Lipid Vesicles and Activity in Biological Membranes

“…The disulfide cross linked, antimicrobial β-hairpin peptide lactoferricin retains its antimicrobial biological activity even after 19 of its 25 residues, including all of the disulfide cross links are removed, requiring only a core linear hexapeptide that is as active as the 25-residue disulfide-cross linked parent β-hairpin (55). In fact many derivatives of lactoferricin are highly active antimicrobial peptides (56)(57)(58). Disulfide cross linked variants of our FSKRGY have antimicrobial activity that is indistinguishable from the non-cross linked peptide (JRM and WCW, unpublished observations).…”

Length Effects in Antimicrobial Peptides of the (RW)nSeries

“…The role of RW-rich motifs in AMPs has been investigated using quantitative-structure-activity relationships (QSAR) (24,45,46,53). Truncated peptide sequences from indolicidin and lactoferricin with conserved RW motifs retain antimicrobial activity even when their original secondary structure is lost (45,53), suggesting that R and W content alone correlates with activity.…”

“…The role of RW-rich motifs in AMPs has been investigated using quantitative-structure-activity relationships (QSAR) (24,45,46,53). Truncated peptide sequences from indolicidin and lactoferricin with conserved RW motifs retain antimicrobial activity even when their original secondary structure is lost (45,53), suggesting that R and W content alone correlates with activity. QSAR analysis of the antimicrobial activities of R and W peptides suggests that charge and multiple W side chains are necessary, while W can be replaced by analogs with bulkier side chains (24,46,48).…”

“…The guanidinium group of R has a more dispersed positive charge than the single amine of K, possibly enhancing electrostatic interactions between peptides and the negatively charged bacterial membrane surface (39,53). On the other hand, the bulkier W side chain may ensure more efficient interaction with membrane surfaces, allowing peptides to partition in the bilayer interface, in contrast with other nonpolar side chains such as F, P, or Y (46,56).…”

“…On the other hand, the bulkier W side chain may ensure more efficient interaction with membrane surfaces, allowing peptides to partition in the bilayer interface, in contrast with other nonpolar side chains such as F, P, or Y (46,56). Recently, consideration of electrostatic effects, including dipole and quadrupole moments of R and W side chains, respectively, suggests that these properties may be involved in the ability to form hydrogen bonds once peptides associate with membranes (27,53,58).…”