Protegrins are small peptides containing 16-18 amino acid residues. Protegrins were first discovered in porcine leukocytes and were found to have antimicrobial activity against bacteria, fungi, and some enveloped viruses.^[1] The amino acid composition of protegrins contains six positively charged arginine residues and four cysteine residues.^[2] Their secondary structure is classified as cysteine-rich β-sheet antimicrobial peptides, AMPs, that display limited sequence similarity to certain defensins and tachyplesins. In solution, the peptides fold to form an anti-parallel β-strand with the structure stabilized by two cysteine bridges formed among the four cysteine residues.^[3] Recent studies suggest that protegrins can bind to lipopolysaccharide, a property that may help them to insert into the membranes of gram-negative bacteria and permeabilize them.^[4]

Structure

There are five known porcine protegrins, PG-1 to PG-5.^[5] Three were identified biochemically and rest of them were deduced from DNA sequences.^[6]

Protegrin structures

The protegrins are synthesized from quadiripartite genes as 147 to 149 amino acid precursors with a cathelin-like propiece.^[5][7] Protegrin sequence is similar to certain prodefensins and tachyplesins, antibiotic peptides derived from the horseshoe crab.^[1] Protegrin-1 that consists of 18 amino acids, six of which are arginine residues, forms two antiparallel β-sheets with a β-turn. Protegrin-2 is missing two carboxy terminal amino acids. So, Protegrin-2 is shorter than Protegrin-1 and it has one less positive charge. Protegrin-3 substitutes a glycine for an arginine at position 4 and it also has one less positive charge. Protegrin-4 substitutes a phenylalanine for a valine at position 14 and sequences are different in the β-turn. This difference makes protegrin-4 less polar than others and less positively charged. Protegrin-5 substitutes a proline for an arginine with one less positive charge.^[5]

Mechanism of action

Protegrin-1 induces membrane disruption by forming a pore/channel that leads to cell death.^[8][9] This ability depends on its secondary structure.^[10] It forms an oligomeric structure in the membrane that creates a pore. Two ways of the self association of protegrin-1 into a dimeric β-sheet, an antiparallel β-sheet with a turn-next-to-tail association or a parallel β-sheet with a turn-next-to-turn association,^[11] were suggested. The activity can be restored by stabilizing the peptide structure with the two disulfide bonds.^[12] The interacts with membranes depends on membrane lipid composition^[13] and the cationic nature of the protegrin-1 adapts to the amphipathic characteristic which is related to the membrane interaction.^[9] The insertion of Protegrin-1 into the lipid layer results in the disordering of lipid packing to the membrane disruption.^[13]

Antimicrobial activity

The protegrins are highly microbicidal against Candida albicans,^[14] Escherichia coli,^[15] Listeria monocytogenes, Neisseria gonorrhoeae,^[16] and the virions of the human immunodeficiency virus in vitro under conditions which mimic the tonicity of the extracellular milieu.^[1][5][17] The mechanism of this microbicidal activity is believed to involve membrane disruption, similar to many other antibiotic peptides ^[5][18]

Mimetics as antibiotics

Protegrin-1 (PG-1) peptidomimetics developed by Polyphor AG and the University of Zurich are based on the use of the beta hairpin-stabilizing D-Pro-L-Pro template which promote a beta hairpin loop structure found in PG-I. Fully synthetic cyclic peptide libraries of this peptidomimetic template produced compounds that had an antimicrobial activity like that of PG-1 but with reduced hemolytic activity on human red blood cells.^[19] Iterative rounds of synthesis and optimization led to the pseudomonas-specific clinical candidate Murepavadin that successfully completed phase-II clinical tests in hospital patients with life-threatening Pseudomonas lung infections. ^[20]

References