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-sheetantimicrobial 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]
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]
↑ Panchal RG, Smart ML, Bowser DN, Williams DA, Petrou S (June 2002). "Pore-forming proteins and their application in biotechnology". Current Pharmaceutical Biotechnology. 3 (2): 99–115. doi:10.2174/1389201023378418. PMID12022262.
1 2 Sokolov Y, Mirzabekov T, Martin DW, Lehrer RI, Kagan BL (August 1999). "Membrane channel formation by antimicrobial protegrins". Biochimica et Biophysica Acta (BBA) - Biomembranes. 1420 (1–2): 23–9. doi:10.1016/S0005-2736(99)00086-3. PMID10446287.
↑ Drin G, Temsamani J (February 2002). "Translocation of protegrin I through phospholipid membranes: role of peptide folding". Biochimica et Biophysica Acta (BBA) - Biomembranes. 1559 (2): 160–70. doi:10.1016/S0005-2736(01)00447-3. PMID11853682.
↑ Zerbe K, Moehle K, Robinson JA (June 2017). "Protein Epitope Mimetics: From New Antibiotics to Supramolecular Synthetic Vaccines". Accounts of Chemical Research. 50 (6): 1323–1331. doi:10.1021/acs.accounts.7b00129. PMID28570824.
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