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Halovir^[1][2][3] refers to a multi-analogue compound belonging to a group of oligopeptides designated as lipopeptaibols (chemical features including lipophilic acyl chain at the N-terminus, abundant α-aminoisobutyric acid content, and a 1,2-amino alcohol located at the C-terminus)^[4] which have membrane-modifying capacity^[5] and are fungal in origin.^[1][6] These peptides display interesting microheterogeneity;^[5] slight variation in encoding amino acids gives rise to a mixture of closely related analogues and have been shown to have antibacterial/antiviral properties.^[2][6]

Nonribosomal peptides compose a significant group of secondary metabolites in bacterial/fungal organisms (though Drosophila melanogaster and Caenorhabditis elegans both exhibit products of nonribosomal peptide synthetases);^[7][8][9] having been observed functioning as self-defense substances/iron-chelating siderophores, they serve as coping mechanisms for environmental stress, perform as virulence factors/toxins promoting pathogenesis, and act in signalling (enabling communications within and between species).^[7][8][9] In lieu of these functionalities, many nonribosomal peptides have been utilized in development of medical drugs and biocontrol agents (examples of such include β-lactams, daptomycin, echinocandins, emodepside, bleomycin, cyclosporine, and bialaphos).^[7][10][11]

Peptaibols are a family of linear, amphipathic polypeptides (typically consisting of 4-21 amino acids residues)^[7] that are generated as a result of the assembly of a variety of aminoacyl, ketoacyl or hydroxyacyl monomers by fungal multimodular megaenzymes denoted as nonribosomal-peptide synthetases (NRPSs).^[12] Typically, NRPSs are composed of three highly conserved core domains: an adenylation (A) domain which recognizes, activates and loads monomers onto NRPS, a thiolation (T) domain (also denoted as the peptidyl carrier protein domain) that transports covalently linked monomers/peptidyl intermediates between nearby NRPS domains, and a condensation (C) domain (catalyzes sequential condensation of monomers within the nascent peptide chain).^[7][10][13] In addition, a chain-terminating domain [thioesterase (TE) domain, a terminal C (C_T) or a reductase (R) domain] is commonly observed at the end of an NRPS in order to relinquish full-length peptide chains in linear or cyclic forms. Furthermore, often seen are feature tailoring domains [epimerase, N-methyltransferase (M), oxidase (Ox), ketoacyl reductase (KR) and cyclase (Cy)], allowing for further modification of monomers/polypeptide intermediates.^[7][10][13]

Notable characteristics of peptaibols include: C-terminal alcohol residues (phenylalaninol, leucinol, isoleucinol, valinol),^[7][14] an N-acyl terminus (usually acetyl),^[14] and high levels of α,α-dialkylated non-proteinogenic amino acids [α-aminoisobutyric acid (Aib), isovaleric acid (Iva), hydroxyproline (Hyp)].^[14][15] In most cases, peptaibols form α-helix and β-bend patterns in their 3D structures (α-aminoisobutyric acid is a turn/helix forming agent).^[4] α,α-dialkylated amino acid residues in peptaibols create substantial conformation constrictions in the peptide backbone, resulting in the formation of right-handed α-helical structures.^[16][17] Membrane modification abilities can be attributed to the formation of transmembrane voltage-dependent channels;^[14][18][19] this occurs as the peptide takes on an α-helical conformation upon contact of lipid bilayers, drilling through and forming ion channels with similar electrophysiological configurations of ion channel proteins.^[19][20] The principle functionality of the peptides is to rupture membranes, in turn triggering cytolysis via loss of osmotic balance.^[20]

Structurally speaking, lipopeptaibols are peptaibols with a fatty acyl moiety linked to the N-terminal amino acid^[4] (thusly named), and have been isolated from a number of soil fungi. Their primary structures all have the L-(S-) configuration at the 2-(α-)carbon.^[4] They overwhelmingly display microheterogeneity (being very structurally similar; with a limited pool of conserved variation in natural sample).

${\displaystyle {\ce {C45H83N7O9}}}$-Halovir A: contains L-leucine, L-valine, and L-glutamine

${\displaystyle {\ce {C43H79N7O9}}}$-Halovir B: contains L-alanine, L-leucine, L-glutamine

${\displaystyle {\ce {C45H83N7O8}}}$-Halovir C: contains L-leucine, L-valine, L-glutamine,

${\displaystyle {\ce {C43H79N7O9}}}$-Halovir D

${\displaystyle {\ce {C43H79N7O8}}}$-Halovir E

${\displaystyle {\ce {C42H77N7O8}}}$-Halovir I

${\displaystyle {\ce {C44H79N7O9}}}$-Halovir J

${\displaystyle {\ce {C43H78N7O9}}}$-Halovir K

The antibiotic capabilities of these compounds can be attributed to membrane insertion and pore-forming functionalities,^[12] and typically exhibit antimicrobial activity in Gram-positive bacteria and fungi.^[14][22]

Halovirs A-E (isolated from Scytidium sp.) has displayed potent antiviral activity against HSV-1, and has been observed performing replication inhibition of HSV-1 and HSV-2^[2] in standard plaque reduction assay without cytotoxicity (at concentrations upwards of 0.85μM)^[2] Mechanistic studies suggest that halovirs kills virus in direct contact and in a time-dependent manner before it can affect host cells.

Halovirs I and J were analyzed for antibacterial and cytotoxic activities, and displayed significant growth inhibition against two Gram-positive bacteria (Staphylococcus aureus and Enterococcus faecium), but not Gram-negative Escherichia coli.^[7] Notably, these two halovirs were found to be effective against methicillin-resistant S. aureus (MRSA) and vancomycin-resistant E. faeccium, indicating that the activity against them to be persistent.^[7] Additionally, strong cytotoxic activity was observed against a panel of cancer cell lines, including: human lung carinoma A549, human breast carcinoma MCF-7, and human cervical carcinoma HeLa cells (cytotoxicity was not specific to cancer cells in the referenced study).^[7]