Lantibiotics

Dehydroalanine (Dha) and dehydro-a-aminobutyric acid, a putative dehydration product of threonine occur accumulated in a particular class of antibiotic fungal peptides recently named lantibiotics of which Nisin (Fig. 26) will be mentioned first. The structure of the 34-peptide as revealed by Erhard Gross and coworkers in the beginning of the seventies, contains three dehydro side chains, RCH=C(NH—)CO—, and five thioether amino acids, lanthionine, Ala-S-Ala and its homolog Abu-S-Ala. Total synthesis by the group of Tetsuo Shiba (Plate 43) [61]. 

The thioether moieties are supposedly generated by addition of the thiol groups of cysteines to one of the unsaturated side chains formed by H2O-elimination in the peptides after their biosynthesis (Fig. 27). 

Quite recently it has been found that the sequence of 21 amino acids of a lantibiotic, epidermin from Staphylococcus epidermidis is encoded on a structural gene in the bacterium as a 52-amino-acid pre-peptide which is processed to the 21-peptide amide antibiotic. The actual sequence of epidermin contains the precursor amino acids Ser, Thr and Cys, from which the unusual amino acid constituents are derived [62]. This ribosomal mechanism is a very surprising observation since the accepted view so far has been that microbial peptides are usually biosynthesized by the thiotemplate pathway (p. 208). 

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Nisin acts bactericidally primarily against gram-positive bacteria. It acts best at acid pH and is almost insoluble at physiological pH. Nisin and probably all lantibiotics appear to permeabilize the bacterial ceU membrane to release small molecules, resulting in an immediate coUapse of the membrane... 

Notwithstanding the aforementioned difficulty in detecting specific target proteins other than the types normally observed in the taxonomic fingerprints from whole bacteria MALDI spectra (i.e., ribosomal proteins), some other target proteins and protein-like materials have been studied directly from whole cells. For example, Lantibiotics, antimicrobial peptides secreted by Gram-positive bacteria have been detected directly from whole bacteria by MALDI-TOF MS.51 The lantibiotics nisin and lacticin 481 were detected from whole cells and crude supernatants. Surprisingly, better results were reported from whole cells than the supernatants. In this study the presence of variants... 

Potential therapeutic applications of host defense peptides also include the lantibiotic nisin. Indeed, nisin has had an impressive history as a food preservative with FDA approval in 1988 for use in pasteurized, processed cheese spreads. The attractiveness of nisin as a potential therapeutic is also enhanced due to its relative resistance to proteases and broad spectrum Gram-positive antimicrobial activity including multidrug-resistant strains. Biosynexus Inc. has licensed the use of nisin for human clinical applications and Immucell Corp. has licensed the use of Mast Out, an antimastitic nisin-containing product, to Pfizer Animal Health." Indeed, nisin formulations have been used as an active agent in the topical therapies Mast Out and Wipe-Out for bovine mastitis, an inflammatory disorder of the udder that is the most persistent disease in dairy cows." ... 

Figure 2 Representative examples of the three classes of lantibiotics. The same color-coding and shorthand notation is used as defined in Figure 1. For Lan and MeLan structures, the segments derived from Ser/Thr are in red whereas those derived from Cys are in blue. The ring numbering is shown for some members and is typically alphabetical from the N- to C-terminus.

Figure 4 The biosynthesis of nisin A as a representative example of the posttranslational maturation process of lantibiotics. Following ribosomal synthesis, NisB dehydrates serine and threonine residues in the structural region of the prepeptide NisA. NisC subsequently catalyzes intramolecular addition of cysteine residues onto the dehydro amino acids in a stereo- and regioselective manner. Subsequent transport of the final product across the cell membrane by NisT and proteolytic cleavage of the leader sequence by NisP produces the mature lantibiotic. For the sequence of the leader peptide, see Figure 6. Adapted with permission from J. M. Willey W. A. van der Donk, Annu. Rev. Microbiol. 2007, 61, 477-501.

An alternative classification scheme was introduced in 2002 and this divides lantibiotics into two subgroups, class I and class II. This scheme primarily classifies lantibiotics according to their biosynthetic enzymes and sequence homology of their leader peptides. Similar to most biosynthetic pathways in bacteria, the genes for lantibiotic biosynthesis are clustered. They have been designated the generic locus symbol Ian, with a more... 

Given the large number of structurally similar natural variants of lantibiotics, they have also been grouped... 

Figure 6 Sequence alignment of lantibiotic and nonlantibiotic bacteriocin prepeptides. The residues in red indicate those positions that are fully conserved within that class, and those in blue are highly conserved. For the nonlantibiotic bacteriocins, only the leader sequences are shown. The site of proteolysis is indicated by the arrow. For cytolysin, the additional six residues removed by CylA are indicated in green.

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