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Bacillibactin is a catechol-based siderophore secreted by members of the genus '' Bacillus'', including ''
Bacillus anthracis
''Bacillus anthracis'' is a gram-positive and rod-shaped bacterium that causes anthrax, a deadly disease to livestock and, occasionally, to humans. It is the only permanent ( obligate) pathogen within the genus ''Bacillus''. Its infection is a ...
'' and '' Bacillus subtilis''. It is involved in the chelation of ferric iron (Fe3+) from the surrounding environment and is subsequently transferred into the bacterial cytoplasm via the use of ABC transporters
The ATP synthase, ATP-binding cassette transporters (ABC transporters) are a transport system superfamily that is one of the largest and possibly one of the oldest gene family, gene families. It is represented in all extant taxon, extant Phylum ...
.
Biosynthesis
The biosynthetic pathway of bacillibactin was first identified by May et al. in the Gram-positive ''B. subtilis
''Bacillus subtilis'', known also as the hay bacillus or grass bacillus, is a Gram-positive, catalase-positive bacterium, found in soil and the gastrointestinal tract of ruminants, humans and marine sponges. As a member of the genus ''Bacillus' ...
''. The siderophore is synthesized through multimodular non ribosomal peptide synthetases (NRPS), similar to enterobactin. However, unlike enterobactin, the genes responsible for encoding the bacillibactin synthetases are all located in one operon. This gene cluster is termed ''dhb'' – cognate to the catecholic structure of 2,3-dihydroxybenzoate (DHB) – and it can be divided into the specific genes responsible for encoding the enzymes. The three genes are ''dhbE, dhbB,'' and ''dhbF'', which get translated into DhbE, DhbB, and DhbF synthetases. Notably, DhbF was characterized as a dimodular NRPS, unlike the monomodular EntF synthetase for enterobactin.
The structure of bacillibactin consists of three 2,3-dihydroxybenzoate (DHB) groups attached to a cyclic amino acid core synthesized by multimodular NRPS. It is the condensation of three DHB- Glycine-Threonine
Threonine (symbol Thr or T) is an amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH form under biological conditions), a carboxyl group (which is in the deprotonated −COOâ ...
units that ultimately leads to the formation of bacillibactin. In the first step of NRPS, the relevant amino acid is adenylated and transferred to the thiol group of the adjacent synthetases. DhbE is selective for DHB, DhbF1 is selective for glycine, and DhbF2 is selective for threonine. DHB is first adenylated by DhbE and transferred to DhbB's thiol group in the second step of NRPS. Once the relevant compounds are thiolated, the construction of bacillibactin begins.
After DHB is transferred to DhbB, an adjacent synthetase orchestrates the condensation of DHB and glycine onto DhbF1. Then the DHB-Gly unit is further condensed onto the threonine unit on DhbF2, resulting in a DHB-Gly-Thr unit. This process is repeated twice more. However at the end of the third iteration, the hydroxyl group from the first threonine intramolecularly attacks the synthetase-ester bond to create the cyclic amino acid core for bacillibactin.
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References
{{Reflist Depsipeptides Catechols Siderophores