Читать книгу Biomolecular Engineering Solutions for Renewable Specialty Chemicals - Группа авторов - Страница 45

Poly‐ϒ‐glutamic acid is a naturally occurring, abundant poly amino acid which is water‐soluble, anionic, biodegradable, and edible biopolymer produced primarily by Bacillus subtilis. Glutamic acid can be L‐ and D‐ in nature linked by ϒ‐amide bond (Cao et al., 2018). Due to the presence of ϒ‐amide linkage, it is resistant to cleavage by proteases (Candela and Fouet, 2006). It is gaining importance because of being the potential candidate for drug delivery, cryoprotectant, thickening agent, biopolymer flocculant, bioabsorption of heavy metals, etc. (Bhattacharyya et al., 1998; Shih and Van, 2001). It is also widely used in skin serums in combination with vitamin C to increase skin elasticity and making it smooth (Tanimoto, 2010; BEN‐ZUR and GOLDMAN, 2007). Predominantly, Bacillus sp., such as Bacillus licheniformis, Bacillus subtilis, B. megaterium, Bacillus pumilis, Bacillus mojavensis, and Bacillus amyloliquefaciens, are producer of PGA. It is synthesized in ribosome independent manner. Glutamate is the precursor of PGA, which is derived from glutamic acid biosynthesis or glutamate transportation. In turn, α‐keto glutaric acid serves as a precursor for glutamate in tricarboxylic acid cycle (TCA) also known as Krebs cycle. The reaction is catalyzed by glutamate dehydrogenase in absence of glutamic acid, while 2‐oxoglutarate aminotransferase in presence of glutamic acid (Stadtman, 1966; Holzer, 1969). Furthermore, the glutamate units are joined together to produce PGA through racemization, polymerization, and anchoring or releasing. This can be studied in detail in review by Najar and Das (2015). PGA synthetase (Pgs), a membrane associated enzyme polymerizes glutamate to PGA. Genes encoding Pgs are different in different Bacillus species. Like Pgs is encoded by four genes (pgsB, C, A, and E) in B. licheniformis and B. amyloliquefaciens, capB, C, A, and E, in B. anthraci, while in B. subtilis they are ywsC, ywtABC. Role of pgsBCA operon was shown in B. subtilis for PGA production. Mutants with disrupted pgsBCA did not showed PGA synthesis. While the xylose‐induced pgsBCA operon started PGA synthesis (Ashiuchi et al., 2006). Sometimes the overexpression of pgsBCA leads to decrease in PGA production as seen in mutants of B. amyloliquefaciens (Feng et al., 2015).

Under nutrient deprivation conditions PGA is sometimes hydrolyzed by PGA hydrolases. Deletion of PGA hydrolases can increase the yield of PGA. Depending on where the PGA hydrolase will cleave PGA, it can be (i) PgdS (the γ‐DL‐glutamyl hydrolase) cleaves the γ‐glutamyl bond between D‐ and L‐glutamic acids residues of γ‐PGA; (ii) the D/L‐endopeptidase hydrolyzes the peptide bond between the glutamic acid residues; and (iii) the γ‐glutamyltransferase (GGT) possesses a powerful exo‐γ‐glutamyl hydrolase activity. The GGT is different from PgdS and D/L‐endopeptidase as it hydrolyzes the γ‐PGA from the N‐terminal to release both D‐ and L‐glutamic acids (Cao et al., 2018).