Читать книгу Biomolecular Engineering Solutions for Renewable Specialty Chemicals - Группа авторов - Страница 44

Cellulose is the major biopolymer found in nature and commercially extracted from wood (Klemm et al., 2005). It is homopolysaccharide of glucose linked by β‐1,4 glycosidic bonds (Cannon and Anderson, 1991). In 2015, the value of cellulose market calculated was US$20.61 billion and is estimated to reach US$48.37 billion by 2025 (Cellulose Fiber Market Size and Share Industry Report, 2014–2025 market [Accessed 20 December 2017]). To meet such high demands overcutting of trees are being done leading to imbalance in nature and global warming. Bacterial cellulose (BC) is the alternative and green approach toward it which is biodegradable, nontoxic, and biocompatible. Apart from this, BC can be used directly in its native form as it is free from contaminants such as lignin, pectin, hemicellulose, and other constituents of lignocellulosic materials (Rahman and Netravali, 2016). There are several bacterial strains that synthesize BC specially the acetic acid bacteria group as they are generally recognized as safe (GRAS) such as Komagataeibacter xylinus, Komagataeibacter hansenii, Komagataeibacter medellinensis, Komagataeibacter nataicola, Komagataeibacter oboediens (Škraban et al., 2018; Castro et al., 2012,2013). Bacterial cellulose synthase (Bcs) is the primary enzyme for cellulose synthesis which adds glucose monomers to the growing chain. There are two subunits BcsA and BcsB necessary for the formation of polysaccharide chain (Römling and Galperin, 2015). Commercial usage of BC and low yields from the strains producing BC lead to the development of microbial cell factories. These cell factories overexpress genes essential for BC like cmc (carboxymethylcellulose), ccp (cellulose complementing factor protein), cesAB, cesC, cesD, bgl, bcsABCD (BC synthase operon), etc. into the host organism to increase the yields and crystallinity of BC.

Figure 1.2 Representative chemical structures of biopolymers.

Gluconacetobacter xylinus six genes cmc–ccp–cesAB–cesC–cesD–bgl were overexpressed in Synechococcus sp. PCC 7002 and resulted in very high‐yield production of extracellular type‐I cellulose (Zhao et al., 2015). BC synthase operon (bcsABCD) from Gluconacetobacter hansenii was engineered in E. coli with its upstream operon (cmcax and ccpAx) giving BC of 1000–3000 μm and a diameter of 10–20 μm (Buldum et al., 2018). Cellulose synthase D subunit (bcsD) increases the crystallinity structure of BC, but not the yield. G. xylinus bcsD when engineered in E. coli synthesizes BC with high crystallinity. FTIR results showed crystallinity index of 0.84, which was 17% more than the wild‐type strain (Sajadi et al., 2017). Oxygen plays an important role in the synthesis of BC. G. xylinus was engineered with Vitreoscilla hemoglobin (VHb)‐encoding gene vgb. vgb help cells to grow in hypoxic conditions. The mutant strains showed significant increase in BC in oxygen tension also (Liu et al., 2018). Recently, Komagataeibacter sp. nov. CGMCC 17276 genome was completely sequenced. BC operon genes were aligned with other BC producers to find sequence similarity. Apart from this growth rate, substrate utilization, BC production, etc. were evaluated. This gives future opportunity to engineer this strain for better BC synthesis (Jang et al., 2019).