ABSTRACT
MicroorganismsThe accessibility of an adequate amount of oligosaccharides is prerequisite to investigate their biological functions and to exploit their potential as medicaments. The enzymatic synthesis of oligosaccharides possesses several aspects of advantages over chemical synthesis, including straightforward process without multi-step protection and deprotection reactions, higher regio-and stereo-selectivity, and so on (Palcic, 1999). However, the production of enzymes is generally a laborious process and an obstacle for in vitro enzymatic synthesis. Furthermore, in most cases, glycosyltransferases of the Leloir pathway are used and sugar nucleotides are required for their reactions as donor substrates (Palcic, 1999). To avoid the strenuous work of the production of enzymes and sugar nucleotides, genetically engineered microorganisms harboring genes encoding all
necessary enzymes for the synthesis or recycle of sugar nucleotides and the formation of glycosidic bonds were used as biocatalysts or cell factories instead. The proof-of-principle experiment of this technology has first been performed by Samain and coworkers (Samain et al., 1997). In their work, gene nodC and nodB from Azorhizobium caulinodans, which encode a chitooligosaccharide synthase (NodC) and a chitooligosaccharide N-deacetylase (NodB), respectively, were heterologously expressed in Escherichia coli DH1 strain. By high-cell-density cultivation of this recombinant strain, penta-N-acetyl-chitopentaose and tetra-N-acetyl-chitopentaose were produced with a yield up to 2.5 g per liter of culture medium (Fig. 15.2).
