Enzymes in the synthesis of monosaccharides and their analogs
Aldolases have immense utility in the production of monosaccharides and their derivatives, and synthetic applications have been reviewed extensively (Wong and Whitesides, 1994; Gijsen et al., 1996; Fessner, 1998; Machajewski and Wong, 2000). The use of pyruvate-dependent aldolases such as NeuAc aldolase has allowed the preparation of various NeuAc derivatives, including imino-sugars (Zhou et al., 1993; Fitz et al., 1995). The NeuAc aldolase reaction is a substrate-control process, i.e., using L-sugars as substrates, L-NeuAc derivatives are obtained. However, employing the technique of directed evolution, pyruvate-dependent D-KDPG aldolase has been altered to a KDG aldolase which accepts both L- and D-glyceraldehyde as substrate with the same facial selectivity (Fong et al., 2000). Thus, the scope of enzymatic aldol addition reactions can be extended through enzyme engineering.
The acetaldehyde-dependent aldolase DERA is unique in that it catalyzes the condensation of two aldehydes. DERA-catalyzed reactions have provided many unnatural sugar derivatives, including iminocyclitols and deoxy sugars. DERA has also been employed in tandem with either FDP aldolase or NeuAc aldolase to afford novel deoxysugars and deoxy-NeuAc derivatives (Gijsen and Wong, 1995).
DHAP-dependent aldolases have been utilized for the synthesis of disaccharide mimetics (Eyrisch and Fessner, 1995), as well as carbohydrates containing 13C labels, heteroatoms, and deoxygenated sites. Furthermore, the condensation of DHAP with pentoses or hexoses affords NeuAc and KDO analogs, and iminocyclitols and L-sugars can also be readily obtained (Wong et al., 1995a; Moris-Varas et al., 1996; Takayama et al., 1997). Recently, a four-enzyme one-pot synthesis of 5-deoxy-5-ethyl-xylulose utilizing FDP aldolase as one of the catalysts has been reported (Schoevaart et al., 1999). Although the enzymes employed in the scheme had a range of pH optima, enzyme activities were controlled by variation of pH over the course of the reaction.
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