ABSTRACT
Due to excellent control over stereochemistry and mild reaction conditions, aldolases exhibit unrivaled efficiency in the synthesis of carbohydrates and complex polyhydroxylated molecules, which are difficult to prepare and handle by conventional chemical synthesis. Among known aldolases, dihydroxyacetone phosphate (DHAP)-dependent aldolases are most intensively studied and widely used so far, as configurations of two newly generated stereogenic centers can be chosen and controlled by an appropriate choice of four known DHAP-dependent aldolases. (Machajewski et al., 2000; Brovetto et al., 2011). DHAP-dependent aldolases comprise fructose-1,6-bisphosphate aldolase (FruA), Fuculose-1-phosphate aldolase (FucA), Tagatose-1,6-bisphosphate aldolase (TagA) and Rhamnulose-1-phosphate aldolase (RhuA), which catalyze the reversible aldol addition reaction of DHAP to aldehyde acceptors. These four DHAP-dependent aldolases are stereocomplementary, thus a complete set of four stereoisomers could be accessed (Fig. 6.1). 6.2 Synthetic ApplicationsWith respect to synthetic application of DHAP-dependent aldolases, over 200 research papers have been published, ranging from synthesis of monosaccharides (Wong and Whitesides, 1994; Fessner, 2008; Bednarski et al., 1989), iminocyclitols (Whalem and Wong, 2006; Laborda et al., 2014; Concia et al., 2014; Sugiyama et al., 2010), higher carbon sugars (Lin et al., 2013; Jarosz, 2008), carbocycles
(El et al., 2004, 2006, 2009; Gijsen and Wong, 1995), and natural products (Fessner and Helaine, 2001; Chenevert et al., 1997; Phung et al., 2003) to active pharmaceutical intermediates (Charmantray et al., 2006a; Gomez et al., 2012a; Concia et al., 2014) (Fig. 6.2). More than 100 aldehydes have been used as acceptors for DHAP-dependent aldolases. These aldehydes can be unhindered aliphatic and aromatic aldehydes, α-heteroatom substituted aldehydes, azido aldehydes, protected amino aldehydes, monosaccharides, and their derivatives. The use of multienzyme systems and engineering of DHAP-dependent aldolases further broaden the substrate scope of aldehyde acceptors to unprecedented α, β-unsaturated aldehydes (Sanchez-Moreno et al., 2009) and complex synthetic
aldehydes (Laborda et al., 2014). Recent work on combining DHAP-dependent aldolase-mediated aldol reactions with other enzymatic transformations allows rapid construction of complex molecules with multiple stereogenic centers and diverse functionalities (Soler et al., 2014). For a comprehensive coverage of synthetic applications of DHAP-dependent aldolases, we refer the readers to previous reviews (Brovetto et al., 2011; Clapes and Garrabou, 2011; Samland and Sprenger, 2006). 6.3 DHAP GenerationHowever, the strict specificity toward donor substrate DHAP greatly hampers the large-scale synthetic utility of DHAP-dependent aldolases, due to high cost and lability of DHAP (Brovetto et al., 2011; Sugiyama et al., 2007). Therefore, effective production of DHAP is instrumental, and several chemical and enzymatic approaches have been developed for its synthesis. Chemical approaches focus on producing storable precursors that can be easily converted to DHAP immediately before its use (Fig. 6.3) (Bednarski et al., 1989; Colbran et al., 1967; Pederson et al., 1991; Jung et al., 1994; Gefflaut et al., 1997; Ferroni et al., 1999; Meyer et al., 2004; Charmantray et al., 2004; Meyer et al., 2006). However, they suffer from low yields, complicated work-up, or toxic reagents or catalysts (Schuemperli et al., 2007). Enzymatic approaches generate
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DHAP in situ and follow three general routes: phosphorylation of dihydroxyacetone (DHA) (Sanchez-Moreno et al., 2009; Wong and Whitesides, 1983; Crans and Whitesides, 1985; Yanase et al., 1995; Itoh et al., 1999; Sanchez-Moreno et al., 2004), oxidation of glycerol 3-phosphate (Fessner and Sinerius, 1994; Schoevaart et al., 2000; Hettwer et al., 2002; Charmantray et al., 2006b; Li et al., 2012), and mimicking glycolysis. (Bednarski et al., 1989; Fessner and Walter, 1992). Though enzymatic approaches start from cheap non-phosphorylated precursors (DHA, glycerol, sucrose), they employ multiple costly isolated enzymes (Fig. 6.4). Both chemical and enzymatic approaches require further improvement to serve as a basis for scalable and cost-effective production of DHAP. Another issue associated with large-scale synthetic utility of DHAP-dependent aldolases is that an additional step is required to remove phosphate group of aldol adducts, which necessitates the use of phosphatase and thus causes non-productive waste of phosphate component.
