Tagatose synthetic

Four DHAP converting aldolases are known, these can synthesize different diastereomers with complementary configurations D-fructose (FruA EC 4.1.2.13) and D-tagatose 1,6-bisphos-phate (TagA, F.C 4.1.2.-), L-fuculose (FucA EC 4.1.2.17) and L-rhamnulose 1-phosphate aldolase (RhuA EC 4.1.2.19)3. The synthetic application of the first (class 1 or 2) and the latter two types (class 2) has been examined. 

While the lyases that transfer a pyruvate unit form only a single stereogenic center, the group of dihydroxyacetone-phosphate-(DHAP, 41)-dependent aldolases create two new asymmetric centers, one each at the termini of the new C-C bond. A particular advantage for synthetic endeavors is the fact that Nature has evolved a full set of four stereochemically-complementary aldolases [189] (Scheme 6) for the retro-aldol cleavage of diastereoisomeric ketose 1-phosphates— D-fructose 1,6-bisphosphate (42 FruA), D-tagatose 1,6-bisphosphate (43 TagA), L-fuculose 1-phosphate (44 FucA), and L-rhamnulose 1-phosphate (45) aldolase (RhuA). In the direction of synthesis this formally allows the... 

In this section, a highly efficient synthetic system for the preparation of 2-C-hydroxymethylaldopentoses is discussed. The reactions of four different keto-hexoses (o-psicose, D-fructose, L-sorbose and D-tagatose) were studied imder various conditions in order to elucidate the relationship between the two key processes, the coordination of the sugar to the nickel complex, and the rearrangement of the carbon skeleton of the substrate ketose. The system was then compared with that of the epimerization of the aldose/nickel(II)/ethylenediamine complex. 

Apparently, all DHAP aldolases are highly specific for the donor component 22 for mechanistic reasons [29]. For synthetic applications, two equivalents of 22 are conveniently generated in situ from commercial fructose 1,6-bisphosphate 23 by the combined action of FruA and triose phosphate isomerase (EC 5.3.1.1) [93,101]. The reverse, synthetic reaction can be utilized to prepare ketose bisphosphates, as has been demonstrated by an expeditious multienzymatic synthesis of the (3S,4S) all-cis-configurated D-tagatose 1,6-bisphosphate 24 (Fig. 13) from dihydroxyacetone 27, including a cofactor-dependent phosphorylation, by employing the purified TagA from E. coli (Fig. 13) [95,96]. 

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