Rhamnulose

It is well known that the 1-phosphates of the ketoses, L-fuculose (51) and L-rhamnulose (52) have considerable biochemical interest. Their chemical synthesis has not been described as far as is known to the writer, but the rate of acid hydrolysis of L-fuculose 1-phosphate, obtained by enzymatic synthesis, has been determined by Heath and Ghalambor (20) and that of L-rhamnulose 1-phosphate by H. Sawada (48) and by Chiu and Feingold (II). They found that the rate of... 

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. 

A solution of 2.25 g (25 mmol) of D-glyccraldehyde in 300 mL of water is combined with a solution of 20 mmol of dihydroxyacetonc phosphate (DIIAP) in 200 mL of water freshly adjusted to pH 6.8. The mixture is incubated with 100 U of L-rhamnulose 1-phosphate aldolase at r.t. for 24 h with monitoring of conversion by TLC (2-propanol/sat. ammonia/water 6 4 2) and by enzymatic assay for DHAP55. 

In the case of L-rhamnulose-1-phosphate aldolase (RhaD), we found that the problem of phosphorylated substrate requirement (dihydroxyactone phosphate (DHAP)) could be overcome by a simple change in buffer. Thus, when using borate buffer, reversible borate ester formation created a viable substrate out of dihydroxyacetone, which is not otherwise accepted by the wild-type enzyme (Figure 6.6) [23]. The process was used in a one-step synthesis of... 

Franke, D., Machajewski, T., Hsu, C.-C. and Wong, C.-H. (2003) One-pot synthesis of L-fructose using coupled multienzyme systems based on rhamnulose-1-phosphate aldolase. The Journal of Organic Chemistry, 68 (17), 6828-6831. 

Mg2+ binds directly to the phosphate oxygen atoms and through a water molecule to the carboxylate group. Since Mg2+ prefers an octahedral coordination sphere, it requires a water structure different from that observed in holo rhamnulose kinase. 

Grueninger, D. and Schultz, G.E. (2006) Structure and reaction mechanism of L-rhamnulose kinase from Eschericia coli, J. Mol. Biol., 359, 787-797. 

One-step Synthesis of L-Fructose Using Rhamnulose-l-phosphate Aldolase in Borate Buffer... 

A practical, inexpensive one-step procedure was developed for the RhaD-catalyzed gram-scale synthesis of L-fructose. The requirement for DHAP as the donor substrate was circumvented by use of borate buffer, presumably by in situ formation of borate esters as a phosphate ester mimic. Racemic glyceraldehyde was also used, as the enzyme preferentially accepted the L-enantiomer as a substrate. The method can also be apphed to other products, including L-rhamnulose, and towards a two-step synthesis of L-iminocychtols. ... 

L-Rhamnose isomerase catalyzes the interconversion of L-rhamnose and L-rhamnulose. L-Rhamnose, a deoxy sugar, is found in bacteria and plants and it plays an essential role in many pathogenic bacteria. The pathway for the metabolism of this sugar does not exist in humans, and this makes enzymes of this pathway attractive targets for therapeutic intervention. " Rhamnose isomerase from E. coli is a tetramer of (/ /a)8-barrels similar to xylose... 

This enzyme [EC 5.3.1.14] catalyzes the interconversion of L-rhamnose and L-rhamnulose. 

This enzyme [EC 4.1.2.19] catalyzes the reversible conversion of L-rhamnulose 1-phosphate to dihydroxyace-tone phosphate (or, glycerone phosphate) and (5)-lactal-dehyde. 

RHAMNOSE ISOMERASE RHAMNOSE ISOMERASE a-L-RHAMNOSIDASE a-L-Rhamnosides, a-L-RHAMNOSIDASE t-Rhamnulose,... 

The synthehc applicability of arsenates is restricted by their toxicity that avoids the green aspect of the enzymatic processes. Wong et al. have shown that the use of inorganic borate buffer allows L-rhamnulose-l-phosphate aldolase (Rha-IPA) to accept DHA as substrate, although the of the reaction is about 50 times lower than with the natural substrate [10]. In spite of this fact, these authors have successfully used this approach for the one-step synthesis of L-fructose and L-rhamnulose, and for the facile two-step synthesis of several L-iminocyclitols. 

D-fructose 1,6-bisphosphate 2 (FruA E.C. 4.1.2.13), D-tagatose 1,6-bisphosphate 4 (TagA E.C. 4.1.2.40), L-fuculose 1-phosphate 5 (FucA, E.C. 4.1.2.17), and L-rhamnulose 1-phosphate 4 (RhuA, E.C. 4.1.2.19). From previous studies, we have DHAP aldolases with all four possible specificities readily available, we have demonstrated their broad substrate tolerance for variously substituted aldehydes, and we have investigated their stereoselectivity profile with non-natural substrates [3-6]. 

Besides FDPA, three other aldolases using DHAP as the donor are known each aldolase generates a new C3-C4 bond with a different stereochemistry u-erythro for fuculose-l-phosphate aldolase, i.-threo for rhamnulose 1-phosphate aldolase, and D-erythro for tagatose 1,6-diphosphate aldolase [7]. These aldolases accept a great variety of electrophilic substrates, which has been widely exploited in synthesis of sugar analogues [8,9]. 

W. D. Fessner, G. Sinerius, A. Schneider, M. Dreyer, G. E. Schulz, J. Badia, and J. Arguilar, Diastereoselective enzymatic aldol additions L-Rhamnulose and L-fuculose 1-phosphate aldolases from E. coli, Angew. Chem, lnt. Ed. Engl. 30 555 (1991). 

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... 

Fig. 8. Stability of the rhamnulose 1-phosphate aldolase from Escherichia coli (RhuA) vs. that of the fructose 1,6-bisphosphate aldolase from rabbit muscle (FruA) in phosphate buffer (pH 7.2 25°C ca. 1 Uml ) a) RhuA b) O RhuA, 30% EtOH c) RhuA, 50% DMSO d) FruA...

The L-rhamnulose 1-phosphate aldolase (RhuA EC 4.1.2.19) is found in the microbial degradation of L-rhamnose which, after conversion into the corresponding ketose 1-phosphate 44, is cleaved into 41 and L-lactaldehyde (l-16). The RhuA has been isolated from E. coli [336-339], and characterized as a metallo-protein [194,340,341]. Cloning was reported for the E. coli [342,343] and Salmonella typhimurium [344] genes, and construction of an efficient overexpression system [195,220] has set the stage for crystallization of the homotetrameric E. coli protein for the purposes of an X-ray structure analysis [345]. 

Table 5. Substrate tolerance of L-rhamnulose 1-phosphate and L-fuculose 1-phosphate aldolases [195,347]...

Enzymology,29 techniques of isolation, and descriptions of a number of them. Apparently, only three have been considered for preparative chemistry, that is, aldolase, sialyl aldolase, and Kdo synthetase. However, whole cells of some strains of Escherichia coli have been used as sources of fucu-lose 1-phosphate aldolase (E.C. 4.1.2.17) or rhamnulose 1-phosphate aldolase (E.C. 4.1.2.19).30 Extraction, and concentration to a suitable degree of homogeneity, of noncommercially available aldolases are not difficult. The examination of their synthetic possibilities could be very rewarding for we already observe that the wealth of chemicals prepared with the help of aldolase and sialyl aldolase far exceeds what they make in Nature. Still, not any aldehyde, however hydrophilic, is a substrate for aldolases. 

For the described approach, it is important to note that aldolases of different origin were tested and that, in contrast to L-rhamnulose-1-phosphate aldolase (RhuA), the D-fructose-1,6-biphosphate aldolase from rabbit muscle and L-fucu-lose-1-phosphate aldolase from E. coli were not active for DHAP/(R)-N- and (S)-iV-Cbz-prolinal condensation. Since RhuA accepts both, (S)-N- and (R)-N-Cbz prolinals, the chemo-enzymatic synthesis of both, hyacinthacines A and A2 isomers could be achieved. In conclusion, the origin and the particular enzyme itself... 

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