Sialic acid reversible cleavage

In vivo, pyruvate lyases perform a catabolic function. The synthetically most interesting types are those involved in the degradation of sialic acids or the structurally related octulosonic acid KDO, which are higher sugars typically found in mammalian or bacterial glycoconjugates [62-64], respectively. Also, hexose or pentose catabolism may proceed via pyruvate cleavage from intermediate 2-keto-3-deoxy derivatives which result from dehydration of the corresponding aldonic acids. Since these aldol additions are freely reversible, the often unfavourable equilibrium constants require that reactions in the direction of synthesis have to be driven by an excess of one of the components, preferably pyruvate for economic reasons, in order to achieve a satisfactory conversion. 

In considering the application of enzyme catalysis to DCC, we were encouraged by the thermodynamic resolution of a dynamic mixture of aldol products by Whitesides and co-workers through the use of a broad-specificity aldolase to lead to reversible formation of carbon-carbon bonds under mild conditions.35 For the current investigation36 we chose a related enzyme, N-acetylneuraminic acid aldolase (NANA aldolase, EC 4.1.3.3), which catalyzes the cleavage of N-acetylneuraminic acid (sialic acid, 27a) to A-acetylmannosamine (ManNAc, 28a), and sodium pyruvate 29 in the presence of excess sodium pyruvate, aldol products 27a-c are generated from... 

Thus mass spectrometry showed the GPI was heterogeneous with at least five separate species and its composition was consistent with known GPI structures but with sialic acid as a novel component. The heterogeneity was confirmed by various separative techniques including capillary electrophoresis, Dionex high performance anion exchange chromatography (HPAEC), and reverse phase HPLC at pH 7. Cleavage... 

Baumann W, Freidenreich J, Weisshaar G, Brossmer R, Friebolin H. Reversible cleavage of sialic acids with aldolase IH-NMR investigations on stereochemistry, kinetics and mechanism. Biol Chem H-S 1989 370 141-149. 

The extended nine-carbon backbone of sialic acids can be constructed from hexose building blocks by aldol addition of a pyruvate unit. Synthetic studies for sialic acid and its modifications have extensively used the catabolic enzyme NeuA, which catalyzes the reversible addition of pyruvate (5) to N-acetyl-D-mannosamine (ManNAc, 4) to form the parent siahc acid NeuSAc (1 Scheme 17.4) [16, 17, 19]. These freely reversible aldol additions have equilibrium constants in favor of cleavage direction [20], which requires that synthetic reactions have to be driven by an excess of one substrate to achieve satisfactory conversions for economic reasons, this usually is 5. In contrast, NeuS utilizes PEP (6) as a high-energy nucleophile, which upon C-C bond formation releases inorganic phosphate and thus renders the addition essentially irreversible [21]. Despite its considerable synthetic potential, NeuS still is an orphan catalyst which so far has been less studied for preparative applications [22]. 

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