Forms 7-6>-acetylneuraminic acid

N-Acetylneuraminic acid aldolase (or sialic acid aldolase, NeuA EC 4.1.3.3) catalyzes the reversible addition of pyruvate (2) to N-acetyl-D-mannosamine (ManNAc (1)) in the degradation of the parent sialic acid (3) (Figure 10.4). The NeuA lyases found in both bacteria and animals are type I enzymes that form a Schiff base/enamine intermediate with pyruvate and promote a si-face attack to the aldehyde carbonyl group with formation of a (4S) configured stereocenter. The enzyme is commercially available and it has a broad pH optimum around 7.5 and useful stability in solution at ambient temperature [36]. 

Total Synthesis of the Biologically Active Form of A -Acetylneuraminic Acid... 

Figure 2.19 Sialylation ofN-acetyl lactose by cytidyl monophosphate-N-acetylneuraminic acid using Of 2,3-neuraminic acid transferase as catalyst (upper box). Regeneration of the sugar nucleotide is shown in the lower box. CMP is converted into CTP in two steps using two different kinases. In the final step CMP-A -acetylneuraminic acid is synthesised from CTP and A -acetylneuraminic acid (sialic acid) using the appropriate synthetase. The formed pyrophosphate is converted into inorganic phosphate. Altogether five different enzymes are involved in the process.

Synthetic studies for sialic acid and its modifications have extensively used the catabolic enzyme N-acetylneuraminic acid aldolase (NeuA E.C. 4.1.3.3), which catalyzes the reversible addition of pyruvate (70) to N-acetyl-D-mannosamine (ManNAc, 11) to form the parent sialic acid N-acetylneuraminic acid (NeuSNAc, 12 Scheme 2.2.5.23) [1, 2, 45]. In contrast, the N-acetylneuraminic acid synthase (NeuS E.C. 4.1.3.19) has practically been ignored, although it holds considerable synthetic potential in that the enzyme utilizes phosphoenolpyruvate (PEP, 71) as a preformed enol nucleophile from which release of inorganic phosphate during... 

Two biologically important ketoaldonic acids should be mentioned here. N-Acetylneuraminic acid (5-acetamido-3,5-dideoxy-D-gh/cero-D-gaiocto-nonulosonic acid) is homomorphous with 3-deoxy-g(uco(and monno)-heptulose, and therefore, in solution, would be expected to be overwhelmingly in the / -D -pyranose form (14). Actually, although the... 

The six-carbon chain of ManNAc 6-P can be extended by three carbon atoms using an aldol-type condensation with a three-carbon fragment from PEP (Eq. 20-7, step c) to give N-acetylneuraminic acid (sialic acid).48 Tire nine-carbon chain of this molecule can cyclize to form a pair of anomers with 6-membered rings as shown in Eq. 20-7. In a similar manner, arabi-nose 5-P is converted to the 8-carbon 3-deoxy-D-manno-octulosonic acid (KDO) (Fig. 4-15), a component of the lipopolysaccharide of gram-negative bacteria (Fig. 8-30), and D-Erythrose 4-P is converted to 3-deoxy-D-arafrmo-heptulosonate 7-P, the first metabolite in the shikimate pathway of aromatic synthesis (Fig. 25-1).48a The arabinose-P used for KDO synthesis is formed by isomerization of D-ribulose 5-P from the pentose phosphate pathway, and erythrose 4-P arises from the same pathway. 

Two of the most frequent monosaccharide components of bacterial polymers belonging to this group have been the subjects of articles in this Series. They are 3-deoxy-D-manno-2-octulosonic acid,247 a normal constituent of the core region of bacterial lipopolysaccharides that is also present in some other polymers, and N-acetylneuraminic acid,248 found in several capsular polysaccharides. Enolpyruvate phosphate serves as the precursor of the C-l-C-3 fragment of the monosaccharides, with D-arabinose 5-phosphate or 2-acetamido-2-deoxy-D-mannose 6-phosphate being an acceptor for transfer of the three-carbon unit. Characteristic, activated forms of these monosaccharides are the CMP derivatives. 

In the trisialoglycolipid from D. nipon hepatopancreas, some of the N-acetylneuraminic acid residues are in the form of the 8-O-methyl derivative, whose structure was proved by mass spectrometry.192... 

The N-acetylneuraminic acid residue situated closer to the nonreducing end of the chain is present in the form of its 8-O-methyl derivative. 

Further developments are shown in Figure 4. On the basis that glucosamine reacted with pyruvic acid in the presence of alkali to yield pyrrole-2-carboxylic acid, in 1% yield, Gottschalk (21) proposed that sialic acid was formed by an aldol condensation reaction between N-ace-tylglucosamine and pyruvic acid. Kuhn and Brossmer (15) and Zilliken and Glick (22) showed that the reverse reaction also took place under alkaline conditions. Cornforth, Firth, and Gottschalk (23) synthesized crystalline N-acetylneuraminic acid (NANA) from N-acetylglucosamine and oxaloacetic acid (pH 11, 20°C). Under conditions less subject to misinterpretation, Heimer and Meyer (24) found that Vibrio cholerae enzymes cleaved NANA into an N-acetylhexosamine and pyruvic acid. 

V,2. This is not the case with cytidine monophosphate JV-acetylneuraminic acid (49) (see Scheme 18), the activated form of JV-acetylneuraminic acid for sialoside synthesis, as no sialylation cycle has so far been achieved, and thus this precursor must be added to the system in stoichiometric quantity. Thus, the availability of 49 is still the limiting factor in the large-scale synthesis of sialosides. 

Sialyl residues in oligosaccharides are introduced by the reaction of cyti-dine monophosphate-V-acetylneuraminic acid (49) as the sugar donor with the appropriate substrate, in the presence of specific transferases. Three of these have been utilized in syntheses which may be considered to be preparative. None are readily available. The most common, which we have called STA (see Table I), catalyzes the transfer of a 5-acetamido-3,5-di-deoxy-D-g/ycm>-a -D-ga/arfo-2-hexulopyranosonic acid unit (the a-D-pyra-nose form of JV-acetylneuraminic acid) to the primary position of D-galactose in a JV-acetyllactosamine residue.86 This enzyme also transfers vV-acetyl-9-O-acetylneuraminic acid (20) and V-glycolylneuraminic acid (12) from the corresponding cytidine monophosphate derivatives.16 The commercial enzyme is rather expensive, but pork liver from a butcher is a... 

With respect to polysaccharides, N-acetylneuraminic acid and KDO are unusual components. Rather, N-acetylneuraminic acid is a constituent of glycoproteins and certain glycolipids - and KDO is characteristically found in enterobacterial lipopolysaccharides, where it forms the linkage region between the lipid and carbohydrate moieties. 

Fio. 1. (a) A view of the influenza virus hemagglutinin trimer showing jV-acetylneuraminic acid (3, in CPK form) bound, (b) The tetrameric unit of influenza A virus sialidase. The figures were generated using the PyMOL Molecular Graphics System (Delano, W.L. (2002) at http // www.pymol.org). 

By analogy with the synthesis of /V-acetylneuraminic acid,63 di-A-acetyl derivatives of 5,7-diamino-3,5,7,9-tetradeoxynon-2-ulosonic acids could be obtained by condensation of 2,4-diacetamido-2,4,6-trideoxyhexoses with oxaloacetic acid under basic conditions. Four chiral centers in the C precursors, C-2 C-5, correspond to the centers C-5-C-8 in the target C9 products, and the fifth asymmetric center, C-4, is formed upon condensation. At present, derivatives of twelve 2,4-diamino-2,4,6-trideoxy-hexoses with the d-gluco, o-manno, L-allo, r>-galacto, D- and L-altro, D- and L-talo, D- and l-gulo, D- and L-ido configurations have been prepared by multistep chemical syntheses.11,17,18,64,65... 

---