A-acetyl-mannosamine

Kinetic parameters for glucose and other usual sugars have been reported.46 Kinetic and tracer studies of the equilibrium between A-acetyl-mannosamine and pyruvate, and IV-acetylneuraminic add shown in Scheme 9, from Brossmer et al.47 led them to the conclusion that substrate and product are both a-pyranose tautomers. 

Fig. 12.6 Sialic acid synthetase. Right Reaction of the substrates, phopshoenolpyruvate and A-acetyl mannosamine. Left Sialic acid (product). (Original Figure)...

Free hydroxyl group at C-3 is a condition sine qua non for aldol condensation [36,58]. Substitution at C-2 of A/-acetyl mannosamine is tolerated, and the enzyme exhibits only a slight preference for the defined stereochemistry at the other centers. 

A-acetyl-mannosamine is an important precursor for the biosynthesis of the nine carbon sugar acid, iV-acetyl-D-neuraminic acid (see section 10.5 and Fig. 10.8A) that is frequently found as a monosaccharide in glycoproteins and glyco-lipids (see Chapter 9). 

Commercial A -acetylneuraminic acid aldolase from Clostridium perfringens (NeuAcA EC 4.1.3.3) catalyzes the addition of pyruvate to A-acetyl-D-mannosamine. A number of sialic acid related carbohydrates are obtained with the natural substrate22"24 or via replacement by aldose derivatives containing modifications at positions C-2, -4, or -6 (Table 4)22,23,25 26. Generally, a high level of asymmetric induction is retained, with the exception of D-arabinose (epimeric at C-3) where stereorandom product formation occurs 25 2t The unfavorable equilibrium constant requires that the reaction must be driven forward by using an excess of one of the components in order to achieve satisfactory conversion (preferably 7-10 equivalents of pyruvate, for economic reasons). 

In nature, NANA arises through condensation of phosphoenolpyruvic acid with A-acetyl-D-mannosamine (NAM) catalysed by the biosynthetic enzyme NANA synthase. Owing to the labile nature of phosphoenolpyruvate, the use of this reaction in the synthesis of NANA has been limited to whole-cell systems where this substance can be generated biosynthetically in situ Most recently, the NANA synthase reaction forms the basis of fermentation processes for total biosynthesis of NANA. ... 

We propose that the first step in NDP-kasugamine biosynthesis is 2-epimeriza-tion of the postulated UDP-A -acetyl-D-glucosamine precursor, which is suggested by the similarity of the KasQ protein with known UDP-(Af-acetyl-)D-glucosamine 2-epimerases and catalyzes the conversion to UDP-N-acetyl-D-mannosamine. To... 

In an interesting extension of this work, the Neu5Ac aldolase from E. coli was subjected to directed evolution to expand its catalytic activity for enantiomeric forms of the usual substrates to include A -acetyl-L-mannosamine and L-arabinose with formation of the synthetically important products L-sialic add and L-3-deoxy-L-manno-oct-2-ulosonic add (l-KDO) (163). The evolved Neu5Ac aldolases were characterized by sequence analysis, kinetics, stereoselectivity, and in one case even by an X-ray structure analysis. Again, remote mutations were identified. It is significant... 

N-acetyl D-glucosamine A/-acetyl D-mannosamine N-acetyl D-galactosamine... 

A-Acetyl neuraminic acid aldolase [from Clostridium perfringens, A-acetylneuraminic acid pyruvate lyase] [9027-60-5] [EC 4.1.3.3]. Purified by extraction with H20, protamine pptn, (NH4)2S04 pptn, Me2CO pptn, acid treatment at pH 5.7 and pptn at pH 4.5. The equilibrium constant for pyruvate + n-acetyl-D-mannosamine ++ /V-acetylneuraminidate at 37° is 0.64. The Km for A-acetylneuraminic acid is 3.9mM in phosphate at pH 7.2 and 37°. [Comb and Roseman Methods in Enzymology 5 391 1962). The enzyme from Hogg kidney (cortex) has been purified 1700 fold by extraction with H20, protamine sulphate pptn, (NH4)2S04 pptn, heat treatment between 60-80°, a second (NH4)2S04 pptn and starch gel electrophoresis. The Km for A-acetylneuraminic acid is 1.5mM. [Brunetti et al. JBC 237 2447 1962). 

UDP-GlcNAc can be converted to UDP-N-acetyl-mannosamine (UDP-ManNAc) with concurrent elimination of UDP (Eq. 20-7). , 7b This unusual epimeri-zation occurs without creation of an adjacent carbonyl group that would activate the 2-H for removal as a proton. As indicated by the small arrows in Eq. 20-7, step a, the UDP is evidently eliminated. In a bacterial enzyme it remains in the E-S complex and is returned after a conformational change involving the acetamido group. This allows the transient C1-C2 double bond to be protonated from the opposite side (Eq. 20-7, step a).47 In bacteria the UDP-ManNAc may be dehydrogenated to UDP-N-acetylmannos-aminuronic acid (ManNAcA). Both ManNAc and ManNAcA are components of bacterial capsules.47... 

By virtue of the aldehyde at the reducing end, sugars are susceptible to deprotonation and isomerization. The rearrangement from an aldose sugar to a ketose sugar, shown in Scheme 6.80, is a direct result of this property [123]. Based on the initial enolization step, this chemistry is easily applied to the direct C-2 epimerization of 2-deoxy-2-aminosugars. Scheme 6.81 illustrates this reaction in the conversion of A-acetyl-D-glucosamine to A-acetyl-D-mannosamine [124,125]. 

The nitroaldol condensation with nitromethane (Henry s reaction), followed by Nef decomposition of the resultant nitronate under strongly acidic conditions, has been used to elongate aldehydes. For instance, A-acetyl-D-mannosamine has been converted into A-acetylneuraminic acid applying this method iteratively [69]. Chikashita and coworkers [70] have reported good levels of anti diastereoselectivity better than 99% in an iterative homologation sequence using 2-lithio-l,3-dithiane [71] with 2,3-O-cyclohexylidene-D-glyceraldehyde R)-62. In the case of the BOM-protected tetrose derivative, the addition of 2-lithio-l,3-dithiane was syn selective (synlanti 82 18) (Scheme 13.30). 

The nitroaldol condensation with nitromethane (Henry s reaction), followed by Nef decomposition of the resultant nitronate under strongly acidic conditions has been used to elongate aldehydes. For instance, A-acetyl-D-mannosamine has been converted into A-acetylneu-... 

Generally, syntheses of DAHP, KDO and Neu5Ac are based on chain extension of an appropriate sugar unit electrophiles by a C3 nucleophiles, according to the biosynthetic pathway, shown in Scheme 1. Thus, the reaction of D-erythrose 4-phosphate (13) acting as a C4 electrophile with phosphoenolpyruvate (14, C3 nucleophile) creates DAHP [6]. Mutual relationship to this pathway concerns the biosynthesis of KDO and Neu5Ac, which are produced on the reaction of phosphoenolpyruvate with D-arabinose 5-phosphate (C5 electrophile) or A-acetyl-D-mannosamine 6-phosphate (C6 electrophile), respectively [5,12]. 

In contrast, Neu5Ac aldolase turned out to be very useful for synthetic purpose because it tolerate wide range of unnatural substrates [21,23]. The enzyme also named sialic acid aldolase (EC 4.1.3.3.) catalyze reversible reaction of A/-acetyl-D-mannosamine (15) and pyruvate (Scheme 3) [33-35],... 

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