Lewis X structure

An associated enzyme, al-3-fucosyltransferase, has also been used in syntheses to provide the Lewis x and sialyl Lewis x structural motifs [65]. Increasing availability of fucosyltransferases via cloning techniques are expected to have a strong influence on oligosaccharide synthesis in the near future. 

An elegant synthesis of the Lewis X structure was reported that utilized an efficient convergent approach.71 First, the azido-lactose acceptor 52, obtained via... 

Lewi.x structure —N=0 . The lone pair on N will favor ortho and para substitution through the following re.sonance forms ... 

Figure 1 Structure of L-selectin ligand and MECA-79 epitope. L-Selectin recognizes sulfated sialyl Lewis X structure (27) presented by O-glycans and A-glycans, marked by dotted-lines. MECA-79 antibody is specific to sulfated extended core 1 O-glycan marked by solid-lines, but MECA-79 antibody also recognizes this oligosaccharide with sialic acid and fucose (24,28,29).

A novel type of a-L-fucosidases have been prepared from X. manihotis [8] and Streptomyces sp. [31]. These two fucosidases exhibit high specificity toward the a-1—>3 and a-1 4 L-fucosidic linkages [8]. Although the enzymes cannot hydrolyze p-nitrophenyl u-l-Fuc, they are able to cleave Fucal- 4GlcNAc in the sialyl-Lewis X structure. 

N-Glycopeptides Carrying the Sialyl Lewis A Structure The regioisomeric tetrasaccharide structures sialyl Lewis A (sLea) (NeuAca(2—>3)Galp( 1 —>3) [Fuca(l— 4)]GlcNAc) and sialyl Lewis X (sLex)50 (NeuAca(2—>3)Galp( 1 —>4)... 

A similar approach was used in the synthesis of the sialyl Lewis X mimetics of type 85 (Scheme 13.22).66 Protein crystallization,67 conformational studies of sLex in solution68 and in bound form to E- and P-selectin69 as well as the study of structure-function relationships70,71 gave information about the functional groups of the sLex-epitope essential for the binding to the selectins. Synthesized mimetics must contain the three essential hydroxyl functions of the fucose. Sialic acid, galactose, and... 

Fig. 21.1. Glycosphingolipids of S. mansoni cercariae. Data on the glycosphingolipid structures are compiled as described in Wuhrer etal. (2000b) [1] and (2000c) [2], Lewis X and pseudo-Lewis Y epitopes are boxed. Cer, ceramide.

Khoo, K.H., Huang, H.H. and Lee, K.M. (2001) Characteristic structural features of schistosome cercarial N-glycans expression of Lewis X and core xylosylation. Clycobiology 11, 149-163. 

Wuhrer, M., Dennis, R.D., Doenhoff, M.J., Lochnit, G. and Geyer, R. (2000c) Schistosoma mansoni cercarial glycolipids are dominated by Lewis X and pseudo-Lewis Y structures. Clycobiology 10, 89-101. 

Why does the charge on the central alkyl group reduce the resonance energy of Sn2 transition states The answer is straightforward, once it is realized that the X A 1 X structure commonly contributes to the two Lewis structures. Thus, the higher the contribution of the triple ionic structure, the more similar the two Lewis structures, and the lower the resonance energy becomes. In the theoretical limit, where the X A+ X structure becomes the dominant structure in the TS, say 100% of the TS wave function, then the resonance energy should go to zero. It is seen that the semiempirical expression in Equation 6.20 mimics this limit. Other semiempirical expressions make similar predictions (5,53), but Equation 6.20 is the simplest one. 

We now know that many vital activities as diverse as healing, blood clotting, infection, prevention of infection, and fertilization all involve carbohydrates. Mysterious compounds such as sialyl Lewis-X , unknown a few years ago, are now known to be vital to our health and happiness. Far from being dull, carbohydrates are exciting molecules and our future depends on them, ft is well worthwhile to spend some time exploring their structure and chemistry. 

Also present are both four- and three-coordinate Al centers, the latter probably giving rise to high Lewis acidity. Structures of some aluminoxane clusters have also been determined in the solid state by single-crystal X-ray studies. One such structure, that of [Al706Me16] anion, is shown by 6.30. Another structure, 6.31, has been proposed on the basis of mass spectroscopic data. All the evidences taken together point to open-cage rather than dense mineral structures for the aluminoxanes in MAO solutions. 

B, Imberty A. Crystal and molecular structure of a histo-blood group antigen involved in cell adhesion the Lewis x trisaccharide. Glycobiology 1996 6 537-542. 46. 

YveUn F, Zhang Y-M, Mallet J-M, Robert F, Jeannin Y, Sinay P. Crystal structure of the Lewis x trisaccharide. Carbohydr. Lett. 

The majority of more recent structures relate to the sialyl Lewis X blood group carbohydrate receptor function for the selectin family of eukaryote lectins that mediate leucocyte-endothelial cell interaction during the initial stages of extravasation ( neutrophil rolling ) (20). These structures are the sialyl Le , Le, and Le sequences and several 6 sulfo GlcNAc and 3 sulfo Gal derivatives found in both protein and sphingolipid glycoconjugates (21-24). 

Lin, C-C, ShimazaM, M, Heck, M-P, Aoki, S, Wang, R, Kimura, T, Ritzen, H, Takayama, S, Wu, S-H, Weitz-Schmidt, G, Wong, C-H, Synthesis of sialyl Lewis X mimetics and related structures using the glycosyl phosphite methodology and evaluation of E-selectin inhibition, J. Am. Chem. Soc., 118, 6826-6840, 1996. 

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