GlcNAc units

One result from the analysis of the MD simulation was the proposal of a new enzymic pathway for hydrolysis by lysozyme. We begin with a description of the alternative mechanism, and the basis on which it was proposed. The energetics of the individual GlcNAc units in the lysozyme cleft are then presented, followed by a graphical representation of the correlation between the atomic fluctuations of the substrate and those of the enzyme. Of particular interest is the fact that the binding interactions stabilize a bound state conformation for the two glycosides involved in hydrolysis that is optimum for catalysis by the alternative mechanism and which differs from the conformations of the other glycosides. These conformational features are described in the final two sections. 

Some hereditary diseases are characterized by lack of two or more lysosomal enzymes. In I-ceII disease (mucolipidosis II), which resembles the Hurler syndrome, at least ten enzymes are absent or are present at much reduced levels.350,361 The biochemical defect is the absence from the Golgi cisternae of the N-acetylglucosaminyl phosphotransferase that transfers P-GlcNAc units from UDP-GlcNAc onto mannose residues (Eq. 20-22) of glycoproteins marked for use in lysosomes. 

The cultivation of BC in the presence of N-acetyl glucosamine (GlcNAc) causes a variation of the polymer formation by the insertion of GlcNAc units. It is possible to produce very thin membranes of the nanocellulose-chitin hybrid formed in this way [36]. 

Figure 9.11 Structure of an oligosaccharide unit of human transferrin. The linkage between the oligosaccharide and polypeptide chains is of the N-asparaginyl type. Dotted arrows indicate positions in which one sometimes finds NeuNAc-Gal-GlcNAc units. Gal is galactose, GIcNAc is N-acetylglucosamine, NeuNAc is N-acetylneuraminic acid (sialic acid), and Man is mannose. (Reproduced with permission from Bezkorovainy A. Biochemistry of Nonheme Iron. New York Plenum Press, 1980, p. 175.)...

SLe mimics 46 to 48, which had the GlcNAc unit replaced by (R,/ )-cyclohexan-l,2-diol and sialic acid substituted by glycolic acid or (5)-cyclohexyl lactic acid, were active as antagonists (Figure 16.21). These observations are in agreement with predictions based on computational studies that suggested these compounds are able to adopt the bioactive conformation. These computational simulations further predicted that the (/f)-lactic acid isomers were inactive because of their inability to properly orient the pharmacophores. Indeed, replacement of sialic acid by... 

Fig. 8. Sttucture of the core-related oligosaccharide (OS-4) that originated from chemical and enzymatic degradation of S-form LPS from H. pylori type strain. OS-4 is composed of the core OS and one GlcNAc unit from the Le O-chain region.

The allyl /i-glycoside of sialyl LacNAc 240 is converted into the sLe derivative by the enzyme-catalyzed transfer of a fucose unit from GDP-fucose to position 3 of the GlcNAc unit under catalysis by -1,3-fucosyltransferase (compare Schemes 37 and 38). The glycosyl donor GDP-fucose may be regenerated by a two-step sequence of enzyme-catalyzed reactions. Under catalysis by pyruvate kinase, spent GDP accepts one phosphate group from enolpyruvate phosphate. The GTP formed reacts with fucose 1-phosphate under catalysis by GDP fucose pyrophosphorylase from porcine thyroid to form GDP-fucose and inorganic pyrophosphate. 

XylMan3GlcNAc2) also non-xylosylated and a-l,6-fucosylated core structures occur. It has to be mentioned that in plant glycoproteins the frequently found xylosylated trimannosyl-A,A -diacetylchitobiosyl core structure can occur with a 1,3-linked in stead of a 1,6-linked oFuc residue, attached at the asparagine-bound GlcNAc unit (for a review, see ref [38]). 

Fuc(al-4) were also observed in A. maculatum (Fig. 4). Other unusual sequences, namely the dimer H epitope Fuc(al-2)Gal(Pl-3)[Fuc(al—2)]Gal, and blood group A determinant terminated with an additional a-l,3-linked GlcNAc unit, were found in X. laevis (Fig. 5). 

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