A/-glycosylation

Biopharmaceuticals in Plants Toward the Next Century of Medicine 

3 DIFFERENCES IN N-GLYCOSYLATION PATTERNS BETWEEN PLANTS AND MAMMALS 

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Transferases. Enzymes which transfer a group, possibly a methyl group or a glycosyl group, from one compound to another. The name of the group transferred is usually in-... 

Bu3SnH, AIBN, PhH, 3 h. A, >72% yield. The thiol is released as a stannyl sulfide that was used directly in a glycosylation. " ... 

Figure 25.7 Glycoprotein formation occurs by initial phosphorylation of the starting carbohydrate to a glycosyl phosphate, followed by reaction with UTP to form a glycosyl uridine 5 -diphosphate. Nucleophilic substitution by an -OH (or -NH2) group on a protein then gives the glycoprotein.

A patient with diabetes received a glycosylated hemoglobin test result of 10%. This indicates... 

In the Koenigs-Knorr method and in the Helferich or Zemplen modifications thereof, a glycosyl halide (bromide or chloride iodides can be produced in situ by the addition of tetraalkylammonium iodide) is allowed to react with a hydrox-ylic compound in the presence of a heavy-metal promoter such as silver oxide, carbonate, perchlorate, or mercuric bromide and/or oxide,19-21 or by silver triflu-oromethanesulfonate22 (AgOTf). Related to this is the use of glycosyl fluoride donors,23 which normally are prepared from thioglycosides.24... 

In the halide-assisted method,25 a glycosyl halide (normally bromide) with a nonparticipating 2-substituent and with the thermodynamically more stable axial orientation at C-l is treated with an excess of the corresponding halide anion by the addition of a soluble tetraalkylammonium salt. This sets up an equilibrium between the axial and the (much less stable) equatorial glycosyl halide. The lat-... 

Glycosides may also be prepared by enzyme-catalyzed condensation reactions utilizing a glycosyl fluoride. Thus 6-0-a-maltosylcyclodextrins were prepared enzymically from a-maltosyl fluoride (obtained from the corresponding heptaacetate by Zemplen deacetylation) and cyclodex-trins. " ... 

The type of intermediate that is formed in the slow inhibition with D-gly-cals was identified, with the aid of the ) -D-glucosidase A3 from Asp. wentii, as an ester of 2-deoxy-D-araA/ o-hexose with an aspartic acid side-chain. The same aspartoyl residue had already been shown, by labeling with con-duritol B epoxide (see Section 111,1), to be essential for -D-glucoside hydrolysis. In addition, this aspartate was found to form a glycosyl -enzyme... 

Hehre and coworkers showed that beta amylase from sweet potatoes, an inverting, a-specific exo-(l 4)-glucanase, catalyzes the hydrolysis of jS-maltosyl fluoride with complex kinetics which indicated the participation of two substrate molecules in the release of fluoride ion. Furthermore, the reaction was strongly accelerated by the addition of methyl ) -maltoside. Hydrolysis of a-maltosyl fluoride, on the other hand, obeyed Michaelis-Menten kinetics. The main product with both a- and yj-maltosyl fluoride was )S-maltose. The results with )3-maltosyl fluoride were interpreted by the assumption of a glycosylation reaction preceding hydrolysis by which a malto-tetraoside is formed by the replacement of fluoride ion by a second substrate molecule or added methyl -maltoside (see Scheme 5). 

If k2 > kj, the glycosyl-enzyme intermediate will accumulate, and may be trapped by the rapid denaturation of the enzyme in the presence of (saturating) amounts of substrate. With -glucoside Aj from Asp. wentii and 4-nitrophenyl [ C]-2-deoxy-) -D-irra />jo-hexopyranoside, it was possible to identify the intermediate as a glycosyl ester (acylal) of 2-deoxy-D-arabino-hexose bound to the same aspartate residue that had previously been labeled with the active-site-directed inhibitor conduritol B epoxide and with D-glucal." This constituted an important proof that the carboxylate reacting with the epoxide is directly involved in catalysis. 

Evidence for a glycosyl-enzyme intermediate of finite lifetime with inverting a-D-glycosidases, and details of its reaction, came from studies with 2,6-anhydro-l-deoxyhept-l-enitols and glycosyl fluorides. - Analysis of hydration and hydrolysis products on the one hand, and of glycosyla-tion products on the other, indicated an intermediate that could be approached by water from the yff-face only of the ring, and by other glycosyl acceptors only from the a-face (see Schemes 4 and 5 This can be considered a proof of the principle of microscopic reversibility of chemical reactions. 

The hydrolysis of zeaxanthin esters by a carboxyl ester lipase indeed enhanced both the incorporation of zeaxanthin in the micellar phase and uptake of zeaxanthin by Caco-2 cells. As mentioned earher, carotenoids can also be linked to proteins by specific bindings in nature and these carotenoid-protein complexes may slow the digestion process and thus make their assimilation by the human body more difficult than the assimilation of free carotenoids. Anthocyanins are usually found in a glycosylated form that can be acetylated and the linked sugars are mostly glucose, galactose, rhamnose, and arabinose. 

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