Galactosyl -xylose

D-Lyxose yielded a D-lyxosyl chloride 2,3,4-tri(chlorosulfate) which, on treatment with chloride ion, led to a dichlorodideoxy compound, most probably 2,4-dichloro-2,4-dideoxy-L-arabinose.353 D-Glucose gave a compound presumed to be 4,6-dichloro-4,6-dideoxy-a,/3-D-galactosyl chloride 2,3-di(chlorosulfate),360 and D-xylose afforded a monochloromonodeoxy derivative formulated, on indirect evidence, as 4-chloro-4-deoxy-L-arabinopyranosyl chloride 2,3-di(chlorosulfate).360 3,4-Dichloro-3,4-dideoxy-/3-D-ribopyranosyl chloride 2-(chlorosulfate) was the major, and 4-chloro-4-deoxy-a-D-xylopyranosyl chloride 2,3-di(chlorosulfate) the minor, product from the reaction of L-arabinose with sulfuryl chloride at room temperature for 24 hours.357,361 It has been established that, on reaction with sulfuryl chloride at low temperature, crystalline a-D-xylopyranose and /3-D-lyxopyranose afford, respectively, the 2,4,6-tri(chlorosul-fate)s of /3-D-xylopyranosyl chloride and a-D-lyxopyranosyl chloride,362 363 confirming that substitution at C-l occurs by an Sn2 process on a l-(chlorosulfuric) ester intermediate. 

The enzymes involved are nucleoside diphosphohexosetransferases, probably GDP-D-mannosyltransferase (EC 2.4.1.32) and UDP-D-galactosyl-transferase. For a structurally related polymer, (1 - 6)-a-D-xylosyl-(l - 4)-/3-D-glucan, synthesis was shown to involve concurrent incorporation of D-xylose and D-glucose, and not substitution by D-xylose of a preformed (1 - 4)-/3-D-glucan chain.34 The insolubility of (1 - 4)-/3-D-glucan and man-nan would make the latter mechanism unlikely. 

Using the enzyme from E. coli, a systematic study of the galactosylation of y3-D-xylopyranosides differently substituted at the anomeric position was carried out [23, 24]. The regioselectivity of the reaction depended on small variations in the xylose acceptor, such as the type of atom linked to the anomeric carbon (X atom in compound 1, Scheme 3). The initial rate of formation of y3( 1,3) disaccharides decreased with the electronegativity of this atom, while the rate for )3(1,4) disaccharides remained constant [24]. To interpret this result it was postulated that two different complexes between the xyloside and the... 

DS and CS chains connect to the proteoglycan protein cores by a tetrasac-charide (o-glucuronosyl- (3-1,3-D-galactosyl- (3-1,3-o-galactosyl- (3-1,4-o-xylose). The single GAG chain of decorin is attached, via an O-glycoside link from this xylose, to residue 4 (serine) of the mature protein core (12). 

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