Carbohydrates groups, removal

Immunoglobulins are glycoproteins. The carbohydrate moiety is attached to the heavy chain (usually the Ch2 domain) via an N-linked glycosidic bond. Removal of the carbohydrate group has no effect upon antigen binding but does affect various antibody effector functions and alters its serum half-life. 

The related reaction foro-nttrobenzyl esters results in hydrolysis of the ester, and this has been developed for use in the protection of alcohols, carboxylic acids and amines. For example, the C-1 hydroxyl group in carbohydrates can be protected as its o-njtrobenzyl ester, and the ester group removed under the very mild conditions of irradiation in neutral solution (5.51). Similarly, the carboxylic acid... 

Not all sequences -Asn-X-Thr( or Ser)- are glycosylated in proteins, however. Hunt and Dayhoff (168) searched the amino acid sequences of 264 proteins. In only 101 cases did they find the occurrence of the sequence -Asn-X-Thr(or Ser)-, and in only 20 cases did the sequence occur in glycosylated form. They did conclude that this sequence is probably the key recognition sequence of the glycosyltransferase. Glyco-sylation may not occur because of the absence of one or more of the required enzymes, because the protein becomes folded into tertiary structure before glycosylation can occur or the carbohydrate groups have been removed prior to isolation. 

The red kidney bean a-amylase inhibitor contains 9-10% covalently bound carbohydrate. Removal of up to 70% of the carbohydrate does not affect the activity of the inhibitor (110). The glyco groups, removed from the protein, do not inhibit a-amylase at 3.5 X 10 times the concentration of the inhibitor (110). Chemical modification studies indicate that histidine and tyrosine residues in the inhibitor may be important for its activity (110). 

F,2 -Ethylenenucleosides. 6-Pivalamido-9-(2-iodo-2-deoxy-5>0-pivalyl-/ -D-arabino-furanosyl)purine treated with N,0-bis(trimethylsilyl)acetamide in pyridine, then l,5-diazabicyclo[4.3.0]non-5-ene added, stirred 1.5 hrs. at room temp., and the trimethylsilyl blocking group removed by methanolysis (cf. 829) 6-pivalamido-9-(2-deoxy-5-0-pivalyl-D-eryr/iro-pent-l-enofuranosyl)purine. Y 98%.-This method prevents epoxide formation. M. J. Robins and R. A. Jones, J. Org. Chem. 39, 113 (1974) unsatd. carbohydrates, review, s. Y. A, Zdanov and V. G. Alekseeva, Russ. Chem. Rev. 42, 477 (1973) (Eng. transl.). 

A new method for the protection of carbohydrate groups in carbohydrates involves the use of propenylidene acetals (Scheme 4). The protecting group can be removed using Wilkinson s catalyst in ethanol, sometimes with the addition of one equivalent of trifluoroacetic acid. Benzoyl groups are labile under these conditions, but in then-presence 1% sulfuric acid in dioxane can be used. 

C1O3/ACOH, 25°, 50% yield, [- ROCOPh (- ROH + PhC02H)]. This method was used to remove benzyl ethers from carbohydrates that contain functional groups sensitive to catalytic hydiogenation or dissolving metals. Esters are stable, but glycosides or acetals are cleaved. 

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