Glycosyl excisions

Base-Excision Repair Every cell has a class of enzymes called DNA glycosylases that recognize particularly common DNA lesions (such as the products of cytosine and adenine deamination see Fig. 8-33a) and remove the affected base by cleaving the Af-glycosyl bond. This cleavage creates an apurinic or apyrimidinic site in the DNA, commonly referred to as an AP site or abasic... 

AP sites may be spontaneous or induced, and can block the replication. Instability of the glycosylic site constitutes the principal cause of AP sites formation. Cellular DNA can undergo puric base losses, estimated at between 250 and 500 per hour. AP sites can be regenerated during reparation processes by base excision and free radicals. 

Base-excision repair requires the recognition and removal of inappropriate bases such as uracil, hypoxanthine, and xanthine from DNA. The DNA glycosylases are enzymes that remove the aberrant base by cleaving the N-glycosyl bond. Direct repair is different than the other repair mechanism in that the mutated base is repaired directly, without removal of the nucleotide. The protein 06-methylguanine DNA methyltransferase, for example, carries our direct repair. 

Base excision repair (BER). The N-glycosyl linkages of the pmine and pyrimidine bases to the deox-ribose residues of the sugar-phosphate backbone of DNA are subjecf to sponfaneous hydrolysis, one important source of damage to DNA. Similar hydrolytic reactions are catalyzed by DNA glycosylases, which remove many mismatched or damaged bases. " ... 

If the virus was grown in a strain of cell that could not add the first residue of the outer chains, which is N-acetylglucosamine, trimming ceased at GlcNAc2Man5. This implied that addition of A -acetylglucosamine and excision of mannose were linked processes. Such is now known to be the case and Tabas and Kornfeld (1978) have demonstrated the enzymatic basis of this. The glycosylation of G-protein is also modified in glucose-deprived, infected BHK cells (Turco and Pickard, 1982). 

The intimate association between the enzymes of glycosyl transfer or excision and the membrane systems of eukaryotic cells has grown ever more apparent over the last 20 years. No discussion of the synthesis of polymeric saccharides can neglect current ideas of the function and interrelationships of the exo- and endomembranes of cells. 

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