Carbohydrate chains, covalent modification

B. Effects of Covalent Modifications of the Carbohydrate Side Chains 238... 

Carbohydrates exhibit enormous diversity because of variation in monosaccharide composition, glycosidic linkage positions, and chain branching [17], Further diversity is achieved by covalent modification of carbohydrates via acetylation, methyl-ation, phosphorylation, and sulfation [1]. Sulfation can be incorporated into the carbohydrates of glycolipids and glycoproteins. Modifications of both A-linked and 0-linked chains are found. 

Posttranslational modifications include many covalent alterations Polypeptide processing, attachment of carbohydrate or lipid groups to specific side chains, and addition of many other low-molecular-weight ligands to side chains. 

Table III lists the most commonly encountered amino acid derivatives formed by in vivo modification of proteins during or after the completion of their ribosome-directed biosynthesis. In contrast to the strict alphabetical listing of Table II, the entries of this table have been organized in groups relative to the parent amino acid. It should be noted that this list is not exhaustive. There are several reports of covalent prosthetic group attachment to proteins through the side chains of a variety of residues (see Glazer et a/., 1975). In addition, carbohydrate is often found attached to proteins (Kornfeld and Kornfeld, 1976 Waechter and Lennarz, 1976) and there are reports of covalently bound lipid as well (Braun and Bosch, 1972 Stoffyn and Folch-Pi, 1971). However, for the most part, these amino acids have not been isolated as defined entities. A brief description of amino acid derivatives formed in vivo and their origin is given below.

Many proteins contain carbohydrates and lipids, covalently bound to amino acid side-chains. Others contain covalently bound cofactors and prosthetic groups, such as vitamins and their derivatives, metal ions or haem. Again the attachment of these non-amino acid parts of the protein is part of the process of post-translational modification to form the active protein. 

Proteins are also covalently modified by specific enzymes that act on side-chain functional groups or on the N- or C-termini. More than 150 types of side-chain modifications are known fhese include glycosylafions, metiiy-lations, and acefylations, among otiiers. For insfance, a variety of carbohydrates are added to proteins, primarily at asparagine residues, and serve as recognition markers on cell surfaces. 

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