Glycosylation is one of the most frequent and important post-translational modifications. In this process oligosaccharides are covalently attached to proteins forming glycoproteins. The capacity of carbohydrate can vary from less than one per cent to more than 90 per cent for different glycoproteins. They have typically one or more chains of monosaccharide units (1 to 30 units long) which can be straight or branched. There are numerous different glycoproteins and they are abundant in living organisms appearing in nearly every biological process. Their functions span the entire spectrum of protein activities, including those of enzymes, transport proteins, receptors, hormones and structural proteins. Most plasma membrane and secretory proteins contain carbohydrate chains. There is also a very important group of proteoglycans formed by covalent aggregation of proteins and glycosaminoglycans. The distinction between glycoproteins and proteoglycans resides in the level and types of carbohydrate modifications. Oligosaccharides can be also attached to lipids but to a lesser extent than in case of proteins.

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Structure of glycoproteins