Febs Letters Volume 97, Number 1-2./Volume 98, Number 1-2. [antikvár]

Volume 97, number 1FEBS LETTERSJanuary 1979Review LetterSTRUCTURAL SIMILARITY OF THE TERMINAL CARBOHYDRATE SEQUENCES OF GLYCOPROTEINS AND GLYCOLIPIDSHeilcki RAUVALA and Jukka FINNE Department of Medical Chemistry, University of Helsinki, Siltavuorenpenger 10, SF-00170 Helsinki 17, FinlandReeeived 4 September 19781. IntroductionTlie carbohydrate chains of animal cell membranes are covalently linked to either protein or lipid. Both glycoproteins and glycolipids are asymmetrically oriented in cell membranes, with their saccharide chains projecting outwards from the membrane [1]. The carbohydrate chains of both classes of gly co-conjugates have been suggested to be involved in many cell surface-associated phenomena, such as cell adhesion and recognition [2-4]. They have also been suggested to function as receptors for various biologically-active agents, such as hormones and toxins [5-7].In spite of these similarities, glycoproteins and glycolipids have been generally considered two independent classes of glycoconjugates. They have been studied by different investigators, and consequently their structure, metabohsm and function have generally been separately discussed. Due to recent advances in the analysis of complex carbohydrates, some structural similarities between glycoproteins and glycolipids have been discovered. In order to make the evaluation of these observations possible, it was found necessary to collect the scattered information concerning the structure of both classes of glycoconjugates. From the data presented a striking similarity between the glycoproteins and glycolipids becomes evident. The similarities occur in the terminal carbohydrate sequences of the molecules, whereas the portions near to protein or lipid carrier structure are different. The structural similarities may indicate that the terminal carbohydrate sequences are synthesized by common glycosyl transferases. It is furthermore possible that the structural similarities reflect similar functional properties.2. Terminal sugar sequences of protein- and lipid-bound carbohydratesThe internal or 'core' portions of the carbohydrate chains of glycoproteins and glycolipids are different. Most glycolipids have as their carbohydrate core a lactose unit, which is linked to the lipid [8]. In the A^-glycosidic type of glycoprotein saccharides a rather invariable core structure is also found, which consists of mannose and A^-acetylglucosamine [9]. To this core structure are linked different carbohydrate chains, whereas these are directly Unked to the peptide, without a distinct core portion, in the alkali-labile 0-glycosidic type of heteroglycans [10]. Most of the structural variation in the carbohydrate chains of both glycoproteins and glycolipids is due to the structure of the nonreducing terminal sugar sequences. For the comparison of these structures, they have been collected in table 1. To facilitate the description of the structures, they are presented as derivatives of basic disaccharides. For brevity, the various types of sialic acids are not specified, although it should be realized that they may contribute significantly to the biological properties of the saccharide structures [11].A. Structures related to galactosyl(pi -4)N-acetyl-glucosamineSaccharide chains structurally related to Gal(/31-4)GlcNAc (structures 7-75) are commonly encountered in animal glycoproteins and glycolipids. The corresponding A^-acetylglucosamine-terminated precursor structures also occur in glycoproteins [10] and glycolipids [59]. Most of the so-called acidic or complex A^-glycosidic saccharides of glycoproteins [9,10] contain terminal sequencesElsevier/North-Holland Biomedical Press1