Glycosylation is the process or result of addition of saccharides to proteins and lipids. The process is one of four principal co-translational and post-translational modification steps in the synthesis of membrane and secreted proteins and the majority of proteins synthesized in the rough ER undergo glycosylation. It is an enzyme-directed site-specific process, as opposed to the non-enzymatic chemical reaction of glycation. Two types of glycosylation exist: N-linked glycosylation to the amide nitrogen of asparagine side chains and O-linked glycosylation to the hydroxy oxygen of serine and threonine side chains. 
Glycoconjugates are simply molecules which have a carbohydrate bonded to a fat or protein. When bonded to a fat, glycoconjugates are called ‘glycolipids’. When carbohydrates are bonded to proteins, they are called ‘glycoproteins’.
Although some ABO antigens are glycolipids, the vast majority are glycoproteins. About 11% of the human body is composed of glycoconjugates.
Factors Influencing Membrane Glycosylation in the Gut
- Animal Species
- Blood Group Specificity
- Site in the Intestines
- Position Along the Crypt/ Villus Axis
- State of Differentiation and Maturation
- Bacterial Status
Altered Glycosylation In Disease States
- Glucose flux leads to altered glycosylation and enhanced lectin sesitivity Enhanced O-GlcNAc protein modification is associated with insulin resistance in GLUT1-overexpressing muscles. (Am J Physiol Endocrinol Metab 2002 Aug;283(2):E241-50)
- Chronically increased glucose flux results in enhanced O-GlcNAcylation of GLUT4 and/or GLUT4-associated proteins and may contribute to insulin resistance.
- Consumption of a diet containing kidney bean caused an increase in the level of mRNAs for the insulin receptor (327%) and GLUT-4? (185%) in the gastrocnemius muscle. (Int J Biochem 1992 Jun;24(6):897-902)
- Immunoglobulins with affinity for the Fc region of IgG molecules are found in rheumatoid arthritis and linked with the severity of the disease.
- Agalactosylated glycoforms of Immunoglobulin G may induce association with the mannose-binding lectin and contribute to the pathology. This may well be the ‘smoking gun’ linking wheat? consumption to increased RA symptomatology.
- Altered glycosylation also affects immune complex formation. The abnormality of lectin binding seen in IgA nephropathy is typical of Henoch-Schonlein purpura with nephritis.
- Aberrant glycosylation is a hallmark of malignancy. Some products are truly tumor-specific antigens that have not been found in normal tissue. (PSA, CA15-3, CA-125 etc.)
- Other antigens are present in normal tissue, but over expressed on tumor cells (ABH antigens, Sialyl Lewis X (SLeX))
- Others are displayed during fetal development, disappear in the adult, only to arise again during malignant transformation. CEA
Lectins and also bacteria modify the glycosylation of gut surface receptors in the rat.
Glycoconj J 1995 Feb;12(1):22-35 Pusztai A, Ewen SW, Grant G, Peumans WJ, Van Damme EJ, Coates ME, Bardocz S
Rowett Research Institute, Bucksburn, Aberdeen, UK.
- Oral exposure to lectins or the presence or absence of bacteria in the small intestine were shown by histological methods using anti-lectin antibodies or digoxigenin-labelled lectins to have major effects on the state of glycosylation of lumenal membranes and cytoplasmic glycoconjugates of epithelial cells. Taken together with the dramatic effects of exposure to lectins on gut function, metabolism and bacterial ecology, this can be used as a basis for new perspectives of biomedical manipulations to improve health.