2010م - 1444هـ
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Structural and Functional Diversity

of Glycoconjugates

A Formidable Challenge to the Glycoanalyst

Gerald W. Hart

1. Overview of Glycosylation

Glycoconjugates represent the most structurally and functionally diverse

molecules in nature. They range in complexity from relatively simple

glycosphingolipids and nuclear or cytosolic glycoproteins with dynamic

monosaccharide modifications to extraordinarily complex mucins and

proteoglycans (for review, see refs. 1,2). Some of the proteoglycans are perhaps the most complex molecules in biology, with more than 100 different

saccharide side chains on a single polypeptide. We now realize that most

proteins, even those within intracellular compartments, are co- and/or posttranslationally modified by covalent attachment of saccharides (3).

1.1. The Glycocalyx and Extracellular Matrix

Many early electron microscopic studies using cationic stains, such as

ruthenium red or alcian blue, documented that virtually all cells are surrounded

by thick carbohydrate coats (4,5), termed the “glycocalyx.” The glycocalyx is

comprised of protein- and lipid-bound oligosaccharides and polysaccharides

attached to membrane-associated proteins and lipids. Although electron

micrographs visualize the glycocalyx as a distinct boundary many times the

thickness of the lipid bilayer of the plasma membrane, in reality, the glycocalyx

is probably even larger and is contiguous with the extrinsically associated

extracellular matrix glycoconjugates, which are washed away during sample

preparation for microscopy. Even the simplest eukaryotic cell, the erythrocyte,

has a large and complex glycocalyx