Glycolipids - The Sugar Code - page 177

10
Glycolipids
J ü rgen Kopitz
The preceding chapters have provided detailed insights into the glycosylation of
proteins. In addition to glycoproteins as a large subgroup of cellular glycoconju-
gates, glycans can also be a part of conjugates with lipids, as already indicated by
the glycosylphosphatidylinositol (GPI) anchors (please see Chapter 9). In this case,
the glycan serves as a bridge between the lipid anchor and a protein. Evidently,
lipids as membrane constituents present glycans. This chapter - in conjunction
with Chapter 30 emphasizing their status as major components in the nervous
system - will focus on lipid -glycan conjugates, where the glycans as headgroups
are freely accessible for intermolecular interactions, that is the glycolipids. In
detail, this chapter will give an overview of glycolipid structures in bacteria, plants
and vertebrates, describe the range of their specifi c cellular functions, and put
them into a context to cell biology and medical sciences.
10.1
Classifi cation and General Structures of Glycolipids
The term glycolipid designates any compound containing one or more monosac-
charides bound by a glycosidic linkage to a hydrophobic membrane- anchoring
compound such as an acylglycerol, a sphingoid, a ceramide ( N - acylsphingoid) or
a prenylphosphate [1]. The primary classifi cation of glycolipids is based on the
aglycon part (http://www.lipidlibrary.co.uk/lipids.html). Consequently, the term
glycoglycerolipid is used to name glycolipids containing one ore more glycerol
residues. Most bacterial and plant glycolipids are such glycerol-containing glyco-
lipids [2] (examples are given in Table 10.1 ). Glycosphingolipid s ( GSL s) contain a
sphingoid or a ceramide. Sphingoids are long-chain aliphatic amines, containing
two or three hydroxyl groups, and often a distinctive trans -double bond in position 4.
The most abundant of these in animal tissues is sphingosine (2S,3R,4E)- 2 - amino -
4 - octadecen - 1,3 - diol) or 4 - sphingenine (Table 10.1 ). The etymology of the term is
depicted in the Info Box. The carbohydrate residue is attached by a glycosidic
linkage to O1 of the sphingoid. Fatty acids of varying length are linked via amide