Animal Glycolipids

Abstract Glycolipids are integral components of animal cell membranes, composed of a glycan covalently linked to a lipid. They include glycosphingolipids (GSLs) (ceramide‐linked), glycoglycerolipids (diacylglycerol‐ or alkylacylglycerol‐linked) and glycosylphosphatidylinositols (GPIs) (phosphatidylinositol‐linked). Their lipid components are embedded in a cell membrane with their glycans extending outwards where they interact with components in the plane of the same membrane or with glycan‐binding proteins, including soluble proteins and those on surfaces of other cells. Most animal glycolipids are GSLs, which can be as simple as ceramide monosaccharides or as large and complex as branched ceramide octadecasaccharides. Animal glycoglycerolipids are a simpler family, primarily galactose or sulphogalactose linked to glycerolipid. GPIs are glycans attached to the common membrane lipid phosphatidylinositol at one end and to a protein at the other, anchoring the protein to the membrane. Animal glycolipids serve diverse and essential functions in cell physiology in animals as simple as Caenorhabditis elegans and as complex as humans. Key Concepts Animal glycolipids are amphipathic molecules composed of a lipid embedded in a cell membrane and a glycan extending outwards. Most animal glycolipids are glycosphingolipids (GSLs), glycans carried on a ceramide lipid core. GSL glycans can be as simple as a single neutral sugar (glucosylceramide and galactosylceramide) or can be large, complex, branched and/or charged. GSLs are categorised as neutral GSLs, sulphated GSLs or gangliosides (sialic acid containing GSLs). Animal GSLs are named based on seven distinct neutral tetrasaccharide cores referred to as: ganglio‐, lacto‐, neolacto‐, globo‐, isoglobo‐, mollu‐ and arthro‐series. Animal glycoglycerolipids include galactose and 3‐sulphogalactose bound to diacylglycerol or alkylacylglycerol. They are scarce except in testes, where they are the major glycolipids and are required for spermatogenesis. Animal glycosylphosphatidylinositols (GPIs) consist of a core tetrasaccharide in glycosidic linkage to the inositol of phosphatidylinositol, a common membrane phospholipid. Animal GPIs carry a distal ethanolamine group, the terminal amine of which is linked via amide linkage to a select group of proteins, anchoring them to the membrane. Animal glycolipids tend to associate into functional microdomains in the plane of the membrane, often with sphingomyelin, cholesterol and select signalling proteins. GSLs function by binding laterally to molecules in the same membrane to regulate their activities (cis regulation) or by acting as receptors for soluble or cell surface glycan binding proteins (trans recognition).