Glycolipids

The synthesis of gangliosides GA and GM are clearly associated to membrane flow from the endoplasmatic reticulum to the Golgi to the plasma membrane because blockade of vesicle flow inhibits ganglioside synthesis. The different transferase enzymes are localized in different subcellular compartments as indicated. Note that the Golgi apparatus itself is subdivided into cis and trans-Golgi vesicular compartments.

The degradation of gangliosides occurs in lysosomes catalyzed by exohydratases which remove saccharide units in a stepwise manner form the non-reducing end. Defects in these enzymes lead to glycolipid storage diseases associated with some inherited neuro-degenerative diseases such as Tay-Sachs disease, a defect in ganglioside metabolism. The disease is characterized by the missing of an enzyme involved in ganglioside degradation. Gangliosides thus accumulate and cause cerebral impairments, blindness, and early death. GM2 or N- Acetyl- D- galactosaminyl- (N-acetylneuraminyl)- D-galactosyl- D-glucosylceramide (C04884) is the culprit in Tay-Sachs disease.

One lysosomal protein involved in GM2 degradation is hexosaminidase A (EC 3.2.1.52). This enzyme hydrolyzes the glycosidic bonds of N-acetylgalactosamine (C01132) and sialic acid (C03525 Acetylneuraminic acid) residues of ganglioside GM2. Like for all aminidases, GM2 alone is not the substrate, but needs to be 'activated' by binding to a protein of 162 amino acids that extracts the ganglioside from the membrane and presents it to hexosaminidase A. The complex of GM2, GM2-activator serves as aminidase substrate.

Glycoproteins

Many cell surface proteins and secretory proteins carry polysaccharide moieties which are either used as signaling devices during the biosynthetic pathway (e.g. N-linked glycosylation) or are involved in the extra-cellular matrix (ECM) function of proteins (O-linked glycosylation). Glycosylation of newly synthesized membrane and secretory proteins (e.g. blood serum proteins) is part of the sorting mechanism within the cell and transport to their final destination. The cellular location of glycosylation are the lumen of the endoplasmatic reticulum and Golgi membrane stacks.

ER core glycosylation
(KEGG pathway MAP00510)

N-linked glycosylation for all proteins shares a common pathway involving about 50 enzymes in 3 subcellular compartments. The resulting carbohydrate moiety varies widely and influences protein solubility, protein structure, protein turnover, and compartmentalization (sorting). Glycosylation includes four important steps, with the first three steps known as core glycosylation and catalyzed by ER resident enzymes:

1. Synthesis of the carrier lipid dolichol-PP (C00621)
2. assembly of oligosaccharide-lipid intermediate dolichol-PP-oligosaccharide
3. transfer of oligosaccharide from dolichol to enzyme carrier
4. oligosaccharide modification in ER and Golgi membranes

The enzyme oligosaccharyl transferase (EC 2.4.1.119) transfers the activated core oligosaccharide from its dolichol pyrophosphate anchor to an asparagine residue on the recipient protein. This reaction occurs on the lumenal side of the membrane and requires the protein recognition sequence Asn-X-Ser/Thr. Asparagine residue not followed by a serine or threonine separated by any amino acid will not be recognized by the transferase.

Dolichol is a polyprenyl moiety synthesized from mevalonate (C00418; see cholesterol synthesis). It is used for the synthesis of dolichol-phosphate-monosaccharide, the activated monosaccharide precursor (e.g. C00043; UDP-N-acetylglucosamine) for protein glycosylation and dolichol- PP- (core)oligosaccharide formation (glycoprotein metabolism).

The dolichol- PP- oligosaccharide synthesis in the ER is catalyzed by sugar transferases. They use nucleotide activated monosaccharides as substrate which are synthesized in the cytoplasm. Upon transfer to the dolichol unit, the sugars are transported across the endoplasmatic reticulum membrane because the transferase activity is found on the lumenal side of this membrane. The first two reactions transfer N-acetyl glucosamine onto dolichol-PP forming N,N'-Diacetylglucosamine diphosphodolichol (C04537). The remaining monosaccharide units are added sequentially by membrane bound transferases until the the dolichol -oligosaccharide unit is complete and can be transferred to the enzyme acceptor.

The final transfer of the oligosaccharide unit to the protein occurs during the translocation of the protein across or into the ER membrane. This process is also known as co-translational modification as opposed to post-translational modification.

Fatty acylation of proteins

Cell membranes contain at least 25% proteins. These membrane proteins are active components of membranes for transport, signaling, and cell-cell communication (receptors, adhesion). Most membrane proteins are transmembrane proteins having functional domains on either side of a membrane allowing interaction between both sides (metabolic compartments). Some membrane proteins are attached to the membrane surface through lipid anchors or electrostatic binding and are known as peripheral membrane proteins. Lipid anchors are fatty acids or isoprenoids (geranyl, farnesyl) covalently lin ked to amino acid residues to provide a close attachment, yet lateral mobility along the membrane surface on both the cytoplasmic and extra-cellular (lumenal) side of a membrane.

More than 50 proteins have thus far been described having one or another form of lipid modification. They are found in all forms of life  -  yeast, plants, bacteria, animals, and viruses. Fatty acylation is a common modification for proteins involved in transmembrane regulatory pathways. The lipid anchor appears to mediate protein-protein interaction of several membrane proteins that act together in the signaling mechanism.

Palmitoylation is acquired post-translationally in the cytoplasm and does not make use of the ER secretory pathway. Instead, palmitoylated proteins appear to be routed directly to the inner leaflet of the plasma membrane. Although commonly found on the cytoplasmic surface of membranes, palmitoylation has been described for cell surface proteins. The responsible palmitoyl transferase for the latter glycoproteins is an ER resident enzyme (lumenal side of membrane). For all palmitoylation sites, a common recognition sequence has been identified  -  Cys-A-A-X, with A denoting an aliphatic amino acid and X any C-terminal amino acid. Many cytoplasmic proteins associated with cell surface receptors are linked by palmitoyl chains to the membrane. Often, deacylation inactivates the proteins because they are now released from the membrane. G-proteins and kinases are thought to be activated by C16 acylation during protein synthesis in the cytoplasm.

Myristoylation is coupled to protein translation. This is known as co-translational modification. The enzyme N-myristoyltransferase (EC 2.3.1.97; NMT) appears to be bound to the ribosomal complex and modifies the emerging N-terminal end of the new protein. The substrate is myristoyl-CoA which is linked to a glycine N-terminal amino group. Beside the glycine at the very N-terminus, the NMT is stimulated if the next amino acid is either a Asn, Gln, Ser, Val or Leu, and inhibited by Asp, D-Asn, Phe, or Tyr. These sequence requirement are found to be very efficient. Every protein with an N-terminal glycine followed by one of the stimulatory residues is myristoylated. In fact, myristoylation is found in many proteins on cytoplasmic surfaces of either cell membranes or subcellular compartments. One enzyme involved in the desaturation of elongated fatty acids, NADH-cytochrom b5 reductase is linked to the ER membrane surface through a myristate anchor.

GPI anchors are found in extra cellular proteins linked to the cell surface. They are synthesized and linked to glycoproteins inside the ER lumen (on membrane). The GPI anchor consists of:

1. ethanolamine attached via amide linkage to C-terminus
2. phosphodiester linkage of ethanolamine to the C6 hydroxyl of a mannose unit
3. a heterogeneous glycan moiety (oligosaccharide) linking the mannose to the inositol headgroup of the phosphatidylinositol

During synthesis, the attachment of the GPI anchor is preceded by the removal of a short, hydrophobic C-terminal peptide segment by either a transamidase which switches the peptide fragment for the GPI anchor, or a sequential reaction involving a protease to remove amino acid residues from the C-terminus and the following linkage of the GPI unit by a transferase. The length of the cleaved hydrophobic peptide fragment varies and its function is to first anchor the protein in the ER membrane to avoid the release of this protein into the ER lumen (would become a secretory protein). GPI anchor structures, because they are phospholipids, provide a high mobility of those cell surface proteins in the membrane. Normal transmembrane proteins with one or more transmembrane peptide domain have a considerably reduced mobility and tend to cluster and are often further immobilized by interaction with cytoskeletal proteins. A second use of GPI anchors is the potential activation of cell attached proteins that can be released from the cell surface by a signaling mechanism involving GPI specific phospholipase C (EC 3.1.4.3; PLC).

The product of PLC activity is a free, extra cellular glycoprotein and a membrane bound diacylglycerol (DAG), which is a major signaling molecule in certain second messenger pathways.

Go to table of contents