Glycolipids and Glycoproteins
Membrane lipids other than phospholipids include the glycolipids
glycosphingolipids (GSL) in animals. They contain a hydrophobic
ceramide anchor N-acylsphingosine
(C00195) and a hydrophilic headgroup composed of saccharides.
They are normally found at the outer surface of cell membranes.
The composition of the saccharide moiety is cell type specific,
depends on the developmental stage of the organism, or can change
with the oncogenic state of a cell.
The central and peripheral nervous systems are rich in many
specialized lipids, but have a very low percentage of triglycerides
(fat). Brain lipids are complex lipids involved in signaling
mechanism: sphingolipids containing the long chain amino alcohol
sphingosine, such as the acylated sphingosine ceramide
(C00195). All lipids may be synthesized from glucose and other
metabolites provided by the blood. The brain thus has fatty
acid synthesis capacity, but all fatty acids are used for membrane
lipid synthesis, and not for beta-oxidation or fat storage.
Neurons have a capacity to utilize ketone bodies for energy
production at restricted glucose supply (e.g. starvation) analogous
to the mechanism in muscle tissue.
Brain lipids fall into two categories: (i) gray-matter
lipids from neurons, and (ii) white-matter lipids from the neuron
protective myelin sheath. While neuronal membranes are
similar to all other tissue membranes, the myelin cells contain
sphingolipids, cholesterol, and phospholipids (phospholipids are
always needed for the formation of a stable bilayer structure). Myelin
sheath are multilayered membrane structures wrapped around axonal
and dentritic cell body extensions. This supporting and electrically
insulating layer is part of glial cells. One unique lipid in glial
cell membranes are the mono-acylglycerol derivative ethanolamineplasmalogen
(C04756). Plasmalogen is also an active factor in blood clotting
through regulation of platelet aggregation. Sphingomyelin appear
to be present in both gray and white-matter tissue, whereas gangliosides
are specific for neurons.
( PATH: MAP00604)
A well studied group of glycosphingolipids are the
gangliosides. They compose a chemically and structurally
diverse group of neuronal cell membranes. They have been shown
to be important in membrane turn over mechanisms through recycling
of plasma membrane through the lysosomal compartments (endocytosis).
Ganglioside lipid components (ceramide) like that of GM1
(Systematic name: D- Galactosyl- N-acetyl- D-galactosaminyl- (N-acetylneuraminyl)-
D-galactosyl- D-glucosylceramide; KEGG C04911) are synthesized
in the ER and glycosylated with glucosyl-UDP
(C00029) by glucosyl-transferase
(EC 184.108.40.206) and subsequently with galactosyl-UDP
(C00052) as activated precursor by galactosyl-transferase
I (EC 220.127.116.11). All other glycosylation reactions are
catalyzed by Golgi resident transferases. There GM1 synthesis
is completed by the sequential addition of galactosyl-UDP, N-acetylgalactosyl-UDP,
and sialicylate-UDP (N-acetylneuraminyl-UDP).
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
One lysosomal protein involved in GM2 degradation is hexosaminidase
A (EC 18.104.22.168). This enzyme hydrolyzes the glycosidic bonds
(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
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
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
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 22.214.171.124) 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,
monosaccharide precursor (e.g. C00043; UDP-N-acetylglucosamine)
for protein glycosylation and dolichol- PP- (core)oligosaccharide
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.
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 126.96.36.199; 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
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 188.8.131.52; 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.
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