Transcript i 10/01/07

COMPLEX LIPID METABOLISM - I
10/01/07
I. LEARNING OBJECTIVES
1) To identify the basic structure of phospholipids and to be able to
distinguish the two classes, phosphoglycerides and sphingomyelins
2) To be able to name and recognize the major phosphoglycerides
3) To differentiate how phosphoglycerides are synthesized, and which of
the building blocks contain the high energy bond
4) To summarize the way in which sphingomyelin is synthesized
5) To explain how phosphoglycerides and sphingomyelin are degraded
II. INTRODUCTION TO PHOSPHOLIPIDS
A) Polar, ionic compounds (Fig. 17.1A)
B) Contain an alcohol (diacylglycerol or
sphingosine) and a phosphodiester linkage
III. STRUCTURE
A) Two Classes
1) glycerophospholipids (phosphoglycerides) –
glycerol backbone; function in membrane
structure, anchoring proteins in the cell
membrane, signal transduction across
membranes, bile composition, lipoprotein
particles
2) sphingomyelins - sphingosine backbone primarily in membrane structure
B) Phosphoglycerides (Fig. 17.1)
1) glycerol backbone
2) phosphate esterified at the three position
3) fatty acids esterified at the one and two
positions [1) + 2) + 3) = phosphatidic acid
(PA)] (Fig. 17.1B) – simplest phosphoglyceride
4) an alcohol component esterified on the
phosphate
5) common phosphoglycerides: (Fig. 17.1A)
PA + serine = phosphatidylserine (PS)
PA + ethanolamine = phosphatidylethanolamine
(PE, cephalin)
PA + choline = phosphatidylcholine (PC, lecithin)
PA + glycerol = phosphatidylglycerol (PG)
PA + inositol = phosphatidylinositol (PI)
6) cardiolipin (Fig. 17.2) - important inner mitochondrial membrane
component = two PA’s esterified to 1 and 3 position of another
glycerol. The only phosphoglyceride that is antigenic.
7) plasmalogens (Fig. 17.3) - fatty acid at carbon 1 position is an
ether instead of an ester linkage
a) three classes - phosphatidalcholines, phosphatidalethanolamines, and phosphatidalserines
b) myelin - has much ethanolamine plasmalogen
c) heart muscle - has much choline plasmalogen
d) platelet-activating factor (PAF) – strong
biological mediator; causes platelet activation;
increases pulmonary and airway edema in
lungs; mediates hypersensitivity, acute
inflammatory reactions, and anaphylactic
shock; stimulates
neutrophils and
macrophages to produce
superoxide radicals; binds
to membrane receptors
C) Sphingomyelins (Fig. 17.4)
1) backbone is sphingosine
2) fatty acid forms amide linkage with
amino group of sphingosine = a
ceramide (this is also a precursor for
glycolipids)
3) esterification of carbon 1 of ceramide
by phosphorylcholine = sphingomyelin
(important component of myelin in
nerves)
IV. PHOSPHOGLYCERIDE SYNTHESIS
(Fig. 17.5) - two methods
A) Donation of phosphatidic acid from
CDP~diacylglycerol to an alcohol
B) Donation of phosphomonoester of the
alcohol from the CDP~alcohol to
1,2-diacylglycerol
C) CDP-derivative is “activated” (has a high
energy bond)
D) Synthesis occurs in smooth ER and
goes through the Golgi apparatus to
membranes or cell exterior (exocytosis)
E) Phosphatidic acid synthesis
(see Fig 16.14); occurs in all tissues
except rbc’s
F) Phosphatidylethanolamine (PE) and
phosphatidylcholine (PC) synthesis
1) ethanolamine and choline obtained
from the diet or from phospholipid
turnover; phosphatidylcholine can be
synthesized de novo from PS
2) preexisiting - phosphorylation of
ethanolamine or choline; activation to
CDP-ethanolamine or CDP-choline;
ethanolamine phosphate or choline
phosphate is transferred to
diacylglycerol (salvage pathway)
a) choline reutilization - de novo choline
synthesis needs three methyl groups from
methionine (Fig. 17.6, bottom). Because
methionine (an essential amino acid) is often
deficient in the diet, choline reutilization
prevents possible choline deficiency
b) lung surfactant – dipalmitoylphophatidylcholine (DPPC) is an important
lung surfactant; hyaline membrane disease
(respiratory distress syndrome, RDS) in
premature infants = low lung surfactant
production. Glucocorticoids (stimulate DPPC
synthesis) can be given to the mother prior to
delivery to reduce the possibility of RDS in the
premature infant. Immune suppressive
medication and chemotherapy – adult RDS
3) de novo synthesis of PC from PS
(Fig. 17.6) - PS is decarboxylated and then
methylated three times
G) Phosphatidylserine (PS) (Fig. 17.6, top) – calciumactivated “base exchange” – the ethanolamine of
PE is exchanged with free serine
H) Phosphatidylinositol (PI)
1) inositol + CDP-diacylglycerol  PI and
CMP (Fig. 17.5)
2) signal transduction - PI becomes
phosphorylated (for example, to PIP2;
Fig. 17.7).
When cells are treated with hormones, neurotransmitters, and
growth factors, the degradation of PIP2 by phospholipase C is
stimulated. This yields inositol-1,4,5-trisphosphate (IP3; mobilizes
intracellular calcium) and diacylglycerol (which activates protein
kinase C) (Figs. 17.7 and 17.8).
DAG
IP3
3) membrane protein anchoring (Fig. 17.9) –
important for anchoring of alkaline
phosphatase, acetylcholine esterase, and
lipoprotein lipase
4) A deficiency in glycosyl phosphatidylinositol
(GPI) in hematopoietic cells causes a
hemolytic disease, paroxysmal nocturnal
hemoglobinuria.
5) phospholipase C cleavage of protein and
inositol phosphate leaves diacylglycerol
(which activates protein kinase C)
I) Phosphatidylglycerol (PG) – mitochondrial
membranes; precursor of cardiolipin; made from
CDP-diacylglycerol and glycerol-P (Fig. 17.5)
J) Cardiolipin (diphosphatidylglycerol) –
diacylglycerolphosphate transferred
from CDP-diacylglycerol to PG (Fig. 17.5)
V. SPHINGOMYELIN SYNTHESIS (Fig. 17.10)
A) palmitoyl CoA and serine are used as
substrates. A series of reactions that include
decarboxylation, loss of CoA, reduction by
NADPH + H+ and oxidation by FAD results in
the synthesis of sphingosine.
B) An amide bond is formed between
sphingosine and a fatty acid (using
fatty acyl CoA) to produce a ceramide.
C) Using phosphatidylcholine, choline
phosphate is transferred to the ceramide to
form a sphingomyelin.
VI. PHOSPHOLIPID DEGRADATION (Fig. 17.11)
A) Degradation of phosphoglycerides - various phospholipases cleave
the phosphodiester bonds at specific sites
1) phospholipase A1 - removes fatty acid at position 1
2) phospholipase A2 - removes fatty acid at position 2; releases
arachidonic acid for prostaglandin, leukotriene, and thromboxane
synthesis; high in pancreatic secretion, activated by trypsin, and
requires bile salts for activity; inhibited by glucocorticoids
3) phospholipase C - in liver lysosomes; plays role in producing
second messengers in the PIP2 pathway; leaves free hydroxyl at
position 3 of glycerol
4) phospholipase D - removes alcohol from the phosphoglyceride,
leaving phosphatidic acid
B) Degradation of sphingomyelin (Fig. 17.12)
1) degraded by sphingomyelinase - lysosomal enzyme; removes
phosphorylcholine to yield a ceramide. Niemann-Pick disease –
sphingomyelinase deficiency. Severe form (Type A) has large
accumulation of sphingomyelin and phosphatidylcholine in liver and
spleen (both very enlarged) (Fig. 17.13). Results in severe mental
retardation and death in early adulthood. “High” in Ashkenazi Jews.
2) ceramidase - cleaves ceramide into sphingosine and a free fatty
acid.
COMPLEX LIPID METABOLISM - II
10/01/07
I. LEARNING OBJECTIVES
1) To identify the structure of glycosphingolipids and to be able to
distinguish the two classes, neutral glycosphingolipids and acidic
glycosphingolipids
2) To be able to name and recognize the major gangliosides and
sulfatides
3) To describe how glycosphingolipids are synthesized
4) To summarize how glycosphingolipids are degraded
5) To list the various sphingolipidoses and the enzymes that are
responsible for the defect.
II. INTRODUCTION TO GLYCOLIPIDS
A) Because they are derivatives of ceramide = “glycosphingolipids”
B) Numerous functions
1) membrane component (particularly outer surface of plasma
membrane; interact with the extracellular environment)
2) very high in nerve tissue
3) roles in cellular interactions, growth, development
4) blood group, embryonic, and tumor antigens
5) receptors for cholera and diphtheria toxins and for viruses
6) genetic disorders in their metabolism cause severe neurological
and developmental problems
III. STRUCTURE
A) General features - contain no phosphate and polar head group is
comprised of mono- or oligosaccharides attached to ceramide by an
O-glycosidic bond
B) Neutral glycosphingolipids
1) cerebroside (Fig. 17.14) (found primarily in brain and peripheral
nervous tissue; high concentrations in myelin sheaths) – contains
monosaccharide unit only
a) galactocerebroside (most common in membranes) contains
galactose: Cer-Gal
b) glucocerebroside (intermediate in glycosphingolipid synthesis)
contains glucose: Cer-Glc
2) ceramide oligosaccharides (globosides;
additional sugars are added to a
glucocerebroside) – includes:
a) lactosylceramide: Cer-Glc-Gal
b) Forssman antigen:
Cer-Glc-Gal-Gal-GalNac-GalNac*
*GalNac = N-acetylgalactosamine
C) Acidic glycosphingolipids – negatively
charged at physiological pH; due to either
N-acetylneuraminic acid (NANA) (sialic
acid) in gangliosides or sulfate in sulfatides
1) gangliosides (Fig. 17.15) - primarily in
ganglion cells of CNS (nerve endings);
derivatives of ceramide oligosaccharides.
Naming includes “G” for ganglioside;
followed by letter that designates how
many NANA molecules are present
(M = mono- = 1; D = di- = 2; T = tri- = 3;
Q = quatro- = 4); additional letters or
numbers in a subscript refer to a
convention for the sequence of
carbohydrate attachment to the ceramide
(e.g., GM2)
2) sulfatides (sulfoglycosphingolipids) –
cerebrosides that contain a sulfated
galactosyl residue. Found mostly in
nervous tissue.
IV. GLYCOSPHINGOLIPID SYNTHESIS
A) Mechanism (Fig. 17.18) - sequential addition of sugar (glycosyl)
residues. Sugar nucleotide donors (high energy bond) used, similar
to glycoprotein synthesis. Occurs in Golgi and smooth
endoplasmic reticulum.
B) Enzymes - glycosyl transferases that are specific for a nucleotide
sugar and an acceptor. Some enzymes are involved in both
glycoprotein and glycosphingolipid synthesis.
X
C) Sulfate group addition - added to galactocerebrosides by transfer
from 3’ phosphoadenosine-5’-phosphosulfate (PAPS). (Fig. 17.16)
Sulfate is added to the 3’ hydroxyl group of galactose by
sulfotransferase (Fig. 17.17).
High Energy Bond
V. GLYCOSPHINGOLIPID DEGRADATION
A) Internalized by endocytosis
B) Degraded by lysosomal enzymes after fusion of endocytic vesicles
with lysosomes
C) Enzymes involved include: a-galactosidase, b-galactosidase,
b-glucosidase, neuraminidase (sialidase), hexosaminidase,
sphingomyelinase, sulfatase (sulfate esterase), and ceramidase.
Remove residues sequentially such that the last one added during
synthesis is the first one removed during degradation. Defects in
the enzymes lead to a large spectrum of
diseases called lipid storage diseases or
sphingolipidoses. Have accumulation of
undigested glycosphingolipids within
cells (Fig. 17.19)
VI. SPHINGOLIPIDOSES (Fig. 17.20)
A) Synthesis and degradation
usually balanced
B) Degradation (hydrolysis)
occurs in lysosomes
C) Deficiency in hydrolase
causes lipid accumulation in
the lysosome – Almost all
show neurological impairment
and most are fatal in early life.
E) Except for Fabry Disease
(X-linked) they are autosomal
recessive.
D) Diagnosis - assay enzyme
activity or accumulated lipid in
tissue biopsies, cultured
fibroblasts, peripheral blood
leukocytes, plasma, and/or
amniotic fluid.
E) Very rare in the general
population. Gaucher and
Tay-Sachs –  Ashkenazi Jews
Sphingolipidoses
Disorder
Lipid Accumulation
Enzyme Deficiency
Primary Organ Involvement
Generalized
Gangliosidosis
Ganglioside GM1
GM1 ganglioside:
b -galactosidase
brain, liver, skeleton
Tay-Sachs Disease
Ganglioside GM2
b-Hexosaminidase A
brain
Gaucher Disease
Glucocerebroside
Glucocerebrosidase
brain, liver, spleen
Metachromatic
Leukodystrophy
b-Sulfogalactocerebroside
Arylsulfatase A
brain
Krabbe Disease
Galactocerebroside
Galactosylceramide
b -galactosidase
brain
Sandoff-Jatzkewitz
Disease
Globoside and
Ganglioside GM2
b-Hexosaminidase
A and B
brain
Fabry Disease
Ceramide trihexoside
a-Galactosidase A
kidney
Niemann-Pick Disease
Sphingomyelin
Sphingomyelinase
brain, liver, spleen
Farber Disease
Ceramide
Ceramidase
joints, liver, spleen
Fucosidosis
Pentahexosylfucoglycolipid
a-Fucosidase
brain, nerves, skin
H) Effects of Eicosanoids (Fig. 17-25) – KNOW ALL OF THESE!!
1) many varied effects
2) Tromboxane A2 (TXA2) – promotes blood clots by causing platelet
aggregation and vasoconstriction. Prostacyclin (PGI2) inhibits
platelet aggregation and causes vasodilation, and inhibits
thrombogenesis, by acting on nucleated endothelial cells. Aspirin
blocks both by covalently attaching to COX-1 and inactivating it
(Fig. 17-24). But endothelial cells can synthesize more COX-1 and
make PGI2, while
anucleated platelets
cannot synthesize
more TXA2. Basis for
low-dose aspirin
therapy for lowering
stroke and heart attack
risk by lowering
thrombi formation.
3) COX-2 inhibitors may
cause increased risk
of heart attack and
stroke (Vioxx recall).