Download Metabolism of Glycerophospholipids

Chapter 19
Lipid Metabolism II:
Membrane Lipids, Steroids,
Isoprenoids, and Eicosanoids
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Biochemistry, 4th Edition
Chapter 19 Outline:
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Metabolism of Glycerophospholipids
Metabolism of Sphingolipids
Steroid Metabolism
Other Isoprenoid Compounds
Eicosanoids: Prostaglandins, Thromboxanes, and
Leukotrienes
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Biochemistry, 4th Edition
Metabolism of Glycerophospholipids
Intracellular synthesis and transport
of membrane phospholipids:
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The most abundant phospholipids are
those derived from glycerol.
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These glycerophospholipids are
found primarily as components of
membranes.
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Biochemistry, 4th Edition
Metabolism of Glycerophospholipids
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Phosphatidic acid is a branch metabolite between triacylglycerol biosynthesis and
phospholipid biosynthesis.
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Biochemistry, 4th Edition
Metabolism of Glycerophospholipids
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Biochemistry, 4th Edition
Metabolism of Glycerophospholipids
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Biochemistry, 4th Edition
Metabolism of Glycerophospholipids
Regulation of membrane phospholipid composition in bacteria:
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Biochemistry, 4th Edition
Metabolism of Glycerophospholipids
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Biochemistry, 4th Edition
Metabolism of Glycerophospholipids
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In eukaryotes, phosphatidic acid has three different origins:
o Glycerol-3-phosphate
o Dihydroxyacetone phosphate
o Diacylglycerol
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Biochemistry, 4th Edition
Metabolism of Glycerophospholipids
Synthesis of phosphatidylcholine and
phosphatidylethanolamine in mammals:
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Metabolism of Glycerophospholipids
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Salvage pathways to phospholipids, starting
with choline or ethanolamine, are
quantitatively important in eukaryotic cells.
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Biochemistry, 4th Edition
Metabolism of Glycerophospholipids
Metabolic activation of phosphocholine:
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CTP:phosphocholine cytidylyltransferase
catalyzes nucleophilic attack at the a-phosphate
of CTP by a phosphoryl oxygen of
phosphocholine, displacing the b,g-phosphates
(pyrophosphate) of CTP, forming CDP-choline.
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Metabolism of Glycerophospholipids
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The second pathway to PE and PC begins with the conversion of
phosphatidylserine (PS) to PE, which is catalyzed by either of two
different enzymes:
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Phosphatidylserine decarboxylase is a mitochondrial enzyme
that, like the corresponding bacterial enzyme, decarboxylates PS to
PE.
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The second enzyme is a calcium-activated transferase,
phosphatidylethanolamine serinetransferase, which exchanges
free ethanolamine
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Biochemistry, 4th Edition
Metabolism of Glycerophospholipids
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S-Adenosyl-L-methionine (AdoMet) is the methyl
group donor in synthesis of phosphatidylcholine and
numerous other methylated metabolites.
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Biochemistry, 4th Edition
Metabolism of Glycerophospholipids
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AdoMet is formed from methionine and ATP in an unusual reaction
catalyzed by methionine adenosyltransferase, in which ATP is
cleaved to yield inorganic triphosphate (PPPi) plus an adenosyl
moiety linked directly via the ribose C5 to the methionine sulfur.
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The triphosphate is hydrolyzed by the enzyme to
pyrophosphate(PPi) and orthophosphate (Pi), drawing the reaction
to completion.
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Biochemistry, 4th Edition
Metabolism of Glycerophospholipids
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Fatty acid chains in phospholipids are remodeled through
the concerted actions of phospholipases and specific
lysophospholipid acyltransferases to meet the needs
of the organism.
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Specificities of phospholipases A1, A2, C, and D:
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Biochemistry, 4th Edition
Metabolism of Glycerophospholipids
Structure of phospholipase A2:
•An enzyme that metabolizes membrane
phospholipids.
•Catabolism of phospholipids at
membrane–water interfaces is important
both in modification of membrane
structure and as a source of second
messengers and other regulators.
•The crystal structure of the porcine
pancreas phospholipase A2 reveals the
catalytic triad (D99-H48-water) and the
active site Ca2+ ion, liganded to a second
water molecule (W2).
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Biochemistry, 4th Edition
Metabolism of Glycerophospholipids
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Biochemistry, 4th Edition
Metabolism of Glycerophospholipids
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Phosphatidylinositol and its phosphorylated
derivatives, collectively termed phosphoinositides,
play important roles as precursors of second
messengers.
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PIP3 plays an important role as a second messenger in
transmembrane signaling, the transmission of an
extracellular signal to some element of the intracellular
metabolic apparatus.
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Biochemistry, 4th Edition
Metabolism of Glycerophospholipids
Biosynthetic route to
alkyl ether
phospholipids:
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Biochemistry, 4th Edition
Metabolism of Glycerophospholipids
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Biochemistry, 4th Edition
Metabolism of Glycerophospholipids
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An unusual ether lipid, called platelet-activating factor,
has the structure 1-alkyl-2-acetylglycerophosphocholine.
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Physiologically, this compound is perhaps the most potent
compound known.
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Biochemistry, 4th Edition
Metabolism of Glycerophospholipids
Synthesis of a plasmalogen from a glyceryl ether:
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Desaturation of 1-alkylacylglycerophosphoethanolamine (the alkyl analog of
phosphatidylethanolamine) yields the corresponding vinyl
ether, or plasmalogen.
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Biochemistry, 4th Edition
Metabolism of Sphingolipids
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Membranes are assembled by membrane vesicles moving
from synthesis sites in the endoplasmic reticulum and Golgi
complex to existing membranes and fusing with them.
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Biochemistry, 4th Edition
Metabolism of Sphingolipids
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Biochemistry, 4th Edition
Metabolism of Sphingolipids
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Biochemistry, 4th Edition
Metabolism of Sphingolipids
A myelinated axon from the spinal cord:
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Myelin, an insulating layer wrapping about the axon,
is rich in sphingomyelin.
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Metabolism of Sphingolipids
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Biochemistry, 4th Edition
Metabolism of Sphingolipids
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Biochemistry, 4th Edition
Steroid Metabolism
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We turn now to an extraordinarily large and diverse
group of lipids, the isoprenoids, or terpenes.
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These compounds are built up from one or more fivecarbon activated derivatives of isoprene.
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The family includes:
o
o
o
o
o
o
o
o
o
o
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Steroids
Bile acids
Lipid-soluble vitamins
Dolichol and undecaprenol phosphates
Phytol
Gibberellins
Insect juvenile hormones
Components of rubber
Coenzyme Q
And many more compounds
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Biochemistry, 4th Edition
Steroid Metabolism
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Ring identification system (a)
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Carbon numbering system (b) used for steroids.
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Structural conventions (c), with cholestanol as the
example.
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a-Substituents project below the plane of the
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The hydrogens at positions 5, 9, and 14 have the
a-configuration, whereas the hydroxyl, the two
methyl groups, the hydrogen at C-8, and the
aliphatic side chain at C-17 are all b-substituents.
steroid ring system (blue dashed wedge), and bsubstituents project above that plane (red solid
wedge).
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Biochemistry, 4th Edition
Steroid Metabolism
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Cholesterol, the precursor to all steroids, derives
all of its carbon atoms from acetate.
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Steroid Metabolism
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The five carbons of isoprene could be derived metabolically from
three molecules of acetate, and the prediction that cholesterol
was a product of the cyclization of the linear hydrocarbon
squalene.
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Squalene contains six isoprene units (delineated by red marks
on the structures below), and its configuration makes it a
plausible steroid precursor.
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Biochemistry, 4th Edition
Steroid Metabolism
Cholesterol biosynthesis can be considered as three
distinct processes:
1.Conversion of C2 fragments (acetate) to a C6 isoprenoid
precursor (mevalonate).
2.Conversion of six C6 mevalonates, via activated
intermediates, to the C30 squalene.
3.Cyclization of squalene and its transformation to the C27
cholesterol.
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Biochemistry, 4th Edition
Biosynthesis of mevalonate and
conversion to
isopentenyl pyrophosphate and
dimethylallyl
pyrophosphate:
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The two carbons of the third acetyl
group are shown in red.
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Biochemistry, 4th Edition
Steroid Metabolism
Stage 2: Synthesis of Squalene from Mevalonate
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The next several reactions occur in the cytosol.
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First, mevalonate is activated by three successive phosphorylations.
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The first two are simple nucleophilic substitutions on the g-phosphorous of
ATP.
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The third phosphorylation, at position 3, sets the stage for a decarboxylation
to give the five-carbon isopentenyl pyrophosphate
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Biochemistry, 4th Edition
Steroid Metabolism
Conversion of isopentenyl pyrophosphate and dimethylallyl
pyrophosphate to farnesyl pyrophosphate:
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Both of these head-to-tail condensations are catalyzed by the
same prenyltransferase (farnesyl pyrophosphate synthase).
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Biochemistry, 4th Edition
Steroid Metabolism
Conversion of farnesyl pyrophosphate to squalene:
•Catalyzed by squalene synthase.
•Dissociation of the PPi group on one of the molecules of
farnesyl-PPi yields an allylic carbocation.
•The double bond nucleophilically attacks the carbocation,
forming a tertiary cation at C3 of the first farnesyl-PPi.
•Loss of a proton from C1 gives the activated cyclopropane
intermediate, presqualene-PPi.
•Dissociation of the second pyrophosphate leads to
rearrangement and formation of another tertiary
carbocation intermediate.
•Hydride transfer from NADPH completes the
rearrangement, giving squalene.
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Biochemistry, 4th Edition
Steroid Metabolism
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Biochemistry, 4th Edition
Steroid Metabolism
Regulation of HMG-CoA
reductase by
ubiquitinmediated
proteolysis:
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Sterol binding leads to
rapid degradation of the
enzyme.
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Biochemistry, 4th Edition
Steroid Metabolism
Insig-mediated regulation of SREBP
activation:
•When cholesterol levels in the cell are low,
Scap transports SREBP to the Golgi.
•Proteolytic processing by membranebound proteases S1P and S2P releases
SREBP’s transcription factor domain
(bHLH), which enters the nucleus and
binds to sterol regulatory elements (SRE)
in the promoters of target genes,
stimulating their transcription.
•High levels of cholesterol block this
process by Insig-mediated retention of
Scap–SREBP in the ER.
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Biochemistry, 4th Edition
Steroid Metabolism
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Once the rate-limiting role of HMG-CoA reductase in cholesterol
biosynthesis was understood, specific inhibitors were sought to
lower blood cholesterol levels.
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The compounds discovered are collectively called statins.
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They act by competitively inhibiting HMG-CoA reductase.
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Lovastatin and simvastatin are fungal polyketides and
atorvastatin (Lipitor®) is synthetic.
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Each statin carries a mevalonate-like moiety (blue), explaining the
competitive nature of their activity.
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Inhibition of HMG-CoA reductase depresses de novo cholesterol
biosynthesis and, hence, intracellular cholesterol levels.
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This in turn leads to increased production of LDL receptors,
allowing more rapid clearance of extracellular cholesterol from the
blood, thus lowering blood cholesterol levels.
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Steroid Metabolism
The protein prenylation pathway:
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aa = amino acid residue
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Biochemistry, 4th Edition
Steroid Metabolism
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Biochemistry, 4th Edition
Steroid Metabolism
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Pregnenolone is an intermediate en route from
cholesterol to all other known steroid compounds.
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Biosynthetic routes
from pregnenolone
to other steroid
hormones:
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Biochemistry, 4th Edition
Steroid Metabolism
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Diethylstilbestrol, a synthetic estrogen, was widely used
to promote growth of beef cattle, until it was found to be
potentially carcinogenic at the levels found in meat from
treated cattle.
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Biochemistry, 4th Edition
Steroid Metabolism
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Oral contraceptives are formulated with compounds
containing progesterone and estrogen activities.
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Two widely used synthetic estrogens are norethynodrel
and mestranol.
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Biochemistry, 4th Edition
Other Isoprenoid Compounds
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There are three active forms of vitamin A: all-transretinol, -retinal, and –retinoic acid. Collectively, these
are referred to as retinoids.
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The vitamin can be either consumed in the diet as
esterified retinol, or biosynthesized from b-carotene, a
plant isoprenoid especially abundant in carrots.
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b-carotene is cleaved in the intestine by a
monooxygenase to form two molecules of all-transretinal (retinaldehyde), which are then reduced to retinol.
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All-trans-retinol is the form which circulates in the blood
and which has the highest biological activity.
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Biochemistry, 4th Edition
Other Isoprenoid Compounds
Schematic drawing of a rod cell:
1. The outer segment is a stack of
membranous disks, which contain the
photoreceptive pigments.
2. This segment is connected by a thin
cilium to the inner segment, which
contains the cell nucleus, cytosol, and
synaptic body.
3. The potential change produced in the
outer segment travels to the synaptic
body and is transmitted to one or more
of the neurons of the retina.
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Biochemistry, 4th Edition
Other Isoprenoid Compounds
The chemical changes in photoreception:
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11-cis-retinal and opsin in a rod cell combine
to form rhodopsin.
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Absorption of a photon of light leads to the
chemical changes shown in steps 2 and 3.
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Step 3 involves at least 3 distinct
conformational changes that occur in ~1 ms.
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Metarhodopsin II is the species that activates
transducin (not shown) to initiate the visual
cascade.
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After about 1 second, metarhodopsin II
dissociates into all-trans-retinal, which
isomerizes as the cycle begins again.
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Other Isoprenoid Compounds
Structures of other
important isoprenoids:
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Biochemistry, 4th Edition
Other Isoprenoid Compounds
•
Terpene is a generic term for all compounds that are
biosynthesized from isoprene precursors.
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Terpenes are biosynthesized ultimately from isopentenyl
pyrophosphate (C5) and dimethylallyl pyrophosphate (C5).
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When these combine to yield geranyl pyrophosphate (C10), any
terpene formed thereby is called a monoterpene.
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When a compound is formed from 1 mole of farnesyl
pyrophosphate (C15), the product is called a sesquiterpene.
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Triterpenes (C30) are formed from 2 moles of farnesyl
pyrophosphate.
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Geranylgeranyl pyrophosphate yields either diterpenes (C20) or
tetraterpenes (C40).
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The dolichols and undecaprenol, are examples of polyprenols
(polyisoprenoid alcohols), which have more than 50 carbons
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Biochemistry, 4th Edition
Other Isoprenoid Compounds
Some terpene
compounds:
•
These examples
are representative
of an enormous
class of natural
products.
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Biochemistry, 4th Edition
Eicosanoids: Prostaglandins,
Thromboxanes, and Leukotrienes
Structures of the major prostaglandins and thromboxane A2:
•The most abundant prostaglandins are those of the 2-series are shown.
•They are derived from arachidonic acid, as is thromboxane A2.
•Numbering of carbons begins with the carboxyl group as shown for the
structure of PGG2.
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Biochemistry, 4th Edition
Eicosanoids: Prostaglandins,
Thromboxanes, and Leukotrienes
Summary of biosynthetic
routes to the major
prostaglandins and
thromboxane A2:
PLA2, phospholipase A2
PLC, phospholipase C
DGL, diacylglycerol lipase
MGL, monoacylglycerol lipase
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Biochemistry, 4th Edition
Eicosanoids: Prostaglandins,
Thromboxanes, and Leukotrienes
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COX-1 is constitutively expressed in most tissues, and is
responsible for the physiological production of prostaglandins.
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COX-2 is induced by cytokines, mitogens, and endotoxins in
inflammatory cells and is responsible for the elevated production
of prostaglandins during inflammation.
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Both isoforms are covalently modified, and hence inactivated, by
reaction with aspirin (acetylsalicylic acid).
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As shown, aspirin acetylates a specific serine residue, which in
turn blocks access of the fatty acid substrate to the
cyclooxygenase active site.
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Biochemistry, 4th Edition
Eicosanoids: Prostaglandins,
Thromboxanes, and Leukotrienes
Nonsteroidal anti-inflammatory drugs
(NSAIDs):
•Ibuprofen and naproxen are examples of
nonselective COX Inhibitors.
•Rofecoxib (Vioxx®) and celecoxib (Celebrex®)
are selective COX-2 inhibitors.
o The phenylsulphonamide moieties,
which contribute to their selectivity,
are highlighted in red.
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Biochemistry, 4th Edition
Eicosanoids: Prostaglandins,
Thromboxanes, and Leukotrienes
Structural basis for selective inhibition of
COX-2:
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Ovine COX-1 with the nonselective NSAID
flurbiprofen bound in the COX active site.
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Arg120, which is important in binding the
normal substrate arachidonic acid, extends
behind flurbiprofen to interact with its
carboxylate.
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Mouse COX-2 with the selective inhibitor SC588 bound in the COX active site.
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The phenylsulfonamide group of SC-588
extends into the side pocket made accessible
by Val523.
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The bulkier isoleucine at this position would
prevent binding of SC-588 to COX-1.
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Biochemistry, 4th Edition
Eicosanoids: Prostaglandins,
Thromboxanes, and Leukotrienes
Biosynthesis of leukotrienes:
•Leukotriene C was originally discovered in the class of white blood cells
called polymorphonuclear leukocytes (PMNs) and was named after the
source (leukocytes) and the triene structure.
•It is a potent muscle contractant that is involved in the pathogenesis of
asthma, through constriction of the small airways in the lung.
•LTs are formed from the initial attack on arachidonate of a lipoxygenase,
which adds O2 to C-5, giving 5-HPETE.
•A dehydration to give the epoxide coupled with isomerization of double
bonds gives leukotriene A4.
•Hydrolysis of the epoxide ring yields leukotriene B4.
•Transfer of the thiol group of glutathione yields leukotriene C4.
•Subsequent modifications of the peptide chain yield related compounds,
leukotrienes D and E.
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