Transcript Slide 1

Fatty Acid Synthesis
Lecture 16
Modified from internet sources,
journals and books
Fatty Acid Synthesis
• Prediction: the pathway for the synthesis of fatty
acids would be the reversal of the oxidation
pathway
• this would not allow distinct regulation of the two
pathways to occur even given the fact that the
pathways are separated within different cellular
compartments
• pathway for fatty acid synthesis occurs in the
cytoplasm (oxidation occurs in the mitochondria)
• the essential chemistry of the two processes 
reversals of each other
continued
• oxidation and synthesis of fats utilize an
activated two carbon intermediate  acetyl-CoA
• acetyl-CoA in fat synthesis  exists temporarily
bound to the enzyme complex as malonyl-CoA
• synthesis of malonyl-CoA  the first committed
step of fatty acid synthesis
• the enzyme that catalyzes this reaction 
acetyl-CoA carboxylase (ACC) = the major site
of regulation of fatty acid synthesis
The rate of fatty acid synthesis
• controlled by the equilibrium between
monomeric ACC and polymeric ACC
• activity of ACC requires polymerization  the
conformational change is enhanced by citrate
and inhibited by long-chain fatty acids
• ACC is also controlled through hormone
mediated phosphorylation (see below).
• The acetyl-CoA and malonyl-CoA are transferred
to ACP (acetyl-CoA phosphatase) by the action
of acetyl-CoA transacylase and malonyl-CoA
transacylase, respectively
continued
• attachment of these carbon atoms to ACP
allows them to enter the fatty acid
synthesis cycle.
• The synthesis of fatty acids from acetylCoA and malonyl-CoA  carried out by
fatty acid synthase (FAS)
continued
• All of the reactions of fatty acid synthesis are carried out
by the multiple enzymatic activities of FAS (fatty acid
synthase)
• like fat oxidation  fat synthesis involves 4 enzymatic
activities:
• β-keto-ACP synthase, β-keto-ACP reductase, 3-OH acylACP dehydratase and enoyl-CoA reductase (the two
reduction reactions require NADPH oxidation to NADP+)
• the primary fatty acid synthesized by FAS is palmitate;
then released from the enzyme and can then undergo
separate elongation and/or unsaturation to yield other
fatty acid molecules
Origin of Cytoplasmic AcetylCoA
• Acetyl-CoA  generated in the mitochondria primarily
from two sources:
• the pyruvate dehydrogenase (PDH) reaction
• fatty acid oxidation
• these acetyl units to be utilized for fatty acid synthesis 
they must be present in the cytoplasm
• shift from fatty acid oxidation and glycolytic oxidation
occurs when the need for energy diminishes
• This results in  reduced oxidation of acetyl-CoA in the
TCA cycle and the oxidative phosphorylation
pathway
• Under these conditions  the mitochondrial acetyl units
can be stored as fat for future energy demands
continued
• Acetyl-CoA  enters the cytoplasm in the
form of citrate via the tricarboxylate
transport system
• In the cytoplasm  citrate is converted to
oxaloacetate and acetyl-CoA (by the ATP
driven ATP-citrate lyase reaction)
• resultant oxaloacetate  is converted to
malate by malate dehydrogenase (MDH)
continued
• The malate produced by this pathway  can
undergo oxidative decarboxylation by malic
enzyme
• co-enzyme for this reaction is NADP+
generating NADPH
• advantage of this series of reactions for
converting mitochondrial acetyl-CoA into
cytoplasmic acetyl-CoA  the NADPH produced
by the malic enzyme reaction can be a major
source of reducing co-factor for the fatty acid
synthase activities
Regulation of Fatty Acid
Metabolism
• must consider the global organismal energy
requirements in order to effectively understand
how the synthesis and degradation of fats (and
also carbohydrates) needs to be exquisitely
regulated
• blood  is the carrier of triacylglycerols in the
form of VLDLs and chylomicrons, fatty acids
bound to albumin, amino acids, lactate, ketone
bodies and glucose
• The pancreas  is the primary organ involved in
sensing the organisms dietary and energetic
states via glucose concentrations in the blood
continued
• The regulation of fat metabolism occurs
via distinct mechanisms:
• short term regulation  regulation effected
by events such as substrate availability,
allosteric effectors and/or enzyme
modification
• ACC (acetyl-CoA carboxylase)  the
rate-limiting (committed) step in fatty acid
synthesis
continued
• two major isoforms of ACC in mammalian
tissues:
• ACC1 and ACC2
• ACC1  is strictly cytosolic and is enriched in
liver, adipose tissue and lactating mammary
tissue
• ACC2  originally discovered in rat heart but is
also expressed in liver and skeletal muscle
• Both isoforms of ACC  allosterically activated
by citrate and inhibited by palmitoyl-CoA and
other short- and long-chain fatty acyl-CoAs
continued
• Citrate  triggers the polymerization of ACC1
which leads to significant increases in its activity
• ACC2  does not undergo significant
polymerization (presumably due to its
mitochondrial association), is allosterically
activated by citrate
• Glutamate and other dicarboxylic acids can also
allosterically activate both ACC isoforms
continued
• ACC activity can also be affected by
phosphorylation
• Glucagon-stimulation  increases in cAMP and
subsequently increasing PKA activity also lead
to phosphorylation of ACC and ACC2
• This insulin-mediated effect  has not been
observed in hepatocytes or adipose tissues cells
• Activation of α-adrenergic receptors in liver and
skeletal muscle cells  inhibits ACC activity as a
result of phosphorylation (undetermined kinase)
continued
• Control of a given pathways' regulatory enzymes
can also occur by alteration of enzyme synthesis
and turn-over rates  these changes are long
term regulatory effects
• Insulin  stimulates ACC and FAS synthesis,
whereas, starvation leads to decreased
synthesis of these enzymes
• Adipose tissue lipoprotein lipase levels  also
are increased by insulin and decreased by
starvation
continued
• in contrast to the effects of insulin and starvation
on adipose tissue  their effects on heart
lipoprotein lipase are just the inverse
• this allows the heart to absorb any available fatty
acids in the blood in order to oxidize them for
energy production
• Adipose tissue  contains hormone-sensitive
lipase (HSL), that is activated by PKAdependent phosphorylation leading to increased
fatty acid release to the blood
continued
• In the liver  the net result of activation of HSL (due to
increased acetyl-CoA levels) is the production of ketone
bodies
• This would occur under conditions where insufficient
carbohydrate stores and gluconeogenic precursors were
available in liver for increased glucose production
• Insulin  has the opposite effect to glucagon and epi
leading to increased glycogen and triacylglyceride
synthesis
• One of the many effects of insulin  to lower cAMP
levels which leads to increased dephosphorylation
through the enhanced activity of protein phosphatases
ChREBP: Master Lipid Regulator
in the Liver
• ChREBP = helix-loop-helix/leucine zipper (bHLH/LZ)
transcription factor, carbohydrate-responsive elementbinding protein  has emerged as a central regulator of
lipid synthesis in liver
• ChREBP  identified as a major glucose-responsive
transcription factor and it is required for glucose-induced
expression of the hepatic isozyme of the glycolytic
enzyme pyruvate kinase (identified as L-PK)
• ChREBP  acts to induce lipogenic genes such as
acetyl-CoA carboxylase (ACC) and fatty acid synthase
(FAS)
continued
• Expression of the ChREBP gene  induced in
the liver in response to increased glucose
uptake
• Under conditions of low (basal) glucose
concentration  ChREBP is phosphorylated and
resides in the cytosol
• An emerging model of the role of ChREBP in
overall glucose and lipid metabolism  indicates
it a master regulator of glucose-mediated lipid
homeostasis not only in the liver but also in
adipose tissue
Elongation and Desaturation
• The fatty acid product released from FAS is palmitate (a
16:0 fatty acid, i.e. 16 carbons and no sites of
unsaturation)
• Elongation and unsaturation of fatty acids  occurs in
both the mitochondria and endoplasmic reticulum
• The predominant site of these processes  the ER
membranes
• Elongation  involves condensation of acyl-CoA groups
with malonyl-CoA
• resultant product  two carbons longer (CO2 is released
from malonyl-CoA as in the FAS reaction) which
undergoes reduction, dehydration and reduction yielding
a saturated fatty acid
• Mitochondrial elongation  involves acetyl-CoA units
and is a reversal of oxidation
continued
• Desaturation occurs in the ER membranes
• involves 4 broad specificity fatty acyl-CoA desaturases
(non-heme iron containing enzymes)
• These enzymes  introduce unsaturation at C4, C5, C6
or C9
• electrons transferred from the oxidized fatty acids during
desaturation  are transferred from the desaturases to
cytochrome b5 and then NADH-cytochrome b5
reductase
• These electrons  are un-coupled from mitochondrial
oxidative-phosphorylation and do not yield ATP
• Since these enzymes cannot introduce sites of
unsaturation beyond C9  they cannot
synthesize either linoleate (18:2Δ9,12) or
linolenate (18:3Δ9,12,15)
• These fatty acids must be acquired from the diet
 referred to as essential fatty acids
• Linoleic  especially important in that it is
required for the synthesis of arachidonic acid
• arachindonate  a precursor for the eicosanoids
(the prostaglandins and thromboxanes)
continued
• role of fatty acids in eicosanoid synthesis
 that leads to poor growth, wound
healing and dermatitis in persons on fat
free diets
• linoleic acid  a constituent of epidermal
cell sphingolipids that function as the skins
water permeability barrier
Synthesis of Triglycerides
• Fatty acids  stored for future use as triacylglycerols in
all cells, but primarily in adipocytes of adipose tissue
• fatty acids present in triacylglycerols  predominantly
saturated
• major building block for the synthesis of triacylglycerols,
in tissues other than adipose tissue, = glycerol
• Adipocytes lack glycerol kinase  dihydroxyacetone
phosphate (DHAP), produced during glycolysis, is the
precursor for triacylglycerol synthesis in adipose tissue
• adipoctes must have glucose to oxidize in order to store
fatty acids in the form of triacylglycerols
continued
• The glycerol backbone of triacylglycerols 
activated by phosphorylation at the C-3 position
by glycerol kinase
• The fatty acids incorporated into triacylglycerols
 activated to acyl-CoAs through the action of
acyl-CoA synthetases
• Two molecules of acyl-CoA  esterified to
glycerol-3-phosphate to yield 1,2-diacylglycerol
phosphate (commonly identified as
phosphatidic acid).
continued
• The phosphate is then removed  to yield
1,2-diacylglycerol, the substrate for
addition of the third fatty acid
• Intestinal monoacylglycerols, derived from
the hydrolysis of dietary fats, can also
serve as substrates for the synthesis of
1,2-diacylglycerols
Phospholipid Structures
• Phospholipids  synthesized by esterification of
an alcohol to the phosphate of phosphatidic acid
(1,2-diacylglycerol 3-phosphate)
• Most phospholipids  a saturated fatty acid on
C-1 and an unsaturated fatty acid on C-2 of the
glycerol backbone
• The most commonly added alcohols = serine,
ethanolamine and choline
• The major classifications of phospholipids are:
Phosphatidylcholine (PC)
PC
• This class of phospholipids  also called the
lecithins
• At physiological pH  phosphatidylcholines are
neutral
• contain primarily palmitic or stearic acid at
carbon 1 and primarily oleic, linoleic or linolenic
acid at carbon 2
• lecithin dipalmitoyllecithin  a component of
lung or pulmonary surfactant
• the major (80%) phospholipid found in the
extracellular lipid layer lining the pulmonary
alveoli
Phosphatidylethanolamine (PE)
PE
• These molecules are neutral at
physiological pH
• contain primarily palmitic or stearic acid on
carbon 1 and a long chain unsaturated
fatty acid (e.g. 18:2, 20:4 and 22:6) on
carbon 2
Phosphatidylserine (PS)
PS
• composed of fatty acids similar to the
phosphatidyl-ethanol-amines
• PE is in the lipid bilayer of the a
membrane
Phosphatidylinositol (PI)
PI
• contain almost exclusively stearic acid at carbon
1 and arachidonic acid at carbon 2
• molecules exist in membranes with various
levels of phosphate esterified to the hydroxyls of
the inositol
• Molecules with phosphorylated inositol 
polyphosphoinositides
• polyphosphoinositides  important intracellular
transducers of signals emanating from the
plasma membrane
continued
• One polyphosphoinositide
(phosphatidylinositol 4,5-bisphosphate,
PIP2)  a critically important membrane
phospholipid involved in the transmission
of signals for cell growth and differentiation
from outside the cell to inside
Phosphatidylglycerol (PG)
PG
• Phosphatidylglycerols  found in high
concentration in mitochondrial membranes and
as components of pulmonary surfactant
• Phosphatidylglycerol  a precursor for the
synthesis of cardiolipin (important component of
the inner mitochondrial membrane, where it
constitutes about 20% of the total lipid)
• vital role of PG  serve as the precursor for the
synthesis of diphosphatidylglycerols (DPGs)
Diphosphatidylglycerol (DPG)
DPG
• These molecules  very acidic
• primarily in the inner mitochondrial
membrane and also as components of
pulmonary surfactant
continued
• The fatty acid distribution at the C-1 and C-2 positions of
glycerol within phospholipids is continually in flux
• phospholipid degradation and the continuous
phospholipid remodeling that occurs while these
molecules are in membranes (= highly dynamic systems)
• Phospholipid degradation  results from the action of
phospholipases
• various phospholipases exhibiting substrate specificities
for different positions in phospholipids
• remodeling of acyl groups in phospholipids = the result
of the action of phospholipase A1 (PLA1) and
phospholipase A2 (PLA2)
Sites of Action of the
Phospholipases A1, A2, C and D.
continued
• products of these phospholipases  called
lysophospholipids and can be substrates for acyl
transferases utilizing different acyl-CoA groups
• PLA2  an important enzyme, whose activity is
responsible for the release of arachidonic acid from the
C-2 position of membrane phospholipids
• released arachidonate  a substrate for the synthesis
of the eicosanoids
• there is not just a single PLA2 enzyme; At least 19
enzymes have been identified with PLA2 activity 
involved in numerous processes including modification of
eicosanoid generation, host defense, and inflammation
• The cytosolic PLA2 family (cPLA2) 
essential component of the initiation of
arachidonic acid metabolism
• the sPLA2 enzymes  tightly regulated by
Ca2+ and by phosphorylation
Plasmalogens
• Plasmalogens are glycerol ether phospholipids
• Three major classes of plasmalogens have been
identified:
• choline, ethanolamine and serine plasmalogens
• Ethanolamine plasmalogen  prevalent in myelin
• Choline plasmalogen  abundant in cardiac tissue.
• One choline (1-O-1'-enyl-2-acetyl-sn-glycero-3phosphocholine)  identified as an extremely powerful
biological mediator  is called platelet activating factor=
PAF
continued
• PAF functions as:
• a mediator of hypersensitivity, acute inflammatory
reactions and anaphylactic shock
• PAF is synthesized in response to the formation of
antigen-IgE complexes on the surfaces of basophils,
neutrophils, eosinophils, macrophages and monocytes
• synthesis and release of PAF from cells  leads to
platelet aggregation and the release of serotonin from
platelets
• PAF also produces responses in liver, heart, smooth
muscle, and uterine and lung tissues
Platelet activating factor
Metabolism of the Sphingolipids
• The sphingolipids (like the phospholipids)
 composed of a polar head group and
two nonpolar tails
• core of sphingolipids  the long-chain
amino alcohol, sphingosine
Sphingosine
Basic composition of a ceramide
"n" indicates any fatty acid may be N-acetylated at this
position
continued
• The sphingolipids  include the sphingomyelins
and glycosphingolipids (the cerebrosides,
sulfatides, globosides and gangliosides)
• Sphingolipids  a component of all membranes
but are particularly abundant in the myelin
sheath
• Sphingomyelins are sphingolipids
• Sphingomyelins  important structural lipid
components of nerve cell membranes
A Sphingomyelin
• Defects in the enzyme acid sphingomyelinase 
result in the lysosomal storage disease known
as Niemann-Pick disease
• NP disease  caused by acid sphingomyelinase
deficiencies
• due to defects in the NPC1 gene and a NPC2
gene
• four principal classes of glycosphingolipids are:
• cerebrosides, sulfatides, globosides and
gangliosides
continued
• Cerebrosides  most common of these is
galactose (galactocerebrosides)
• Galactocerebrosides  found predominantly in
neuronal cell membranes
• glucocerebrosides  not normally found in
membranes; they represent intermediates in the
synthesis or degradation of more complex
glycosphingolipids
• Excess lysosomal accumulation of
glucocerebrosides is observed in Gaucher
disease
A Glucocerebroside
Clinical Significances of
Sphingolipids
• Some of the most devastating inborn errors in
metabolism  those associated with defects in the
enzymes responsible for the lysosomal degradation of
membrane glycosphingolipids (particularly abundant in
the membranes of neural cells)
• Many of these disorders  lead to severe psycho-motor
retardation and early lethality
• the disorders are caused by defective lysosomal
enzymes  result being lysosomal accumulation of
pathway intermediates
• these diseases  often referred to as lysosomal
storage diseases
Pathways and intermediates in
glycosphingolipid metabolism
• Enzymes are indicated in green and the
disease(s) associated with defects in the
indicated enzyme are shown in blue
• SAP-A, SAP-B, SAP-C, and SAP-D = the
saposins which are a family of small
glycoproteins
• The saposins (A, B, C, and D) are all derived
from a single precursor  prosaposin
• mature saposins, as well as prosaposin 
activate several lysosomal hydrolases involved
in the metabolism of various sphingolipids
Disorders Associated with Abnormal
Sphingolipid Metabolism
• Tay-Sachs disease
• infantile form: rapidly progressing mental
retardation, blindness, early mortality
• Gaucher disease
• hepatosplenomegaly, mental retardation in
infantile form, long bone degeneration
continued
•
•
•
•
Fabry disease
kidney failure, skin rashes
Niemann-Pick diseases
type A is severe disorder with
heptosplenomegaly, severe neurological
involvement leading to early death, type B
only visceral involvement
Clinically important classes of
sphingolipids
• One of the most clinically important classes of
sphingolipids  those that confer antigenic
determinants on the surfaces of cells 
particularly the erythrocytes
• ABO blood group antigens  the carbohydrate
moieties of glycolipids on the surface of cells as
well as the carbohydrate portion of serum
glycoproteins
• When present on the surface of cells the ABO
carbohydrates are linked to sphingolipid and are
therefore of the glycosphingolipid class
continued
• When the ABO carbohydrates are associated
with protein in the form of glycoproteins  are
found in the serum and are referred to as the
secreted forms
• Some individuals produce the glycoprotein forms
of the ABO antigens while others do not
• This property distinguishes secretors from nonsecretors, a property that has forensic
importance such as in cases of rape.
RDS
• A significant cause of death in premature infants and, on
occasion, in full term infants = respiratory distress
syndrome (RDS) or hyaline membrane disease
• caused by an insufficient amount of pulmonary
surfactant
• normal conditions  the surfactant is synthesized by
type II endothelial cells and is secreted into the alveolar
spaces to prevent atelectasis
• Surfactant  comprised primarily of dipalmitoyllecithin
(additional lipid components include phosphatidylglycerol
and phosphatidylinositol)
continued
• During the third trimester  the fetal lung
synthesizes primarily sphingomyelin, and type II
endothelial cells convert the majority of their
stored glycogen to fatty acids and then to
dipalmitoyllecithin
• Fetal lung maturity  can be determined by
measuring the ratio of lecithin to sphingomyelin
(L/S ratio) in the amniotic fluid
• An L/S ratio less than 2.0 indicates a potential
risk of RDS
• The risk is nearly 75-80% when the L/S ratio is
1.5
continued
• The carbohydrate portion of the
ganglioside, GM1, present on the surface
of intestinal epithelial cells  the site of
attachment of cholera toxin, the protein
secreted by Vibrio cholerae
• These are just a few examples of how
sphingolipids and glycosphingolipids are
involved in various recognition functions at
the surface of cells