Download Lecture_11

Lipids
Fatty, oily, or waxy organic compound
Mostly made up of hydrocarbons hence they show very
little tendency to dissolve in water and are
“hydrophobic”.
Cells use different kinds as their:
1) Main energy reservoirs.
Fatty acids
Fat : Triglyceride
2) Structural materials e.g. in the
cell membrane and surface
coatings.
3) Signaling molecules.
Phospholipids
Sterols
Waxes
Fatty acids
Organic compound that
consists of a chain of
carbon atoms with an acidic
carboxyl group at one end .
Carbon chain of saturated
types has single bonds only;
that of unsaturated types
has one or more double
bonds
Stearic acid : Saturated fatty acid
Oleic acid : Monounsaturated fatty acid
Linoleic acid : Polyunsaturated fatty acid
Fats:
Lipids that have one, two or three fatty
acids attached to glycerol
Animal fats : butter, lard etc.
Plant fats : groundnut oil, sunflower oil, coconut oil etc.
Triglycerides
Most natural fats from animal and plant sources are made up of triglycerides: fats
having three fatty acid tails attached to glycerol.
http://www.familyhealthavenue.com/2009/
04/know-something-about-triglycerides/
Phospholipids
Lipid with a highly polar phosphate group in its hydrophilic head, and two
nonpolar, hydrophobic fatty-acid tails. Main constituent of eukaryotic cell
membranes which is made up of a bilayer of phospholipids. The heads of one
layer are in the interior of the cell which is a watery environment and the heads
of the other layer are dissolved in the fluid exterior of the cells.
http://academic.brooklyn.cuny.edu/biology/bio4fv/page/phosphb.htm
http://alevelnotes.com/Biological-Membranes/128
Understand the difference between fatty acids and fat
Fatty acids are stored in adipose tissue as triacylglycerols (TAG) in which fatty acids
are linked to glycerol with ester linkages.
Fatty acids have four major functions:
1.
Fatty acids are fuel molecules stored as triacylglycerols.
2.
Fatty acids are components of phospholipids and glycolipids.
3.
Fatty acids are attached to proteins to localize the proteins to membranes.
4.
Fatty acids function as hormones and intracellular messengers.
Fatty acid degradation and synthesis consist of four steps that are the reverse of each
other with regard to their chemistry.
Fatty acid degradation is an oxidative process that yields acetyl CoA.
Fatty acid synthesis is a reductive process that begins with acetyl CoA.
Triacylglycerols (TAG) are energy rich. Because they are hydrophobic and reduced, a
gram of anhydrous fat stores more than six times the energy of a gram of hydrated
glycogen.
TAG are stored in large droplets in the cytoplasm of adipocytes. Adipose tissue is
located throughout the body, with subcutaneous (below the skin) and visceral (around
the internal organs) deposits being most prominent.
Consider a typical 70-kg man, who has fuel reserves of 420,000 kJ (100,000 kcal) in
triacylglycerols, 100,000 kJ (24,000 kcal) in protein (mostly in muscle), 2500 kJ (600 kcal)
in glycogen, and 170 kJ (40 kcal) in glucose. Triacylglycerols constitute about 11 kg
of his total body weight. If this amount of energy were stored in glycogen, his total body
weight would be 64 kg greater.
Migratory birds use fat stores to power long
flights without the opportunity to feed.
Triacylglycerols from the diet form lipid droplets in the stomach. Bile acids, secreted as
bile salts by the gall bladder, insert into the lipid droplets, rendering them more
accessible to digestion by lipases.
Lipases, secreted by the pancreas, convert the triacylglycerols into two fatty acids and
monoacylglycerol.
The digestion products are carried as micelles to the intestinal epithelium cells for
absorption.
In the intestine, triacylglycerols are reformed from free fatty acids and
monoacylglycerol and packaged into lipoprotein particles called chylomicrons.
The chylomicrons eventually enter the blood so that the triacylglycerols can be
absorbed by tissues.
The fatty acids incorporated into triacylglycerols
in adipose tissue are made accessible in three
stages.
1. Degradation of TAG to release fatty acids and
glycerol into the blood for transport to energyrequiring tissues.
2. Activation of the fatty acids and transport into
the mitochondria for oxidation.
3. Degradation of the fatty acids to acetyl CoA
for processing by the citric acid cycle.
Triacylglycerols are stored in adipocytes in a lipid droplet.
Epinephrine and glucagon, acting through 7TM receptors, stimulate lipid breakdown or
lipolysis.
Protein kinase A phosphorylates perilipin, which is associated with the lipid droplet, and
hormone-sensitive lipase.
Phosphorylation of perilipin results in the activation of adipocyte triacylglyceride lipase
(ATGL). ATGL initiates the breakdown of lipids.
The glycerol released during lipolysis is absorbed by the liver for use in glycolysis or
gluconeogenesis.
Fatty acids are transported in the blood bound to albumin. Glycerol is absorbed by the
liver, phosphorylated and converted into dihydroxyacetone phosphate or glyceraldehyde
3-phosphate.
Upon entering the cell cytoplasm, fatty acids are activated by attachment to coenzyme A,
in a reaction catalyzed by acyl CoA synthetase (fatty acid thiokinase). The reaction
proceeds through an acyl adenylate intermediate.
The reaction occurs in two steps:
1. The formation of the acyl adenylate.
2. The reaction of acyl adenylate with CoA to form acyl CoA.
The reaction is rendered irreversible by the action of pyrophosphatase.
After being activated by linkage to CoA, the fatty acid is transferred to carnitine, a
reaction catalyzed by carnitine acyltransferase I, for transport into the mitochondria. A
translocase transports the acyl carnitine into the mitochondria.
In the mitochondria, carnitine acyltransferase II transfers the fatty acid to CoA . The fatty
acyl CoA is now ready to be degraded.
A number of diseases have been traced to a deficiency of carnitine, the
transferase, or the translocase.
Inability to synthesize carnitine may be a contributing factor to the development
of autism in males.
The symptoms of carnitine deficiency range from mild muscle cramping to
severe weakness and even death.
In general, muscle, kidney, and heart are the tissues primarily impaired.
Muscle weakness during prolonged exercise is a symptom of a deficiency of
carnitine acyltransferases because muscle relies on fatty acids as a long-term
source of energy. Medium-chain (C8–C10) fatty acids are oxidized normally in
these patients because these fatty acids can enter the mitochondria, to some
degree, in the absence of carnitine.
These diseases illustrate that the impaired flow of a metabolite from one
compartment of a cell to another can lead to a pathological condition.
Fatty acid degradation consists of four steps that are repeated: an oxidation, a
hydration, another oxidation, followed by thiolysis.
Fatty acid degradation is also called β-oxidation because oxidation occurs at the βcarbon atom.
1. Oxidation of the β carbon, catalyzed by acyl CoA dehydrogenase, generates
trans-Δ2-enoyl CoA and FADH2.
The electrons travel from FADH2 to electron-transferring flavoprotein (ETF).
ETF-ubiquinone reductase transfers electrons from ETF to ubiquinone.
2. Hydration of trans-Δ2-enoyl CoA by enoyl CoA hydratase yields L-3hydroxyacyl CoA.
3. Oxidation of L-3-hydroxyacyl CoA by L-3-hydroxyacyl CoA
dehydrogenase generates 3-ketoacyl CoA and NADH.
4. Cleavage of the 3-ketoacyl CoA by thiolase forms acetyl CoA and a fatty
acid chain two carbons shorter.
The reaction for one round of β-oxidation is
The complete reaction for C16 palmitoyl CoA is
Processing of the products of the complete reaction by
cellular respiration would generate 106 molecules of ATP.
β-oxidation alone cannot degrade unsaturated fatty acids. When monounsaturated fatty
acids are degraded by β-oxidation, cis-Δ3-enoyl CoA is formed, which cannot be
processed by acyl CoA dehydrogenase.
Cis-Δ3-enoyl CoA isomerase converts the double bond into trans-Δ2 enoyl CoA, a
normal substrate for β-oxidation.
When polyunsaturated fatty acids are degraded by β-oxidation, cis-Δ3 enoyl CoA
isomerase is also required. 2,4-Dienoyl CoA is also generated, but cannot be
processed by the normal enzymes.
2,4-Dienoyl CoA is converted into trans-Δ3-enoyl CoA by 2,4-dienoyl CoA reductase,
and the isomerase converts this product to trans-Δ2-enoyl CoA, a normal substrate.
Unsaturated fatty acids with odd numbers of double bonds require only the isomerase.
Those with even numbers of double bonds require both the isomerase and reductase.
β-Oxidation of fatty acids with odd numbers
of carbons generates propionyl CoA in the
last thiolysis reaction.
Propionyl CoA carboxylase, a biotin
enzyme, adds a carbon to propionyl CoA to
form methylmalonyl CoA.
Succinyl CoA, a citric acid cycle component,
is subsequently formed from methylmalonyl
CoA by methylmalonyl CoA mutase, a
vitamin B12-requiring enzyme.
Cobalamin (vitamin B12)-requiring enzymes catalyze three types of reactions:
1. Intramolecular rearrangements
2. Methylations
3. Reduction of ribonucleotides to deoxyribonucleotides
In mammals, vitamin B12 is required only for the conversion of L-methylmalonyl CoA
into succinyl CoA and for the synthesis of methionine.
The core of cobalamin is a corrin ring with a cobalt atom.
Coenzyme B12 catalyzes exchanges of two groups bonded to adjacent carbon atoms.
The mutase reaction begins with the generation of a 5-deoxyadenosyl radical and
the Co2+ form of the coenzyme.
The radical removes a hydrogen atom from the substrate, generating a substrate
radical, which spontaneously rearranges: the carbonyl CoA migrates to the adjacent
carbon atom that relinquished the hydrogen atom.
This product radical removes a proton from 5-deoxyadenosine, regenerating the
deoxyadenosyl radical and forming succinyl CoA.
Oxidation of long-chain fatty acids can occur in peroxisomes. Oxidation halts with the
formation of octanoyl CoA.
The first dehydration in peroxisomal fatty acid degradation requires a flavoprotein
dehydrogenase that generates H2O2, which is converted into water and oxygen by
catalase.
Subsequent steps are identical to β-oxidation.
Peroxisomes do not function in patients with Zellweger syndrome. Liver, kidney, and muscle
abnormalities usually lead to death by age 6. The syndrome is caused by a defect in the import
of enzymes into the peroxisomes. Here we see a pathological condition resulting from an
inappropriate cellular distribution of enzymes.
What happens if Acetyl CoA generated during beta-oxidation
cannot enter TCA cycle?
Glycolysis, gluconeogenesis and fatty acid degradation
Ketone bodies
The acetyl CoA formed in fatty acid oxidation
enters the citric acid cycle only if fat and
carbohydrate degradation are appropriately
balanced. Acetyl CoA must combine with
oxaloacetate to gain entry to the citric acid cycle.
The availability of oxaloacetate, however, depends
on an adequate supply of carbohydrate.
Oxaloacetate is normally formed from pyruvate,
the product of glucose degradation in glycolysis,
by pyruvate carboxylase. If carbohydrate is
unavailable or improperly utilized, the
concentration of oxaloacetate is lowered and
acetyl CoA cannot enter the citric acid cycle. This
dependency is the molecular basis of the adage
that fats burn in the flame of carbohydrates.
Steps of Gluconeogenesis
Ketone bodies—acetoacetate, D-3-hydroxybutyrate and acetone—are synthesized from
acetyl CoA in liver mitochondria and secreted into the blood for use as a fuel by some
tissues such as heart muscle.
D-3-Hydroxybutyrate is formed upon the reduction of acetoacetate. Acetone is
generated by the spontaneous decarboxylation of acetoacetate.
Heart muscle, Kidney (renal cortex), brain – when desperate 
In tissues using ketone bodies, 3-hydroxybutyrate is oxidized to acetoacetate,
which is ultimately metabolized to two molecules of acetyl CoA.
Ketone bodies are moderately strong acids, and excess production can lead
to acidosis. An overproduction of ketone bodies can occur when diabetes, a
condition resulting from a lack of insulin function, is untreated. The
resulting acidosis is called diabetic ketosis.
If insulin is absent or not functioning, glucose cannot enter cells. All
energy must be derived from fats, leading to the production of acetyl CoA.
Acetyl CoA builds up because oxaloacetate, which can be generated from
glucose, is not available to replenish the citric acid cycle.
Moreover, fatty acid release from adipose tissue is enhanced in the absence
of insulin function.
1.
Interestingly, diets that promote ketone-body formation,
called ketogenic diets, are frequently used as a
therapeutic option for children with drug-resistant
epilepsy. Ketogenic diets are rich in fats and low in
carbohydrates, with adequate amounts of protein.
In essence, the body is forced into starvation mode,
where fats and ketone bodies become the main fuel
source. How such diets reduce the seizures suffered by
the children is currently unknown.
Research Atkins Diet.
Fats are converted into acetyl CoA which is then processed by the citric
acid cycle.
Oxaloacetate, a citric acid cycle intermediate, is a precursor to glucose.
However, acetyl CoA derived from fats cannot lead to the net synthesis
of oxaloacetate or glucose because, although two carbons enter the cycle
when acetyl CoA condenses with oxaloacetate, two carbons are lost as
CO2 before oxaloacetate is generated.
Saturated and trans unsaturated fatty acids (“trans fat”) are commercially synthesized
from polyunsaturated fatty acids to increase stability for storage and cooking.
Consumption of large amounts of saturated and trans fat has been linked to obesity, type
2 diabetes and atherosclerosis.
1. Fatty acid synthesis occurs in the cytoplasm, whereas degradation occurs in the
mitochondrial matrix.
2. Intermediates in synthesis are linked to the sulfhydryl group of acyl carrier protein
(ACP), whereas intermediates in degradation are linked to the sulfhydryl group of CoA.
3. Fatty acid synthase in higher organisms is a single polypeptide containing all of the
required enzyme activities. The enzymes for degradation do not appear to be associated
with one another.
4. The new fatty acid grows by the sequential addition of two carbon units from malonyl
ACP, which is derived from acetyl CoA. The decarboxylation of malonyl CoA powers
synthesis.
5. The reductant in fatty acid synthesis is NADPH, whereas the oxidants in degradation are
NAD+ and FAD.
6. The isomeric form of the hydroxyacyl intermediate differs: the L form is found in
degradation and the D form in synthesis.
Malonyl CoA is synthesized by acetyl CoA carboxylase 1, a biotin-requiring
enzyme.
The formation of malonyl CoA occurs in two steps:
Acyl carrier protein (ACP) is a 77 amino acid protein. Phosphopantetheine, a
component of CoA also, is attached to a serine residue of the protein.
Fatty acid synthesis occurs on the acyl carrier protein (ACP), a polypeptide linked to
CoA. Intermediates are linked to the sulfhydryl group of the CoA attached to ACP.
Acetyl transacylase and malonyl transacylase attach substrates to the ACP.
β-Ketoacyl synthase catalyzes the condensation of acetyl ACP and malonyl ACP to form
acetoacetyl ACP.
The next three steps—a reduction, dehydration, and another reduction—convert the keto
group at carbon 3 to a methylene group (-CH2-), forming butyryl ACP. The
corresponding enzymes are β-ketoacyl reductase, 3-hydroxylacyl dehydratase, and enoyl
reductase.
NADPH is the source of reducing power.
The second round of synthesis begins with the condensation of malonyl CoA with the
newly synthesized butyryl ACP, forming C6-β-ketoacyl ACP.
The reduction, dehydration, reduction sequence is repeated.
Synthesis continues until C16-acyl ACP is formed; this is cleaved by thioesterase to
yield palmitate.
The elongation cycles continue until C16-acyl ACP
is formed. This intermediate is a good substrate
for a thioesterase that hydrolyzes C16-acyl ACP to
yield palmitate and ACP. The thioesterase acts as
a ruler to determine fatty acid chain length.
The bacterial enzyme that catalyzes this step, enoyl
reductase, can be inhibited by triclosan, a broad-spectrum
antibacterial agent that is added to a variety of products
such as toothpaste, soaps, and skin creams.
The reactions of fatty acid synthesis are similar in E. coli and animals.
In animals, all of the enzymes required for fatty acid synthesis are components of a single
polypeptide chain.
The functional enzyme is composed of two identical chains.
The enzyme consists of two distinct compartments.
The selecting and condensing compartment binds the acetyl and malonyl substrates
and condenses them.
The modification compartment carries out the reduction and dehydration activities
required for elongation.
A catalytic cycle of mammalian fatty acid synthase involves seven steps.
1. ACP delivers an acetyl unit to the synthase (KS) and accepts a malonyl unit from MAT.
2. ACP delivers the malonyl unit to KS, which forms the keto acyl product, still attached to
ACP.
3. ACP visits the reductase (KR), which reduces the keto group to an alcohol.
4. The alcohol product is delivered to the dehydratase (DH), which introduces a double
bond with the release of water.
5. The enoyl product visits the reductase (ER), which reduces the double bond.
6. ACP hands off the reduced product to KS and receives another malonyl from MAT.
7. KS condenses the two molecules on ACP, which is ready for another reaction cycle.
Function of Fatty Acid Synthase is thematically very similar to Pyruvate Dehydrogenase Complex
The stoichiometry for the synthesis of palmitate is
The synthesis of the required malonyl CoA is described by the following reaction
Thus, the stoichiometry for the synthesis of palmitate from acetyl CoA is
Citrate, synthesized in the mitochondria, is
transported to the cytoplasm and cleaved by ATPcitrate lyase to generate acetyl CoA for fatty acid
synthesis.
Fatty acid synthesis requires reducing power in the form of NADPH.
Some NADPH can be formed from the oxidation of oxaloacetate, generated by ATPcitrate lyase, by the combined action of cytoplasmic malate dehydrogenase and malic
enzyme.
Pyruvate formed by malic enzyme enters the mitochondria where it is converted into
oxaloacetate by pyruvate carboxylase.
The sum of the reactions catalyzed by malate dehydrogenase, malic enzyme, and
pyruvate carboxylase is
Additional NADPH is synthesized by the pentose phosphate pathway.
Tumors require large amounts of fatty acid synthesis to produce precursors for
membrane synthesis.
β-Ketoacyl ACP synthase inhibitors retard tumor growth.
Mice treated with synthase inhibitors also showed dramatic weight loss, suggesting
that such drugs may be used to treat obesity.
Acetyl CoA carboxylase may also be a target for inhibiting cancer cell growth.
Fatty acid synthase cannot generate fatty acids longer than C16 palmitate.
Longer fatty acids are synthesized by enzymes attached to the endoplasmic reticulum.
These enzymes extend palmitate by adding two-carbon units, using malonyl CoA as a
substrate.
The introduction of double bonds is catalyzed by a complex of three membranebound proteins: NADH-cytochrome b5 reductase, cytochrome b5 and a desaturase.
Mammals lack the enzymes that
introduce double bonds beyond carbon
9.
Thus, linoleate and linolenate are
essential fatty acids that must be
obtained in the diet.
Arachidonate, a 20-carbon fatty acid with four double bonds, is derived from linoleate.
Arachidonate is a precursor for a variety of signal molecules 20 carbons long,
collectively called the eicosanoids.
These signal molecules, which include prostaglandins, are local hormones because they
are short-lived and only affect nearby cells.
Aspirin blocks access to the active site of the enzyme that converts arachidonate into
prostaglandin H2. Because arachidonate is the precursor of other prostaglandins,
prostacyclin, and thromboxanes, blocking this step interferes with many signaling
pathways. Aspirin’s ability to obstruct these pathways accounts for its wide-ranging
effects on inflammation, fever, pain, and blood clotting.
The mammalian fatty acid synthase is a member of a family of complex
enzymes called megasynthases.
Such enzymes synthesize polyketides and nonribosomal peptides, some of
which are important antibiotics.
Acetyl CoA carboxylase 1 is subject to regulation on several levels.
Carboxylase 1 is inhibited when phosphorylated by AMP-dependent kinase (AMPK).
Inhibition due to phosphorylation is reversed by protein phosphatase 2A.
Citrate activates carboxylase, in conjunction with a protein MIG12, by facilitating the
formation of active polymers of the carboxylase. Citrate mitigates inhibition due to
phosphorylation.
Palmitoyl CoA, the end product of fatty acid synthase, inhibits carboxylase by causing
depolymerization of the enzyme.
Acetyl CoA carboxylase 2, a mitochondrial enzyme, inhibits fatty acid degradation
because its product, malonyl CoA, prevents the entry of fatty acyl CoA into the
mitochondria by inhibiting carnitine acyltransferase 1.
Glucagon and epinephrine inhibit carboxylase by enhancing AMPK activity.
Insulin stimulates the dephosphorylation and activation of carboxylase.
The enzymes of fatty acid synthesis are regulated by adaptive control. If adequate fats are
not present in the diet, the synthesis of enzymes required for fatty acid synthesis is
enhanced.