Download 18. Metabolism of lipids 1

METABOLISM OF LIPIDS:
DIGESTION OF LIPIDS. TRANSPORT
FORMS OF LIPIDS
PHYSIOLOGICAL ROLE OF LIPIDS
 Energetic role (fuel
molecules)
 Components of
membranes
(structural role)
 Precursors for many
hormones (steroids)
 Signal molecules
(prostaglandins)
 Protective role (lipids
surround important organs)
 Enzyme cofactors (vitamin K)
 Electron carriers (ubiquinone)
 Insulation against
temperature extremes
TRIACYLGLYCEROLS ARE HIGHLY
CONCENTRATED ENERGY STORES
•Triacylglycerols (TGs) and glycogen two major forms of stored energy
TGs which are more efficient energy
stores because:
(1) They are stored in an anhydrous form
(2) Their fatty acids are more reduced
than monosaccharides.
• 1 g of triacylglycerols stores more than six times
as much energy as a 1 g of glycogen
• Glycogen reserves are depleted in 12 to 24 hours
after eating, triacylglycerols within several
weeks.
•Fat breakdown
about 50 % of energy in liver, kidney and skeletal
muscles up to 95 % of energy cardiac muscle
•Fats are the major source of energy for:
fasting animal organism in diabetes
• Fatty acids and glycerol substances that are directly used
as a fuel by mammalian organisms.
• Fatty acids (FA) and glycerol for
metabolic fuels are obtained
from triacylglycerols:
(1) In the diet
(2) Stored in adipocytes (fat
storage cells)
• Free fatty acids occur only in
trace amounts in cells
•For supplying of fatty acids as a fuel for organism, the
triacylglycerols have to be digested
DIGESTION OF DIETARY LIPIDS
Lipids in diet:
 triacylglycerols
 phospholipids
 cholesterol
Digestion – in small intestine.
Enzyme – pancreatic lipase.
Lipase catalyzes hydrolysis at the C1 and C3 positions of
TGs producing free fatty acids and 2-monoacylglycerol.
Colipase – protein which is present in the intestine and helps
bind the water-soluble lipase to the lipid substrates.
Colipase also activates lipase.
Bile salts (salts of bile acids) are required for lipids digestion.
Bile salts are synthesized in the liver from cholesterol.
Taurocholate and glycocholate - the most abundant bile salts.
Amphipathic: hydrophilic (blue) and hydrophobic (black)
TGs are water insoluble and lipase is water soluble.
Digestion of TGs takes place at lipid-water interfaces.
Rate of digestion depends on the surface area of the
interface.
Bile salts are amphipathic, they act as detergent
emulsifying the lipid drops and increasing the surface area
of the interface.
Bile salts also activates the lipase.
Inadequate production of bile salts results in
steatorrhea.
Dietary phospholipids are degraded by
phospholipases
Phospholipases are synthesized in the pancreas.
Major phospholipase is phospholipase A2 (catalyses the
hydrolysis of ester bond at C2 of glycerophospholipids
and lysophosphoglycerides are formed).
Lysophosphoglycerides are
absorbed and in
the intestinal
cells are
reesterified
back to glycerophospholipids.
Lysophosphoglycerides can act as detergent
and therefore in high concentration can
disrupt cellular membranes.
Lysophosphoglyceride is normally present in
cells in low concentration.
Snake venom contain
phospholipase A2 and
causes the lysis of
erythrocytes
membranes.
Dietary cholesterol
• Most dietary cholesterol is unesterified
• Cholesteryl esters are hydrolyzed in the intestine by
an intestinal esterase
• Free cholesterol is solublized by bile-salt micelles for
absorption
• After absorption in the intestinal cells cholesterol
react with acyl-CoA to form cholesteryl ester.
ABSORPTION OF DIETARY LIPIDS
Lipid absorption – passive diffusion process.
2-monoacylglycerols, fatty acids,
lysophosphoglycerides, free cholesterol form
micelles with bile salts.
Micelles migrate to the microvilli and lipids diffuse
into the cells.
Bile acids are actively absorbed and transferred
to the liver via portal vein.
Bile salts can circulate through intestine and liver
several time per day.
In the intestinal cells the fatty acids are converted to fatty
acyl CoA molecules.
Three of these molecules can combine with glycerol, or two
with monoacylglycerol, to form a triacylglycerols.
O
1.
CH2
OH
CH
O
CH2
OH
O
C
R2 + R1
CO
SCoA
CH2
O
C
O
CH
O
C
CH2
OH
2.
O
C
O
CH
O
C
CH2
OH
R2 + HSCoA
O
O
CH2
R1
R1
R2 + R3
CO
SCoA
CH2
O
C
O
R1
CH
O
C
O
R2 + HSCoA
CH2
O
C
R3
1-st reaction is catalyzed by monoacylglycerol acyltransferase
2-nd reaction is catalyzed by diacylglycerol acyltransferase
TRANSPORT FORMS OF LIPIDS
• TGs, cholesterol and cholesterol esters are insoluble in water
and cannot be transported in blood or lymph as free molecules
• These lipids assemble
with phospholipids and
apoproteins
(apolipoproteins) to
form spherical
particles called
lipoprotein
Structure:
Hydrophobic core:
-TGs,
-cholesteryl esters
Hydrophilic surfaces:
-cholesterol,
-phospholipids,
-apolipoproteins
The main classes of lipoproteins
1.Chylomicrons.
2.Very low density lipoproteins (VLDL).
3.Intermediate density lipoproteins (IDL).
4.Low density lipoproteins (LDL).
5.High density lipoproteins (HDL).
Chylomicrons
• are the largest lipoproteins (180 to 500 nm in diameter)
• are synthesized in the ER of intestinal cells
• contain 85 % of TGs (it is the main transport form of dietary TGs).
• apoprotein B-48 (apo B-48) is the main protein component
• deliver TGs from the intestine (via lymph and blood) to tissues (muscle
for energy, adipose for storage).
• bind to membrane-bound lipoprotein lipase (at adipose tissue and
muscle), where the triacylglycerols are again degraded into free fatty
acids and monoacylglycerol for transport into the tissue
• are present in blood only after feeding
exocytosis
Lymphatic
vessel
• are formed in the liver
VLDL
• contain 50 % of TGs and 22 % of cholesterol
• two lipoproteins — apo B-100 and apo E
• the main transport form of TGs synthesized in the organism (liver)
• deliver the TGs from liver to peripheral tissue (muscle for energy,
adipose for storage)
• bind to membrane-bound lipoprotein lipases (triacylglycerols are again
degraded into free fatty acids and monoacylglycerol)
triacylglycerol
cholesteryl esters
Apo B
Apo E
cholesterol
phospholipids
Lipoproteinlipase – enzyme which is located within
capillaries of muscles and adipose tissue
Function: hydrolyses of TGs of chylomicrons and VLDL.
Formed free fatty acids and glycerol pass into the cells
Chylomicrons and VLDL which gave up TGs are called remnants
of chylomicrons and remnants of VLDL
Remnants are rich in cholesterol esters
Remnants of chylomicrons are captured by liver
Remnants of VLDL are also called intermediate density
lipoproteins (IDL)
Fate of the IDL:
- some are taken by the liver
- others are degraded to the low density lipoproteins (LDL)
(by the removal of more triacylglycerol)
LDL
LDL are formed in the blood from IDL and in liver from IDL
(enzyme – liver lipase)
LDL are enriched in
cholesterol and
cholesteryl esters
(contain about 50 % of
cholesterol)
Protein component - apo
B-100
LDL is the major
carrier of cholesterol
(transport cholesterol
to peripheral tissue)
Cells of all organs have LDL receptors
Receptors for LDL are localized in specialized regions called
coated pits, which contain a specialized protein called clathrin
Apo B-100 on the surface of an LDL binds to the receptor
Receptor-LDL complex enters the cell by endocytosis.
Endocytic vesicle is formed
Vesicle fuse with lysosomes
Lysosomal lipases and proteases degrade LDL
LDL receptor itself returns to the plasma membrane
Apo B-100 is hydrolyzed to amino acids
Cholesteryl esters are hydrolyzed to free cholesterol and
fatty acids
Released free cholesterol:
- is incorporated into the membranes or
- is reesterified for storage inside the cell by the enzyme
acyl CoA:cholesterol acyltransferase (ACAT)
Feedback regulation:
abundance of intracellular cholesterol suppresses the
synthesis of LDL receptors and so the uptake of additional
cholesterol from plasma LDL is blocked
LDL uptake by receptor-mediated endocytosis
Familial hypercholesterolemia
 congenital disease when LDL receptor are not synthesized (mutation at a
single autosomal locus)
 the concentration of cholesterol in blood markedly increases
 severe atherosclerosis is developed (deposition of cholesterol in arteries)
 nodules of cholesterol called xanthomas are prominent in skin and tendons
 most homozygotes die of coronary artery disease in childhood
 the disease in heterozygotes (1 in 500 people) has a milder and more
variable clinical course
atherosclerosis
xanthomas
HDL
 are formed in the liver and partially in small intestine
 contain the great amount of proteins (about 40 %)
 pick up the
cholesterol from
peripheral tissue,
chylomicrons and
VLDL
 enzyme
acyltransferase in
HDL esterifies
cholesterols,
convert it to
cholesterol esters
and transport to
the liver
High serum levels of cholesterol
cause disease and death by
contributing to development of
atherosclerosis
Cholesterol which is present in the
form of the LDL is so-called "bad
cholesterol."
Cholesterol in the
form of HDL is
referred to as "good
cholesterol”
HDL functions as a
shuttle that moves
cholesterol
throughout the body
LDL/HDL Ratio
The ratio of cholesterol in the form of LDL to that in the
form of HDL can be used to evaluate susceptibility to
the development of atherosclerosis
For a
healthy
person,
the
LDL/HDL
ratio is
3.5
Transport Forms of Lipids
LIPID METABOLISM:
MOBILIZATION OF
TRIACYLGLYCEROLS;
OXIDATION OF
GLYCEROL
Storage and Mobilization of
Fatty Acids (FA)
• TGs are delivered to adipose
tissue in the form of
chylomicrones and VLDL,
hydrolyzed by lipoprotein
lipase into fatty acids and
glycerol, which are taken up
by adipocytes.
• Then fatty acids are
reesterified to TGs.
• TGs are stored in adipocytes.
• To supply energy demands
fatty acids and glycerol are
released – mobilisation of
TGs.
adipocyte
At low carbohydrate and insulin concentrations (during
fasting), TG hydrolysis is stimulated by epinephrine,
norepinephrine, glucagon, and adrenocorticotropic
hormone.
TG
hydrolysis is
inhibited
by insulin
in fed
state
•Lipolysis - hydrolysis of
triacylglycerols by lipases.
•A hormone-sensitive lipase
converts TGs to free fatty
acids and monoacylglycerol
•Monoacylglycerol is
hydrolyzed to fatty acid
and glycerol or by a
hormone-sensitive lipase or
by more specific and more
active monoacylglycerol
lipase
Transport of Fatty Acids and Glycerol
• Fatty acids and glycerol diffuse
through the adipocyte membrane and
enter bloodstream.
• Glycerol is transported via the blood
in free state and oxidized or converted
to glucose in liver.
• Fatty acids are traveled bound to
albumin.
• In heart, skeletal muscles and liver
they are oxidized with energy release.
Oxidation of Glycerol
Glycerol is absorbed by the liver.
Steps: phosphorylation, oxidation and isomerisation.
Glyceraldehyde 3-phosphate is an intermediate in:
 glycolytic pathway
 gluconeogenic pathways
Isomerase
ATP Generation from Glycerol Oxidation
glycerol – glycerol 3-phosphate
- 1 ATP
glycerol 3-phosphate - dihydroxyaceton
phosphate
2.5ATP (1 NADH)
glyceraldehyde 3-phosphate – pyruvate
4,5 ATP (1NADH + 2 ATP)
pyruvate – acetyl CoA
2.5 ATP (1 NADH)
acetyl CoA in Krebs cycle
10 ATP (3NADH + 1 FADH2 + 1GTP)
Total
19,5-1 = 18,5 ATP
LIPID
METABOLISM:
FATTY ACID
OXIDATION
Stages of fatty acid oxidation
(1) Activation of fatty acids takes place
on the outer mitochondrial membrane
(2) Transport into the mitochondria
(3) Degradation to two-carbon
fragments (as acetyl CoA) in the
mitochondrial matrix (b-oxidation
pathway)
(1) Activation of Fatty Acids
• Fatty acids are converted to CoA thioesters by
acyl-CoA synthetase (ATP dependent)
• The PPi released is hydrolyzed by a
pyrophosphatase to 2 Pi
• Two phosphoanhydride bonds (two ATP equivalents)
are consumed to activate one fatty acid to a
thioester
(2) Transport of Fatty Acyl CoA into Mitochondria
• The carnitine shuttle
system.
• Fatty acyl CoA is first
converted to acylcarnitine
(enzyme carnitine
acyltransferase I (bound to
the outer mitochondrial
membrane).
• Acylcarnitine enters the
mitochondria by a
translocase.
• The acyl group is transferred
back to CoA (enzyme carnitine acyltransferase II).
• Carnitine
shuttle
system
• Path of
acyl group
in red
(3) The Reactions of b oxidation
• The b-oxidation pathway (b-carbon atom (C3)
is oxidized) degrades fatty acids two carbons
at a time
b

1. Oxidation of acyl
CoA by an acyl CoA
dehydrogenase to
give an enoyl CoA
Coenzyme - FAD
2. Hydration of the
double bond between
C-2 and C-3 by enoyl
CoA hydratase with
the 3-hydroxyacyl
CoA (b-hydroxyacyl
CoA) formation
3. Oxidation of
3-hydroxyacyl CoA to
3-ketoacyl CoA by
3-hydroxyacyl CoA
dehydrogenase
Coenzyme – NAD+
4. Cleavage of
3-ketoacyl CoA by
the thiol group of
a second molecule
of CoA with the
formation of
acetyl CoA and an
acyl CoA
shortened by two
carbon atoms.
Enzyme b-ketothiolase.
The shortened acyl
CoA then
undergoes another
cycle of oxidation
The number of
cycles: n/2-1,
where n – the
number of carbon
atoms
b-Oxidation
of
Fatty acyl CoA
saturated fatty
acids
• One round of b oxidation: 4 enzyme steps
produce acetyl CoA from fatty acyl CoA
• Each round generates one molecule each of:
FADH2
NADH
Acetyl CoA
Fatty acyl CoA (2 carbons shorter each round)
Fates of the products of b-oxidation:
- NADH and FADH2 - are used in ETC
- acetyl CoA - enters the citric acid cycle
- acyl CoA – undergoes the next cycle of oxidation
ATP Generation from Fatty Acid Oxidation
Net yield of ATP per one oxidized palmitate
Palmitate (C15H31COOH) - 7 cycles – n/2-1
• The balanced equation for oxidizing one palmitoyl
CoA by seven cycles of b oxidation
Palmitoyl CoA + 7 HS-CoA + 7 FAD+ + 7 NAD+ + 7 H2O
8 Acetyl CoA + 7FADH2 + 7 NADH + 7 H+
ATP generated
8 acetyl CoA
7 FADH2
7 NADH
10x8=80
7x1.5=10.5
7x2.5=17.5
108 ATP
ATP expended to activate palmitate
Net yield:
-2
106 ATP
LIPID METABOLISM:
FATTY ACID
OXIDATION
b-OXIDATION OF ODD-CHAIN FATTY ACIDS
• Odd-chain fatty acids
occur in bacteria and
microorganisms
• Final cleavage product is
propionyl CoA rather
than acetyl CoA
• Three enzymes convert
propionyl CoA to succinyl
CoA (citric acid cycle
intermediate)
Propionyl CoA Is Converted into Succinyl CoA
1. Propionyl CoA is carboxylated to yield the D
isomer of methylmalonyl CoA.
The hydrolysis of an ATP is required.
Enzyme: propionyl CoA carboxylase
Coenzyme: biotin
2. The D isomer of methylmalonyl CoA is
racemized to the L isomer
Enzyme: methylmalonyl-CoA racemase
3. L isomer of methylmalonyl CoA is converted
into succinyl CoA by an intramolecular
rearrangement
Enzyme: methylmalonyl CoA mutase
Coenzyme: vitamin B12 (cobalamin)
OXIDATION OF FATTY ACIDS IN
PEROXISOMES
Peroxisomes - organelles containing
enzyme catalase, which catalyzes
the dismutation of hydrogen
peroxide into water and molecular
oxygen
Acyl CoA
dehydrogenase
transfers electrons
to O2 to yield H2O2
instead of
capturing the highenergy electrons by
ETC, as occurs in
mitochondrial boxidation.
METABOLISM OF
LIPIDS:
SYNTHESIS OF
FATTY ACIDS
Fatty Acid Synthesis
• Occurs mainly in liver and adipocytes, in
mammary glands during lactation
• Occurs in cytoplasm
• FA synthesis and degradation occur by
two completely separate pathways
• When glucose is plentiful, large amounts
of acetyl CoA are produced by glycolysis
and can be used for fatty acid synthesis
Three stages of fatty acid
synthesis:
A. Transport of acetyl CoA into
cytosol
B. Carboxylation of acetyl CoA
C. Assembly of fatty acid chain
A. Transport of Acetyl CoA to
the Cytosol
• Acetyl CoA from catabolism of
carbohydrates and amino acids is
exported from mitochondria via the
citrate transport system
• Cytosolic NADH also converted to NADPH
• Two molecules of ATP are expended for
each round of this cyclic pathway
Citrate transport
system
Sources of NADPH for Fatty Acid Synthesis
1. One molecule of NADPH is generated for each
molecule of acetyl CoA that is transferred from
mitochondria to the cytosol (malic enzyme).
2. NADPH molecules come from the pentose
phosphate pathway.
B. Carboxylation of Acetyl CoA
Enzyme: acetyl CoA carboxylase
Prosthetic group - biotin
A carboxybiotin intermediate is formed.
ATP is hydrolyzed.
The CO2 group in carboxybiotin is transferred to
acetyl CoA to form malonyl CoA.
Acetyl CoA carboxylase is the regulatory enzyme.
C. The Reactions of Fatty Acid Synthesis
• Five separate stages:
(1) Loading of precursors via thioester
derivatives
(2) Condensation of the precursors
(3) Reduction
(4) Dehydration
(5) Reduction
During the fatty acid synthesis all intermediates are linked
to the protein called acyl carrier protein (ACP-SH), which
is the component of fatty acyl synthase complex.
The pantothenic acid is
a component of ACP.
Intermediates in the
biosynthetic pathway
are attached to the
sulfhydryl terminus of
phosphopantotheine
group.
The elongation phase of fatty acid synthesis starts with
the formation of acetyl ACP and malonyl ACP.
Acetyl transacylase and malonyl transacylase catalyze
these reactions.
Acetyl CoA + ACP  acetyl ACP + CoA
Malonyl CoA + ACP  malonyl ACP + CoA
Condensation
reaction.
Acetyl ACP and
malonyl ACP react to
form acetoacetyl
ACP.
Enzyme acyl-malonyl ACP
condensing enzyme.
Reduction.
Acetoacetyl ACP is
reduced to D-3hydroxybutyryl ACP.
NADPH is the
reducing agent
Enzyme: b-ketoacyl
ACP reductase
Dehydration.
D-3-hydroxybutyryl
ACP is dehydrated
to form crotonyl
ACP
(trans-2-enoyl
ACP).
Enzyme:
3-hydroxyacyl ACP
dehydratase
Reduction.
The final step in the cycle
reduces crotonyl ACP to
butyryl ACP.
NADPH is reductant.
Enzyme - enoyl ACP
reductase.
This is the end of first
elongation cycle (first
round).
In the second round
butyryl ACP condenses
with malonyl ACP to
form a C6-b-ketoacyl
ACP.
Reduction, dehydration,
and a second reduction
convert the C6-bketoacyl ACP into a C6acyl ACP, which is ready
for a third round of
elongation.
Final reaction of FA synthesis
• Rounds of synthesis continue until a
C16 palmitoyl group is formed
• Palmitoyl-ACP is hydrolyzed by a thioesterase
Overall reaction of palmitate synthesis from
acetyl CoA and malonyl CoA
Acetyl CoA + 7 Malonyl CoA + 14 NADPH + 14 H+
Palmitate + 7 CO2 + 14 NADP+ + 8 HS-CoA + 6 H2O
Organization of Multifunctional Enzyme
Complex in Eukaryotes
The synthase is dimer with antiparallel subunits.
Each subunit has three domains.
ACP is located in domain 2.
Domain 1 contains transacylases, ketoacyl-ACP
synthase (condensing enzyme)
Domain 2 contains acyl carrier protein, b-ketoacyl
reductase, dehydratase, and enoyl reductase.
Domain 3 contains thioesterase activity.
Fatty Acid Elongation and Desaturation
The common product of fatty acid synthesis is
palmitate (16:0).
Cells contain longer fatty acids and unsaturated
fatty acids they are synthesized in the
endoplasmic reticulum.
The reactions of elongation are similar to the ones
seen with fatty acid synthase (new carbons are
added in the form of malonyl CoA).
For the formation of unsaturated fatty acids there
are various desaturases catalizing the formation of
double bonds.
THE CONTROL OF FATTY ACID METABOLISM
Acetyl CoA carboxylase plays an essential role
in regulating fatty acid synthesis and
degradation.
The carboxylase is controlled by hormones:
 glucagon,
 epinephrine, and
 insulin.
Another regulatory factors:
 citrate,
 palmitoyl CoA, and
 AMP
Global Regulation
is carried out by means of reversible phosphorylation
Acetyl CoA carboxylase is switched off by phosphorylation
and activated by dephosphorylation
Insulin stimulates fatty acid synthesis causing
dephosphorylation of carboxylase.
Glucagon and epinephrine have the reverse effect (keep the
carboxylase in the inactive phosphorylated state).
Protein kinase is
activated by AMP and
inhibited by ATP.
Carboxylase is
inactivated when the
energy charge is low.
Local Regulation
Acetyl CoA carboxylase is allosterically stimulated by
citrate.
The level of citrate is high when both acetyl CoA and ATP
are abundant (isocitrate dehydrogenase is inhibited by
ATP).
Palmitoyl CoA inhibits carboxylase.
Fed state:
Response to Diet
• Insulin level is increased
• Inhibits hydrolysis of stored TGs
• Stimulates formation of malonyl CoA, which inhibits
carnitine acyltransferase I
• FA remain in cytosol (FA oxidation enzymes are in the
mitochondria)
Starvation:
• Epinephrine and glucagon are produced and stimulate
adipose cell lipase and the level of free fatty acids rises
• Inactivate carboxylase, so decrease formation of malonyl
CoA (lead to increased transport of FA into mitochondria
and activate the b-oxidation pathway)
LIPID METABOLISM:
BIOSYNTHESIS OF TRIACYLGLYCEROLS
AND PHOSPHOLIPIDS
Synthesis of Triacylglycerols (TGs)
and Glycerophospholipids (GPLs)
Glycerol 3-phosphate can be obtained either by the
reduction of dihydroxyecetone phosphate (primarily) or
by the phosphorylation of glycerol (to a lesser extent).
Formation of phosphatidate
Two separate acyl transferases (AT) catalyze the
acylation of glycerol 3-phosphate.
The first AT (esterification at C1) has preference for
saturated fatty acids;
the second AT (esterification at C2) prefers
unsaturated fatty acids.
• Phosphatidic acid (phosphatidate) is an
common intermediate in the synthesis of
TGs and GPLs
Phosphatidate can be converted to two precursors:
- diacylglycerol (precursor for TGs and neutral
phospholipids) - cytidine diphosphodiacylglycerol (CDPdiacylglycerol) (precursor for acidic phospholipids)
Synthesis of TGs and neutral phospholipids
Phosphatidylethanolamine
Triacylglycerol
Phosphatidylcholine
Synthesis of TGs
Diacylglycerol can
be acylated to
triacylglycerol (in
adipose tissue
and liver)
Enzyme:
acyltransferase
Synthesis of neutral phospholipids
CDP-choline or CDP-ethanolamine are formed from
CTP by the reaction:
CTP + choline phosphate  CDP-choline + PPi
CTP + ethanolamine phosphate 
CDP-ethanolamine + PPi
Diacylglycerol react with CDP-choline or CDPethanolamine to form phosphatidylcholine or
phosphatidylethanolamine
Synthesis of acidic phospholipids
Phosphatidylinositol can be converted to phosphatidylinositol
4,5-biphosphate which is the precursor of the second
messenger inositol 1,4,5-triphosphate
• Interconver
-sions of
phosphatidylethanolamine and
phosphatidylserine