Download 06_Metabolism of lipid

METABOLISM 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
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.
ABSORPTION OF DIETARY LIPIDS
Lipid absorption – passive diffusion process.
2-monoacylglycerols, fatty acids,
lysophosphoglycerides, free cholesterol form
micelles with bile salts.
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)
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
Transport Forms of Lipids
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
•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
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
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)
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
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
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
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
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.
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).
Synthesis of acidic phospholipids
Functions of Cholesterol
• a precursor of steroid hormones
(progesterone, testosterone, estradiol,
cortisol, etc.)
• a precursor of bile acids
• a precursor of vitamin D
• important component of many mammalian membranes (modulates the
fluidity)
Sources of Cholesterol
• from the diet
• can be synthesized de novo (about 800 mg of cholesterol per day)
- in the liver (major site)
- in the intestine
• Liver-derived and dietary cholesterol
are both delivered to body cells by
lipoproteins
Synthesis of Cholesterol
Three stages of cholesterol biosynthesis
1. Synthesis of isopentenyl pyrophosphate, that is
the key building block of cholesterol, from acetyl
CoA
2. Condensation of six molecules of isopentenyl
pyrophosphate to form squalene
3. Squalene cyclizes and the tetracyclic product is
converted into cholesterol
Acetyl CoA (C2)
Isopentenyl pyrophosphate (C5)
Squalene (C30)
Cholesterol (C27)
THE REGULATION OF
CHOLESTEROL BIOSYNTHESIS
Regulatory enzyme - 3-hydroxy-3-methylglutaryl
CoA reductase.
Tetrameric
enzyme.
NADPH coenzyme
HMG CoA reductase is controlled in multiple ways:
1. The rate of synthesis of reductase mRNA is controlled
by the sterol regulatory element binding protein (SREBP).
When cholesterol levels fall this protein migrates to the
nucleus and enhance transcription.
2. The rate of translation of reductase mRNA is inhibited
by cholesterol
3. The degradation of the reductase is controlled.
The increase of cholesterol concentration makes the enzyme
more susceptible to proteolysis.
4. Phosphorylation decreases the activity of the reductase.
Enzyme is switched off by an AMP-activated protein kinase.
Thus, cholesterol synthesis ceases when the ATP level is
low.
Products of Cholesterol Metabolism
ATHEROSCLEROSIS
The desirable
level of
cholesterol in
blood plasma:
< 200 mg/dl
(< 5 mmol/l)
For a
healthy
person, the
LDL/HDL
ratio is 3.5
KETONE BODIES
The entry of acetyl CoA into the citric acid cycle
depends on the availability of oxaloacetate.
The concentration of oxaloacetate is lowered if
carbohydrate is unavailable (starvation) or improperly
utilized (diabetes).
Oxaloacetate is
normally formed from
pyruvate by pyruvate
carboxylase
(anaplerotic reaction).
Fats burn in the flame
of carbohydrates.
In fasting or diabetes the gluconeogenesis is activated
and oxaloacetate is consumed in this pathway.
Fatty acids are oxidized producing excess of acetyl CoA
which is converted to ketone bodies:
b-Hydroxybutyrate
Acetoacetate
Acetone
Ketone bodies are synthesized
in liver mitochondria and
exported to different organs.
Ketone bodies are fuel
molecules (can fuel brain and
other cells during starvation)
A. Synthesis of ketone bodies
Two molecules
of acetyl CoA
condense to
form
acetoacetyl CoA.
Enzyme –
thiolase.
Acetoacetyl
CoA reacts
with acetyl
CoA and water
to give 3hydroxy-3methylglutaryl
CoA (HMGCoA) and CoA.
Enzyme:
HMG-CoA
synthase
3-Hydroxy-3methylglutaryl
CoA is then
cleaved to
acetyl CoA and
acetoacetate.
Enzyme:
HMG-CoA lyase.
3-Hydroxybutyrate is
formed by the reduction of
acetoacetate by
3-hydroxybutyrate
dehydrogenase.
Acetoacetate also
undergoes a slow,
spontaneous
decarboxylation to
acetone.
The odor of acetone may
be detected in the breath
of a person who has a high
level of acetoacetate in
the blood.
B. Ketone bodies are a major fuel
in some tissues
Ketone bodies diffuse from the liver
mitochondria into the blood and are transported
to peripheral tissues.
Ketone bodies are important molecules in energy
metabolism.
Heart muscle and the renal cortex use
acetoacetate in preference to glucose in
physiological conditions.
The brain adapts to the utilization of
acetoacetate during starvation and diabetes.
3-Hydroxybutyrate is oxidized to produce
acetoacetate as well as NADH for use in
oxidative phosphorylation.
3-hydroxybutyrate
dehydrogenase
Acetoacetate is activated
by the transfer of CoA
from succinyl CoA in a
reaction catalyzed by a
specific CoA transferase.
Acetoacetyl CoA is cleaved
by thiolase to yield two
molecules of acetyl CoA
(enter the citric acid
cycle).
CoA transferase is present
in all tissues except liver.
Ketone bodies are a watersoluble, transportable
form of acetyl units
KETOSIS
The absence of insulin in diabetes mellitus
 liver cannot absorb glucose
 inhibition of glycolysis
 activation of
gluconeogenesis
 deficit of oxaloacetate
 activation of fatty
acid mobilization by
adipose tissue
 large amounts of acetyl CoA which can
not be utilized in Krebs cycle
 large amounts of ketone bodies (moderately strong acids)
 severe acidosis (ketosis)
Impairment of the tissue function, most importantly
in the central nervous system