Lipid metabolism I
Triacylglycerols
Biochemistry I
Lecture 8 2008 (J.S.)
2
Major classes of lipids
Simple lipids Triacylglycerols serve as energy-providing
nutrients, the turnover
about 100 g per day in
an adult person
(Waxes, ceramides)
Complex lipids Phospholipids
Glycolipids - both types are mainly
structural components
of biomembranes, the
turnover about 2 g / d
Derived "lipids" (rather isoprenoid compounds)
Cholesterol and other steroids
Eicosanoids
Carotenoids
3
Triacylglycerols (as well as free fatty acids and both free and
esterified cholesterol) are very hydrophobic. They are not
soluble in water unless they are emulsified or included in micelles
in the presence of tensides.
In the intestine
fat droplets are emulsified in the presence of bile salts and form
mixed micelles from the products of digestion catalysed by the
pancreatic lipase.
Lipid absorption is preceded by dissociation of the micelles and
the components are separately absorbed through the brush
border microvilli of the epithelial cells (enterocytes) lining the
lumen.
In the extracellular fluids
hydrophobic lipids are transported in the form of lipoprotein
particles
4
The mixed micelles
in the chyme are composed, in varying proportions, of the fatty acids (FFA),
mono- and diacylglycerols (MG and DG), perharps some unhydrolysed
triacylglycerol (TG), and anions of bile acids, together with minor components
of the diet such as phospholipids and fat-soluble vitamins.
Intestinal lumen Mucosal cell (enterocyte)
5
Chylomicrons secreted from the mucosal cells enter the chyle of the lymphatic
lacteals. Thoracic duct delivers chylomicrons into the blood.
Triacylglycerols
(resynthesized)
Triacylglycerols
2INTESTINAL
LUMEN
Within the mucosal cells, triacylglycerols are resynthesized (the details are
given in the part Synthesis of triacylglycerols) and embodied into chylomicrons
- a class of lipoprotein particles.
6
Common structure of lipoprotein particles:
Lipoprotein particles transport triacylglycerols
and cholesterol in body fluids
Superficial layer
(hydrophilic surface)
Hydrophobic core
E.g. the diameter of a low-density lipoprotein (LDL) particle is about 30 nm and it
consists of about 50 % cholesterol (both free and esterified), 20 % phospholipids,
20 % apoprotein B-100 and 10 % triacylglycerols.
7
8
Metabolism of triacylglycerols
O CH2–O–C–
O
CH2–O–C–
O–C–O–CH
Lipases
is the group name for enzymes that catalyse hydrolysis of ester
bonds of triacylglycerols releasing so free fatty acids.
Some distinct types of lipases:
Extracellular lipases
- Pancreatic lipase secreted into the duodenum,
- Lipoprotein lipase on the surface of the endothelium lining the capillaries
Intracellular lipases
- „Hormone sensitive" lipase in adipocytes mobilizing fat stores
- Lysosomal lipase
9
Hormone-sensitive lipase in adipocytes
is an intracellular lipase that through hydrolysis of triacylglycerols
mobilizes the fat energy reserves stored in adipose tissue.
The activity of this lipase is controlled by hormones:.
Glucagon (at low blood glucose) and adrenaline/noradrenaline
(in stress) cause an increase in lipase activity by its phosphorylation.
Both free fatty acids and glycerol are released into the blood.
Fatty acids are taken up promptly from the blood plasma by tissues that require
nutrients (fatty acids are transported bound to albumin).
Glycerol cannot be utilized in adipocytes (they are lacking in glycerol kinase) and
serves as the substrate for gluconeogenesis in the liver:
Insulin exhibits opposite effect: it support dephosphorylation of the
lipase in adipocytes and initiates the synthesis of triacylglycerols.
10
Mechanism of the hormone-sensitive lipase activation
11
Degradation of fatty acids
– the β-oxidation pathway
Fatty acids serve as an energy source for most of the cells
(not for the nervous system and for red blood cells).
The tissues gain fatty acids
- either from lipoprotein particles after the triacylglycerols have been hydrolysed
by lipoprotein lipase,
- or as fatty acids mobilized by the action of hormones on the fat stores in
adipose tissue and supplied bound onto albumin.
The utilization of fatty acids in the cells requires three stages of processing
1 Activation by linking to coenzyme A,
2 transport of acyl CoA into the mitochondrial matrix
by conjugating it to carnitine,
3 β-oxidation of acyl CoA in the mitochondrial matrix to
acetyl CoA that enters the citrate cycle.
12
1 Activation of a fatty acid – synthesis of acyl coenzyme A
Acyls can be attached to the sulfanyl group by means of a thioester bond.
~
O
OH
CH2OP
O
O
O
N
N
N
N
NH2
O
P O
O
O
HO
P
O
O
OCH2C
HS CH2 CH2 HN
O
C CH2 CH2 HN
O
C CH
CH3
CH3
Cysteamine β-Alanine Pantoic acid
Pantothenic acid
3´–phospho ADP
Coenzyme A
13
The synthesis of the high-energy acyl-CoA thioester is
catalysed by acyl-CoA synthetases:
R–COO–
+ CoA–SH R–CO–S-CoA
Acyl-CoA synthetases are located on the outer mitochondrial
membrane.
There is a loss of energy equivalent to 2 molecules of ATP,
because the reaction is made irreversible by the hydrolysis of
inorganic diphosphate..
In fact, the activation is accomplished in two steps.
First, the fatty acid reacts with ATP to form an acyl adenylate. In this mixed
anhydride, the acyl is bonded to the phosphoryl group of AMP.
The sulfanyl group of CoA then attacks the acyl adenylate, which is tightly
bound to the enzyme, to form acyl-CoA and AMP.
ATP AMP + 2 Pi
14
Acyl-CoA itself cannot cross the inner mitochondrial membrane;
instead, acyl groups are transferred to carnitine, transported
across the membrane as acylcarnitine, and transferred back to
CoA within the mitochondrial matrix.
Short-chain fatty acids (4 – 10 carbon atoms) do not require the
carnitine shuttle, they can cross the inner mitochondrial membrane..
Trimethyl(2-hydroxy-3-carboxypropyl)ammonium
2 Carnitine carries long-chain activated fatty acids into
the mitochondrial matrix
CH3
CH3
H3C N –
CH2–CH–CH2–COO
OH
Carnitine
(Carnitine may be also seen as β-hydroxybutyric acid, to which
trimethylammonium group was attached.)
15
The transfers of acyls from acyl-CoA to carnitine and from
acylcarnitines to CoA are catalysed by
carnitine acyltransferases I and II.
ester bond
O
C
O
CH3
CH3
H3C N –
CH2–CH–CH2–COO
O-Acylcarnitine
Carnitine is synthesized from lysine bound in body
proteins.
Daily intake in the food is about 100 mg / d (meat, milk and
other foodstuffs of animal origin).
There is no reliable evidence that supplementation of food with carnitine
increases muscle strength.
16
Carnitine shuttle
of the inner mitochondrial membrane
Cytosolic
side
Matrix
side
17
3 The β-Oxidation of acyl CoA
Fatty acyl CoAs are degraded in the mitochondrial matrix by the
repetition of a recurring sequence of four reactions:
- dehydrogenation by FAD,
- hydration,
- the second dehydrogenation by NAD+
, and by
- thiolysis by CoA.
As a result of these reactions, the fatty acyl chain is
- shortened by two carbon atoms, and
- FADH2, NADH + H+
, and acetyl CoA are generated.
This series of reactions is called the β-oxidation pathway,
because oxidation is on the β carbon.
18
Acyl CoA
trans-Alk-2-enoyl CoA
L-3-Hydroxyacyl CoA
3-Oxoacyl CoA
Acyl CoA
SHORTENED
BY TWO CARBONS
Acetyl CoA
CoA–SH
H2O
FAD
FAD
FADH2
NAD+
NADH+H+
19
Configuration trans
Saturated acyl CoA
αβ
FAD
FADH2
The first dehydrogenation
α,β-Unsaturated acyl CoA
(2,3-unsaturated)
C
S–CoA
O
R–CH2–CH2–CH2–
CH–C
S–CoA
O
R–CH2–CH
The reaction is catalysed by acyl CoA dehydrogenase that is
the component of the complex II of the terminal respiratory chain.
20
α,β-Unsaturated acyl CoA
CH–C
S–CoA
O
R–CH2–CH
H2O
β-Hydroxyacyl CoA
(L-3-Hydroxy) C
S–CoA
O
R–CH2–CH–CH2–
OH
Hydration of the double bond between C-2 and C-3
The reaction is catalysed stereospecifically by enoyl CoA hydratase.
The enzyme also hydrates a cis-double bond, but the product is then
the D isomer.
Hydration is not a redox reaction, by addition of water to a double bond the sum of
the oxidation numbers of both carbon atoms remain the same.
21
The second oxidative step (dehydrogenation)
NAD+
NADH + H+
β-Hydroxyacyl CoA
(L-3-Hydroxy)
C
S–CoA
O
R–CH2–CH–CH2–
OH
β-Ketoacyl CoA
(3-Oxoacyl CoA) C
S–CoA
O
R–CH2–C–CH2–
O
The reaction is catalysed by L-3-hydroxyacyl CoA dehydrogenase,
which is stereospecific for the L isomer of the hydroxyacyl CoA.
22
The final step of a recurring sequence –
the thiolysis of 3-oxoacyl CoA by a molecule of CoA-SH:
Acetyl CoA
Substrate for the citrate cycle
S–CoA
O
CH3–C
β-Ketoacyl CoA
(3-Oxoacyl CoA) C
S–CoA
O
R–CH2–C–CH2–
O
ACYL CoA
SHORTENED BY TWO CARBONS
S–CoA
O
R–CH2–C
HS–CoA
Thiolase
23
24
Palmitoyl CoA + 7 FAD + 7 NAD+
+ 7 H2O + 7 CoA
8 acetyl CoA + 7 FADH2 + 7 NADH + 7 H+
8 × 12 ATP = 96 ATP
14 ATP + 21 ATP – 2 ATP + 96 ATP = 129 ATP
The energetic yield of β-oxidation of palmitate
– to eight acetyl coenzymes A
– and eight acetyl CoA in the citrate cycle
Net yield of complete palmitate oxidation to CO2
14 ATP 21 ATP – 2 ATP (activation of
palmitate)
+
25
Net yield of the aerobic breakdown of glucose is
38 mol ATP / mol glucose (M = 180 g / mol; 6 mol C),
i.e. 0.21 mol ATP / g glucose, or
6.3 mol ATP / mol C.
Net yield of complete oxidation of palmitate is
129 mol ATP / mol palmitate (M = 256 g / mol; 16 mol C),
i.e. 0.50 mol ATP / g palmitate, or
8.1 mol ATP / mol C.
Why there is a difference in energetic yields per gram or per
carbon atom? Explain.
26
Certain fatty acids require additional steps
Unsaturated fatty acids
Oleoyl CoA (octadec-cis-9-enoyl CoA) → dodec-cis-3-enoyl CoA
dodec-trans-2-enoyl CoA
Linoleoyl CoA (octadec-cis,cis-9,12-dienoyl CoA) →
→ dec-trans-2-cis-4-dienoyl CoA (inhibits hydratase) that must be reduced
(NADPH) to trans-3-enoyl CoA and then isomerized to trans-2-enoyl CoA.
Odd-chain fatty acids
are uncommon in lipids. If present, the product of β-oxidation will be propionyl CoA that
is carboxylated to methylmalonyl CoA and isomerized (B12 coenzyme) to succinyl CoA,
similarly as in catabolism of valine, isoleucine and methionine.
27
28
β-Oxidation of fatty acids is a powerfull source of energy.
It occurs if the cells require energy and the access to
glucose is not sufficient, i.e.
in the post-absorptive phase, during fasting, and
in stress respectively.
Mobilization of fat stores due to the action of glucagon (or
adrenaline) on adipose tissue increases the plasma level of free fatty acids,
which are taken up by the liver and other peripheral tissues (esp. muscle,
myocard and kidney) at the rates proportional to the plasma concentration.
The special role is appointed to the liver:
Uptake of plasma FFA is in excess of requirements for complete
FA oxidation to CO2 and synthesis of triacylglycerols is
depressed. The liver cells then cover the energy requirements
mostly from β-oxidation of fatty acids to acetyl-CoA. A great part
of acetyl-CoA is diverted to the production of ketone bodies,
which are released in to the circulation and serve as an excellent
nutrient for extra-hepatic tissues.
29
LIVER
NERVOUS SYSTEM
OTHER EXTRA-HEPATIC TISSUES
Free FA
- albumin
Ketone bodies
FA
TG in VLDL
Acetyl-CoA
Acetyl-CoA
CO2
Acetyl-CoA
ADIPOSE TISSUE
Triacylglycerols
Glycerol + FA
FA
CO2
Glycerol-3-P
Glucose
VLDL
Ketone bodies
30
Formation of ketone bodies - ketogenesis
Ketone bodies are formed in the liver mitochondria and released
into blood plasma. The two acids are detectable in plasma at any
time, the usual ratio β-hydroxybutyrate to acetoacetate is 3 – 6 (it
reflects the intramitochondrial NADH/NAD+
ratio). There are always
traces of ketone bodies in urine, since there is no renal threshold for
the two acids.
Ketone bodies are readily metabolised in non-hepatic tissues.
The term "ketone bodies" may be used only for the three compounds:
acetoacetic acid (3-oxobutanoic or β-ketobutyric acid), its reduction product
β-hydroxybutyric acid (3-hydroxybutanoic acid), and the product of
non-catalysed decarboxylation of acetoacetate acetone (propanone).
– CO2
- 2 H
+ 2 H
AcetoneAcetoacetic acid
O O
O–H
CH3–C–CH2–C
β-Hydroxybutyric acid
CH3–CH–CH2–C
O
OH
OH O
CH3–C–CH3
31
The production of ketone bodies increases at high ratios
glucagon / insulin, when fat stores are mobilized (prolonged
fasting, starvation, uncontrolled diabetes mellitus type I).
An extreme production of ketone bodies (ketosis) is very
dangerous, because ketogenesis is a proton-producing process
that disturbs acid-base balance (evoking ketoacidosis) and,
through excretion of the two acids into urine, is a cause of
serious loss of cations.
Acetoacetic acid pKa = 3.52
β-Hydroxybutyric acid pKa = 4.70
32
Ketogenesis in liver mitochondria
Acetoacetyl-CoA Acetyl-CoA
3-Hydroxy-3-methylglutaryl-CoA (HMG-CoA)
Acetoacetate (free) Acetyl-CoA
Acetone
β-Hydroxybutyrate
H2O
33
Utilization of ketone bodies in non-hepatic tissues
β-Hydroxybutyrate
are broken down in the citrate cycle
β-Hydroxybutyrate and
acetoacetate are important in
providing energy for peripheral
tissues.
Acetoacetate is reactivated to
acetoacetyl-CoA not directly in the
reaction with ATP and CoA-SH,
but through the transfer of CoA
from succinyl-CoA.
Acetone is a waste product,
eliminated by the kidney or expired, it
can be smelt on the breath.
34
Fatty acid synthesis
Long-chain fatty acids are synthesized by the sequential
addition of two-carbon units derived from acetyl CoA.
Fatty acid synthesis is not a reversal of the degradative pathway.
There are some important differences between the pathways:
– Synthesis is located in the cytosol.
– Intermediates in fatty acid synthesis are covalently linked to the
-SH groups of phosphopantethein of an acyl carrier protein
(ACP), not to coenzyme A.
– The activated donor of two-carbon units is malonyl CoA,
the elongation reaction is driven by the release of CO2.
– The reductant in fatty acid synthesis is NADPH, whereas the
oxidants in fatty acid degradation are NAD+
and FAD.
35
by NADPH
by NADPH
by FAD
by NAD+
36
A very intensive synthesis of fatty acids takes place in the cytosol of
liver, adipose tissue, and lactating mammary gland.
Conditions favourable to synthesis of fatty acids:
– the fed state – sufficient amounts of glucose are available producing
acetyl CoA,
– low energy expenditure – high ATP concentrations within the cells
inhibit decomposition of acetyl CoA in the citrate cycle,
– absence of stress that activates mobilization of fat stores, free fatty acids
released through the action of catecholamines inhibit fatty acid
synthesis.
Fat synthesis and storage are essential components of fuel metabolism in the
body, but excess accumulation of fat leads to obesity which is becoming
a growing problem.
The control of energy balance depends on many factors, some of which are
– genetically linked (appetite control involves a number of recently
discovered protein messengers and receptors),
– environmental (e.g. the relative abundance of food and the type of
food, esp. the energy-dense foods currently in vogue).
37
Supposing that there is enough ATP in the cell and sufficient
quantity of acetyl CoA produced from glucose (by oxidative
decarboxylation of pyruvate) or amino acids in mitochondria,
acetyl CoA has to be transported from mitochondria into cytosol.
Then fatty acid synthesis can be considered as a two-stage process:
- stage 1 – synthesis of malonyl CoA,
- stage 2 – reactions catalysed by the fatty acid synthase complex.
38
Matrix side Cytosolic side
Citrate synthase Citrate lyase
Transfer of acetyl CoA to the cytosol
Citrate lyase catalyses the reaction
Citrate + ATP + CoA-SH + H2O → acetyl-CoA + ADP + Pi + oxaloacetate
NADP+
-linked malate enzyme
Synthesis of malonyl CoA
is the rate-limiting step in fatty acid synthesis, catalysed by
acetyl-CoA carboxylase:
S
HN NH
O
C O –Enzyme
S
HN N
O
C O –Enzyme
–COOH+ HCO3
–
ATP ADP + Pi
CH2–CO–S–CoA
COO–
CH3–CO–S–CoA
Malonyl CoA
Acetyl CoA
Biotinyl–E Carboxybiotinyl–E
The enzyme complex consists of
several identical subunits, each
containing
biotin,
biotin carboxylase,
biotin carboxyl carrier protein
(BCCP), and transcarboxylase.
The enzyme is inhibited by
phosphorylation catalysed by
AMP-dependent protein kinase.
It is inhibited also by palmitoyl-CoA
(due to dissociation of the active
fibrous enzyme polymer to inactive
octamers).
Enzyme activation by citrate
(polymerizing is promoted) and by
dephosphorylation (dependent on
insulin).
40
The fatty acyl synthase complex
One of the two functional units
ACP domaine with
phosphopantethein arm
Seven enzyme activities:
AT
Acetyl/acyl-CoA transacylase
MT
Malonyl transacylase
CE
Condensing enzyme
(Oxoacyl-PPt synthase)
KR
Oxoacyl reductase
DH
Hydroxyacyl dehydratase
ER
Enoyl reductase
TE
Palmitoyl thioesterase
In mammals, the complex is a homodimer. Each monomer is arranged in
three domains carrying the seven catalytic activities. One domain in both
monomers includes the "acyl carrier protein (ACP)" area to which the
phosphopantethein "arm" is attached. Both monomers cooperate so that each
of them takes part on the synthesis of two fatty acids processed simultaneously,
41
The flexible phosphopantethein "arm" of the synthase
linked to a serine residue of acyl carrier protein ACP is found
also in coenzyme A (as just one half of the coenzyme A molecule):
~O
HO
P
O
O
OCH2C
HS CH2 CH2 HN
O
C CH2 CH2 HN
O
C CH
CH3
CH3
Cysteamine
β-Alanine Pantoic acid
Pantothenic acid
NH
CH2–CH
CO
ACP
The processed acyls attached to the sulfanyl group are
carried from one active site of the synthase complex to another.
42
43
Reactions of fatty acid synthesis
1
The synthesis begins with the transfer of the acetyl group of
acetyl CoA to the sulfur of a cystein residue of the condensing
enzyme. The reaction is catalysed by acetyl transacylase.
ACP
SH
Cys
S
CO–CH3
"Priming“
44
2
Similarly, the malonyl group is transferred to the sulphur atom
of the phosphopantetheine attached to ACP.
The reaction is catalysed by malonyl transacylase.
ACP
S
Cys
S
CO–CH3
COOH
CH2
CO
"Loading“
45
3
Condensation
The beginning of elongation: The joining of the acetyl unit to
a two-carbon part of the malonyl unit on phosphopantetheine.
CO2 is released.
An acetoacetyl unit is formed of PPt.
The reaction is catalysed by condensing enzyme (3-oxoacyl
synthase).
ACP
S
Cys
SH
+ CO2
CH3
C=O
CH2
CO
ACP
S
Cys
S
CO–CH3
COOH
CH2
CO
46
4
The first reduction
catalysed by β–ketoacyl reductase with NADPH.
The product is 3–hydroxyacyl unit.
ACP
S
Cys
SH
CH3
C=O
CH2
CO
+ NADPH+H+
ACP
S
Cys
SH
CH3
CH–OH
CH2
CO
+ NADP+
47
ACP
S
Cys
SH
CH3
CH–OH
CH2
CO
ACP
S
Cys
SH
CH3
CH
CH
CO
+ H2O
5
Dehydration
catalysed by 3-hydroxyacyl dehydratase.
The product is trans–2–enoyl (named crotonyl) unit.
48
ACP
S
Cys
SH
CH3
CH
CH
CO
+ NADPH+H+
ACP
S
Cys
SH
CH3
CH2
CH2
CO
+ NADP+
6
The second reduction
catalysed by enoyl reductase with NADPH.
The product is saturated acyl (now butyryl) unit.
Initial acetyl was elongated by two carbon atoms.
49
ACP
S
Cys
SH
CH3
CH2
CH2
CO
ACP
SH
Cys
S
CH3
CH2
CH2
CO
7
The saturated acyl is transferred to the cysteine sulfur atom
on the condensing enzyme.
The synthase is now ready for another round of elongation
50
After the completion of the first elongating cycle, new malonyl
is "loaded“ on the sulfanyl group of PPt.
In the second round of fatty acid synthesis, butyryl unit
condenses with malonyl to form a C6-acyl, ……
The elongation cycles continue until C16-acyl unit (palmitoyl)
is formed.
Palmitoyl unit is a good substrate for thioesterase that
hydrolyses palmitoyl-PPt to yield palmitate (16:0).
In mammals, palmitate is the major product of FA synthesis.
A minor saturated product is stearate (18:0).
Further elongation of fatty acids is provided by similar
mechanisms, but the elongating system is located on the
membranes of endoplasmic reticulum.
51
7 Malonyl CoA
PALMITATE
8 Acetyl CoA
The fatty acid synthesis
52
NADPH is required in the reductive steps of FA synthesis
The main source of NADPH is the pentose phosphate pathway.
A certain part of NADPH is supplied by the reaction catalysed by
NADP+
–linked malate enzyme ("malic enzyme“):
Malate + NADP+
→ pyruvate + CO2 + NADPH
The reaction takes part on the transport of acetyl-CoA (in the form of
citrate) across the inner mitochondrial membrane.
53
The stoichiometry of fatty acid synthesis
The synthesis of palmitate (C16):
The synthesis of malonyl CoA
7 Acetyl CoA + 7 CO2 + 7 ATP → 7 malonyl CoA + 7 ADP + 7 Pi + 14 H+
The synthesis catalysed by the fatty acid synthase complex
Acetyl CoA + 7 malonyl CoA + 14 NADPH + 20 H+
→
→ palmitate + 7 CO2 + 14 NADP+
+ 8 CoA + 6 H2O
The overall stoichiometry for the synthesis of palmitate is
8 Acetyl CoA + 7 ATP + 14 NADPH + 6 H+
→
→ palmitate + 14 NADP+
+ 8 CoA + 6 H2O + 7 ADP + 7 Pi
54
Control of fatty acid synthesis
Regulation is carried out by means of reversible phosphorylation of
acetyl-CoA carboxylase.
This enzyme phosphorylated by AMP-dependent protein kinase is
inhibited, dephosphorylation – dependent on insulin – activates the
carboxylase.
Local regulation is provided by citrate that activates the carboxylase,
palmitoyl-CoA inactivates this key enzyme.
55
Elongation of fatty acids
Although palmitate (C16) is the major product of the fatty acid synthase
complex, and is the chief saturated fatty acid in human fat,
stearate and oleate (C18) are common and longer-chain fatty acids,
arachidate (C20),
behenate (C22) and
lignocerate (C24) occur in phospholipids.
Elongation by enzymes bound to the endoplasmic reticulum:
– Activation of palmitate by conversion to palmitoyl CoA,
– activation of acetyl CoA by its carboxylation to malonyl CoA,
– elongation similar to synthesis catalysed by FA synthase
complex, but the intermediates are CoA-thioesters, not enzymebound
acyls. The reductant is also NADPH.
Elongation process in mitochondria (for the synthesis of fatty acids
incorporated into mitochondrial lipids):
– Reversal of the β-oxidation.
56
Desaturation of fatty acids
Unsaturated fatty acids of the series n-6 are comprised in all plant oils (olive
oil, sunflower oil etc.).
15-Desaturase is present predominantly in plants growing in cold water
(algae, phytoplankton), then a high concentration of polyunsaturated fatty
acyls of the series n-3 is in fish oils (fish feeds phytoplankton).
A large proportion (> 50 %) of acyl groups in human triacylglycerols contain
double bonds. Such fatty acids are formed by desaturation of long-chain fatty
acyl-CoA.
E.g. Palmitoyl-CoA + NADH + H+
+ O2
Palmitoleoyl-CoA + 2 H2O + NAD+
In higher animals, only the desaturases are known which generate
double bonds at carbons 9, 6, 5, and 4 in the fatty acid chain.
Mammals lack the enzymes to introduce double bonds at carbon
atoms beyond C-9.
Fatty acids containing double bonds beyond C-9 are synthesized by
plants, they contain also 12- and 15-desaturase.
57
Polyunsaturated fatty acids (n-6 and n-3)
are essential for animals
Fatty acids n-6 and n-3 are essential dietary constituents for
animals and serve as precursors of eicosanoids (prostanoids and
leukotrienes).
Providing the dietary intake is sufficient (vegetable seed oils, resp.
fish), linoleate and α-linolenate can act as precursors of other
polyenoic acids such as arachidonate (n-6) and
eicosapentaenoate (n-3), from which eicosanoids are formed.
Linoleate 18:2 (9,12)
γ-Linolenate 18:3 (6,9,12)
Eicosatrienoate 20:3 (8,11,14)
Arachidonate 20:4 (5,8,11,14)
6-desaturation
elongation
5-desaturation
α-Linolenate 18:3 (9,12,15)
Octadecatetraenoate 18:4 (6,9,12,15)
Eicosatetraenoate 18:4 (8,11,14,17)
Eicosapentaenoate 18:5 (5,8,11,14,17)
6-desaturation
elongation
5-desaturation
58
18:0 18:1 (9) 18:2 (9,12) 18:3 (9,12,15)
n-9 series
18:2 (6,9)
20:2 (8.11)
20:3 (5,8,11)
22:3 (7,10,13)
18:3 (6,9,12) 18:4 (6,9,12,15)
20:3 (8,11,14)
20:4(5,8,11,14)
22:4 (7,10,13,16)
20:4 (8,11,14,17)
20:5(5,8,11,14,17)
22:5 (7,10,13,16,19)
plants phytoplankton
6–desaturase 6–desaturase
5–desaturase 5–desaturase
n-6 series n-3 series
elongation
elongation
elongation
elongation
59
Mechanism of long-chain fatty acyl-CoAs desaturation
The enzymatic systems that catalyse desaturation are located in the
smooth endoplasmic reticulum of liver cells.
Desaturases are hydroxylating monooxygenases, although water is
eliminated from the hydroxylated product in the formation of the double
bond. The reductant is NADH+H+
, from which the electrons are carried
by the flavine enzyme and the cytochrome b5 to a desaturase.
Electron transport to desaturase
Saturated acyl-CoA
–CH2–CH2–
–CH=CH–
Unsaturated acyl-CoA
O2
2H2O
60
Example:
9
10
1
S
CoA
O
H
H
HO
H
HH
1
S
CoA
O
1
S
CoA
O
O=O + NADH+H+
+ H2O + NAD+
+ H2O
Stearoyl–CoA
Oleoyl–CoA
61
Synthesis of triacylglycerols
Glycerol-3P
+
NAD+
NADH + H+
is provided by esterification of glycerol 3-phosphate (or dihydroxyacetone
phosphate) by activated fatty acids - acylcoenzymes A.
There are two possible sources of glycerol phosphate:
In liver and small intestine (but not in adipose tissue) is
glycerol phosphorylated by glycerol kinase.
In most other tissues glycerol phosphate originates by reduction
of dihydroxyacetone phosphate, an intermediate of glycolysis, by
the action of glycerol phosphate dehydrogenase
CH2–O–PO
CH2–OH
3
2–
CH–OH
CH 2–OH
CH2–OH
CH–OH
ATP ADP
Dihydroxyacetone-P
CH2–OH
CH2–O–PO 3
2–
C=O
Glycerol
kinase
62
Usually unsaturated FA
NADPH + H+
NADP
R-CO-S-CoA CoA-SH
CH2–OH
CH2–O–PO 3
2–
C=O
CH2–O–CO–R
CH2–O–PO 3
2–
C=O
R-CO-S-CoA CoA-SH
2-LysophosphatidateGlycerol-3P
CH2–O–PO
CH2–OH
3
2–
CH–OH
CH2–O–CO–R
CH2–O–PO 3
2–
CH–OH
Phosphatidate
CH2–O–CO–R
CH2–O–PO 3
2–
CH–O–CO–R
Dihydroxyacetone
phosphate
R-CO-S-CoA CoA-SH
Phosphatidate is an intermediate in the synthesis
of triacylglycerols and glycerophospholipids in the endoplasmic reticulum:
63
Glycerophospholipids
Phosphatidylserine Phosphatidylcholine
Phosphatidylinositol Phosphatidylethanolamine
Cardiolipin
TRIACYLGLYCEROLS
Small intestine → Chylomicrons
Liver cells → VLDL
Adipocytes → Reserve fat
Phosphatidate
CH2–O–CO–R
CH2–O–PO 3
2–
CH–O–CO–R
CH2–O–CO–R
CH2–OH
CH–O–CO–R
CH2–O–CO–R
CH2–O–CO–R
CH–O–CO–R
R-CO-S-CoA CoA-SH
Pi
hydrolase
H2O
1,2-Diacylglycerol