Transcript D) fatty acids

Wayne V. Vedeckis, Ph.D.
Department of Biochemistry and Molecular Biology
And
Stanley S. Scott Cancer Center
Room 4B6, 4th Floor
Clinical Sciences Research Building
533 Bolivar Street
504-599-0572 (Office)
[email protected]
LIPID STRUCTURE AND METABOLISM I
09/26/07
I.
LEARNING OBJECTIVES
1) To identify the basic structure and properties of lipids
2) To list the major pancreatic enzymes involved in lipid metabolism
3) To describe the overall digestion and absorption of lipids
4) To illustrate the role of bile in lipid metabolism
5) To distinguish the forms (chylomicrons, lipoproteins) in which lipids
are transported in the blood
6) To discuss the importance of lipoprotein lipase in the uptake of fatty
acids from the blood
II. INTRODUCTION TO LIPIDS (Fig. 15.1)
A) Water Insoluble, heterogeneous
B) Functions
1) compartmentalize cellular
components and biochemical
pathways
2) provide energy
3) coenzymes, signaling molecules
C) Hydrophobic, lipophilic
D) Major factor in obesity, diabetes,
atherosclerosis
III. DIGESTION (Fig. 15.2)
A) Adults – lingual lipase (acid stable; from tongue) swallowed.
Different acid-stable lipase, gastric lipase, also made in stomach.
Small amount of digestion begins in stomach (triacylglycerols
containing short- and medium-chain fatty acids)
B) Small intestine
1) emulsification
2) bile salts and acids
(Fig. 15.3)
3) peristalsis
C) pancreatic enzymes
1) pancreatic lipase (Fig. 15.2); colipase (anchors pancreatic lipase)
Triacylglycerols  2-monoacylglycerol + 2 FAs
2) cholesteryl esterase
(Fig. 15.2)
3) phospholipase (Fig. 15.2) phospholipase A2 + lysophospholipase
IV. LIPID ABSORPTION
A) Mixed micelles (Fig. 15.5) – discshaped structures with charged,
hydrophilic portions on surface
(water-soluble) and hydrophobic
portions facing interior. Approach
unstirred water layer at brush border
of intestinal mucosa. Aid in transport
of lipids through cell membrane.
B) Short and medium chain fatty acids –
soluble without forming micelles.
Directly absorbed into intestinal cells.
V. LIPID RESYNTHESIS
A) Long chain fatty acids - activated by fatty acyl CoA synthase
(thiokinase) (Fig. 15.6). Very important - 2 high energy bonds are
used to activate a fatty acid. In typical reactions, ATP (2~) is
converted to ADP (1~) and Pi (0~). In the thiokinase reaction, ATP
(2~) is converted to AMP (0~) and PPi (1~). However, there is a
ubiquitous pyrophosphatase present is all cells that converts PPi
(1~) to 2 Pi (0~). Thus FA activation requires 2~ (or 2 ATP
“equivalents”; ATP  ADP + Pi). 2 fatty acids are esterified to
2-monoacylglycerol to re-form triacylglycerol.
B) Short/medium chain fatty acids – pass through cell, bind to albumin
C) Lysophospholipids – reacylated to form phospholipids
D) Cholesterol – reacylated to form cholesteryl esters
2Pi
Pyrophosphatase
VI. LIPID SECRETION (Fig. 15.6)
A) Chyle (contains chylomicrons; not chyme) secreted into the
lymphatics and then the bloodstream
B) Chylomicrons – Large, TAG-rich, lipoprotein particle
2Pi
Pyrophosphatase
VIII. USE OF DIETARY LIPIDS BY TISSUES
A) Skeletal muscle and adipocytes
B) Minor tissues - heart, lung, kidney, liver
C) Lipoprotein lipase - from muscle and adipocytes; digests
triacylglycerols; anchored to the endothelium on the interior of
blood vessels
D) Fate of free fatty acids – energy and resynthesis of triacylglycerols
E) Fate of glycerol – converted to glycerol 3-phosphate in the liver
F) Chylomicron remnants - cholesteryl esters, phospholipids, protein,
some triacylglycerol. Bind to liver and are endocytosed and
metabolized.
LIPID STRUCTURE AND METABOLISM II
09/26/07
I. LEARNING OBJECTIVES
1) To identify the basic structure of a fatty acid, and its state of
saturation
2) To illustrate common fatty acids using the three methods
(common, carbon 1 numbering, w carbon numbering)
3) To identify the citrate shuttle
4) To describe the rate limiting enzyme in fatty acid biosynthesis,
acetyl CoA carboxylase, and how it is positively and negatively
regulated in various ways
5) To distinguish the steps in fatty acid synthesis, and the way in
which it is catalyzed by fatty acid synthase
6) To explain how fatty acids are used for the synthesis of
triacylglycerol
II. INTRODUCTION
A) Fatty acids - free (FFA) and fatty acid
esters
B) Plasma levels - varied with fasting and
starvation
C) Utilization – mostly liver and muscle
D) Important source of energy
E) Structural components of phospholipids
and glycolipids
F) Precursors of signaling molecules
G) Synthesis versus degradation – pathways
(Fig. 16.1)
H) Central role for Acetyl CoA
III. STRUCTURE AND NOMENCLATURE
A) General structure – amphipathic (Fig. 16.2)
B) Features (Fig. 16.3) - saturated; unsaturated; cis double bonds;
rancidification; melting temperature. The melting temperature of a
FA is lower with shorter chain length and with a higher degree on
unsaturation (more double bonds – adds “kink” in the chain).
C) Naming - three methods (Fig. 16.5)
1) common (Table 16.4)
2) carbon 1 numbering - # of carbons:
# of double bonds (carbons at
which double bonds occur); delta
() naming (or N- or n- naming);
carbon #1 is carboxyl, #2 is called a
carbon, # 3 is b carbon, etc.
3) w carbon numbering – terminal
methyl group is always the
omega carbon. Begin counting
at omega carbon (#1); most
common are w-6 or N-6 or n-6
(linoleic) and w-3 (N-3; n-3)
(linolenic) fatty acids. These
two are “essential” fatty acids.
Arachidonic acid becomes
essential in linoleic acid
deficiency.
*To determine the type of omega fatty
acid, subtract the first carbon in the
highest numbered double bond from
the total number of carbons in the FA.
Must number carbons starting at
carboxy carbon!!
IV. FATTY ACID BIOSYNTHESIS
A) Sites - major (liver, lactating
mammary gland) and minor
(adipose tissue, kidney)
B) Precursors and cofactors – acetyl
CoA, ATP, NADPH, CO2
C) Cytoplasm - cellular synthesis site
D) Cytoplasmic acetyl CoA – from
mitochondrial acetyl CoA
equivalents
E) Citrate = form in which acetyl CoA
is transported (Fig. 16.6) mitochondrial citrate synthase
(OAA + Acetyl CoA)  citrate 
cytoplasm  cytoplasmic citrate lyase
(OAA + Acetyl CoA)
F) acetyl CoA is precursor for fatty
acid synthesis; energy charge (ATP
levels) must be high in the cell
G) Carboxylation/decarboxylation –
provides energy and mechanism
for synthesis (Fig. 16.7) – acetyl
CoA carboxylase (covalently bound
biotin); ATP; CO2; product is
malonyl CoA (3 carbons)
H) Regulation of acetyl CoA
carboxylase (rate-limiting,
committed enzyme)
1) short term - polymerization of
dimer (protomer) is stimulated
by citrate; inhibited by malonyl
CoA and palmitoyl CoA
(Fig. 16.7)
(1~)
(1~)
(2~)
(1~)
2) phosphorylation – glucagon and
epinephrine stimulate
phosphorylation = inactive;
high insulin and
carbohydrates promote
dephosphorylation = active
(Fig. 16.8)
3) long term - enzyme levels;
increased by high
carbohydrate, low fat diet;
inhibited by low carbohydrate,
high fat diet. Same is true for
fatty acid synthase (or
synthetase)
I) Fatty acid synthase –
complex, multi-activity
enzyme; seven different
enzyme activities; binding
site for 4’phosphopantetheine in acyl
carrier protein (ACP) portion
J) Reaction sequence –
“enzymes” are activities of the
same protein (Fig. 16.9)
1) acetyl CoA-ACP
transacylase
2) intramolecular transfer
3) malonyl-CoA-ACP
transacylase
4) b-ketoacyl-ACP synthase
5) b-ketoacyl-ACP reductase
6) b-hydroxyacyl-CoA
dehydratase
7) enoyl-ACP reductase
End result – have
synthesized a four carbon,
saturated, fatty acyl ACP
starting with a two carbon
compound (acetyl CoA) and
a three carbon compound
(malonyl CoA)
8) steps 3-6 are repeated until
palmitoyl (16:0) unit is
attached to the enzyme
9) palmitoyl thioesterase
hydrolyzes palmitic acid
from the enzyme
10) overall reaction
NADH + H+
NAD+
8 acetyl CoA + 14 NADPH + 14 H+ + 7 ATP

palmitic acid (16:0) + 8 CoA + 14 NADP+ +
7 ADP + 7 Pi + 7 H20
K) Sources of NADPH - hexose monophosphate
shunt and cytoplasmic malic enzyme
(Fig. 16.10)
cytoplasmic
L) Interrelationship with
glucose metabolism
(Fig. 16.11) – glycolysis
(pyruvate & NADH);
gluconeogenesis (OAA
production); pyruvate
dehydrogenase (acetyl
CoA); citric acid cycle
(citrate)
M) Further chain elongation –
Mitochondria - Acetyl (2C)
units are added, followed
by reduction by NADH and
NADPH.
Endoplasmic reticulum successive condensation of
malonyl CoA and acyl CoA,
and NADPH reductions
(similar to fatty acid
synthase reactions)
N) Desaturation (introduction of double
bonds) – Mammals have 9, 6, 5,
and 4 desaturases. Desaturation past
carbon 9 in humans does not occur.
This is why linoleic acid and linolenic
acid are essential fatty acids in
humans.
O) Storage as triacylglycerol (Fig. 16.12)
1) carboxyl group is esterified to
hydroxyl on glycerol
2) fat = solid at room temperature;
oil = liquid at room temperature
3) fatty acids in triacylglycerol - #1 is
usually saturated, #2 is usually
unsaturated, #3 is either
4) glycerol phosphate - acceptor for triacylglycerol synthesis
(Fig. 16.13)
a) liver - from NADH-dependent reduction of DHAP by glycerol
phosphate dehydrogenase and from phosphorylation of
glycerol by glycerol kinase
b) adipose tissue - only glycerol phosphate dehydrogenase path
5) fatty acid activation - by thiokinase to fatty acyl
CoA (costs 2 ATP “equivalents”)
6) reactions in triacylglycerol synthesis (4)
(Fig. 16.14)
a) acyltransferase produces lysophosphatidic acid
(position #1)
b) acyltransferase produces phosphatidic acid
(position # 2)
c) phosphatase removes phosphate to form
diacylglycerol
d) acyltransferase produces triacylglycerol
7) fates of triacylglycerol - liver: packaged into very
low density lipoprotein (VLDL) particles and
transported to other tissues; adipocytes: stored as
lipid droplets (“depot fat”)
LIPID STRUCTURE AND METABOLISM - III
09/26/07
I. LEARNING OBJECTIVES
1) To describe the role and regulation of hormone sensitive lipase in the
mobilization of fatty acids from triacylglycerol (TAG)
2) To identify the location and steps in the b-oxidization of fatty acids
3) To distinguish how odd chain fatty acids are metabolized
4) To explain the metabolism of unsaturated fatty acids
5) To differentiate a- and peroxisomal b-fatty acid oxidation.
6) To explain the biochemical pathways in ketone body synthesis and
degradation, including the concept of tissue-restricted expression of
different enzyme activities
Glucagon
II. FAT MOBILIZATION
A) fat - major energy store (9 kcal/g when
metabolized to CO2 and H20)
B) Hormone sensitive lipase - releases fatty
acids (#1 and/or #3) from TAG (Fig. 16.15)
1) stimulation - epinephrine and glucagon
through the elevation of cyclic AMP
2) inhibition - high insulin and glucose;
high fat diet
C) glycerol - cannot be rephosphorylated in
adipocyte; released into blood, goes to
liver and is used (glycerol 3-phosphate;
glycolysis, gluconeogenesis)
D) fatty acids - released, bind to serum
albumin, transported to peripheral
tissues for energy production (brain,
nervous tissue, erythrocytes, and
adrenal medulla cannot use fatty acids
for fuel)
III. b-OXIDATION OF FATTY ACIDS
A) Occurs in mitochondrion
B) Two carbon fragments are successively removed from carboxyl end
of fatty acid to yield acetyl CoA
C) Carnitine shuttle – transports fatty acids into mitochondrion
(Fig. 16.16) - free fatty acid enters cell and thiokinase activates it to
fatty acyl CoA; condensed with carnitine by carnitine
palmitoyltransferase I (CPT-I) (aka, carnitine acyltransferase I,
CAT-I) to form O-acylcarnitine; moves into mitochondrion; released
by carnitine palmitoyltransferase II using CoA to produce fatty acyl
CoA and carnitine (energy neutral reactions)
1) carnitine shuttle inhibited by malonyl CoA (FA synthesis,
opposing pathway)
2) Synthesized from lysine or methionine by the liver.
3) carnitine deficiencies – long chain fatty acids (LCFA) not used
well for fuel.
a) congenital CPT-I deficiency - decreases liver’s ability to
synthesize glucose during fasting (liver uses CHO for cell
maintenance). Causes severe hypoglycemia, coma, and death
b) CPT II deficiency – cardiomyopathy, muscle weakness
4) Short chain fatty acids (SCFA) and MCFA – do not use carnitine
shuttle. Pass into mitochondria. Not inhibited by malonyl CoA
D) Reaction sequence - four sequential reactions
are repeated (Fig. 16.17)
1) acyl CoA dehydrogenase - forms double bond
2) enoyl CoA hydratase - adds water across
double bond
3) b-hydroxylacyl CoA dehydrogenase – oxidizes
by removing 2 H+
4) acyl CoA:acyltransferase (“thiolase”) – cleaves
acetyl CoA from the end
5) repeats until the final reaction liberates two
acetyl CoA molecules
6) total energy yield after further metabolism of the acetyl CoA = 131
ATP from one palmitoyl CoA (16:0) (Fig. 16.18). N.B. If you start
with the free FA, you need to invest 2 ATP “equivalents” (high
energy bonds) in the thiokinase reaction needed to activate the
FA. Thus, the net yield of ATP starting with the free fatty acid for
complete b-oxidation is 2 ATP less (129 in the case of palmitic
acid).
E) Medium
chain-length
acyl CoA
dehydrogenase
deficiency –
decreased fatty
acid oxidation;
severe
hypoglycemia;
involved in
sudden infant
death syndrome
(SIDS) and
Reye’s
syndrome
F) Comparison of fatty acid synthesis and degradation (Fig. 16-19) –
this is an excellent resource for reviewing, comparing, and
contrasting fatty acid biosynthesis and degradation
G) Odd chain fatty acid metabolism (Fig. 16.20) –
at first, same as even chain. But propionyl
CoA (3 carbons) is a product
1) Propionyl CoA is converted to
methylmalonyl CoA by propionyl CoA
carboxylase (biotin).
2) Methylmalonyl CoA is converted to
succinyl CoA by methylmalonyl CoA
mutase (cobalamin [vitamin B12] derivative).
3) Succinyl CoA is metabolized in the citric
acid cycle.
4) Deficiency in methylmalonyl CoA mutase
or ability to synthesize cobalamin
derivative causes methylmalonic acidemia
and aciduria (low blood pH) –
developmental retardation in patients
H) Unsaturated fatty acids produce less energy than saturated fatty
acids
1) monounsaturated fatty acids [e.g., palmitoleic acid; 16:1(9)]
first three cycles – same as saturated FA. Now have:
H H H O
| |
| ||
--------C=C – C – C – S–CoA
|
H
original C #
10 9 8 7
actual C #
4 3 2 1
3-cis-enoyl CoA (NOT a substrate for enoyl CoA hydratase; Fig. 16.17)
3,2-enoyl-CoA isomerase
H H
O
|
|
||
--------C – C=C – C – S–CoA
|
|
H
H
4 3 2 1
2-trans-enoyl CoA (a substrate for enoyl CoA hydratase)
2) polyunsaturated fatty acids – require an isomerase plus an
NADPH-dependent reductase to eliminate the double bonds that
are separated by three carbons
IV. OTHER TYPES OF FATTY ACID OXIDATION
A) peroxisomal b-oxidation - can degrade very long chain fatty acids
(VLCFA) fatty acids >20 carbons in length (mitochondrial
b-oxidation cannot). Initial dehydrogenation uses an acyl CoA
oxidase and FAD. Once the carbon chain is reduced to 18,
mitochondrial b-oxidation is used.
1) Defect in peroxisome biosynthesis in all tissues – no metabolism
of fatty acids >18 carbons in length, and these accumulate.
Disease is called Zellweger (cerebrohepatorenal) Syndrome.
2) Defect in ability of peroxisome to activate VLCFA leads to
X-linked adrenoleukodystrophy – mental impairment, motor
problems, etc. (Lorenzo’s Oil)
B) a-oxidation - requires NADPH, molecular
oxygen, and cytochromes (a fatty acid
a-hydroxylase). Uses free fatty acid as
substrate initially. The degradation of
chlorophyll yields phytanic acid, a branched
chain fatty acid (Fig. 16-21). Must use
a-oxidation to metabolize it. Deficiency in
a-hydroxylase causes Refsum's Disease.
(retinitis pigmentosum, peripheral
neuropathy, nerve deafness, cerebellar
ataxia). Dietary restriction needed.
V. KETONE BODIES
A) Excess acetyl CoA - can be metabolized by liver mitochondria into
ketone bodies (acetoacetate, b- (or 3-) hydroxybutyrate, acetone).
B) Transported to other tissues, converted to acetyl CoA and
metabolized by the citric acid cycle.
C) Importance - water-soluble; produced in liver when excess acetyl
CoA exceeds the oxidative capacity; can be used by skeletal and
cardiac muscle, renal cortex, and even brain (when prolonged fasting
causes large elevation in ketone bodies)
blood concentration [mM]
OVERALL METABOLIC CHANGES THAT OCCUR DURING STARVATION
7
ketone bodies
glucose from
glycogenolysis
6
5
blood glucose
4
3
glucose from
gluconeogenesis
2
free fatty acids
1
0
0
5
10
15
20
25
30
urinary excretion [g/day]
Days of Starvation
20
15
total urinary nitrogen
10
5
urinary NH4+
0
0
5
10
15
20
25
30
Days of Starvation
The data points represent: 0, 6 hr, 12 hr, 18 hr, and
1, 2, 4, 6, 8, 10, 20, and 30 days of starvation.
D) Synthesis (Fig. 16.22)
1) production of acetoacetyl CoA
a) incomplete breakdown of a fatty acid
b) reversal of the thiolase reaction
2) Hydroxymethylglutaryl CoA (HMG CoA; 6
carbons) is synthesized by the condensation of
acetoacetyl CoA and acetyl CoA by
mitochondrial HMG CoA synthase (rate-limiting,
committed enzyme in ketone body synthesis;
only found in liver).
3) acetoacetate is produced by HMG CoA
cleavage
4) acetoacetate is reduced to b-hydroxybutyrate
by b-hydroxybutyrate dehydrogenase (NADH)
or spontaneously decarboxylates to form
acetone
mitochondrial
E) Utilization by extrahepatic tissues (Fig. 16.23) - b-hydroxybutyrate is
oxidized to acetoacetate by b-hydroxybutyrate dehydrogenase.
Acetoacetate receives CoA from succinyl CoA (succinyl
CoA:acetoacetate CoA transferase [thiophorase]; lacking in liver).
Acetoacetyl CoA is converted into two acetyl CoA molecules.
F) Because mitochondrial HMG CoA synthase is only found in liver,
only the liver can make ketone bodies. Because most other tissues
except liver have thiophorase, the liver cannot use the ketone bodies
that it synthesizes. Tissue-specific separation of opposing metabolic
pathways.