Transcript CHAPTER 6

Chapter 23
Fatty Acid Catabolism
Biochemistry
by
Reginald Garrett and Charles Grisham
Outline
1. How Are Fats Mobilized from Dietary Intake
and Adipose Tissue?
2. How Are Fatty Acids Broken Down?
3. How Are Odd-Carbon Fatty Acids Oxidized?
4. How Are Unsaturated Fatty Acids Oxidized?
5. Are There Other Ways to Oxidize Fatty
Acids?
6. What Are Ketone Bodies, and What Role Do
They Play in Metabolism?
23.1 – How Are Fats Mobilized from
Dietary Intake and Adipose Tissue?
1. Triacylglycerols represent the major energy
input in the modern American diet
•
Provides 30-60 % of calories
1. Triacylglycerols represent the major energy
input in the modern American diet
• Provides 30-60 % of calories
2. Triacylglycerols are also the major form of
stored energy in the body (80%)
Why Fatty Acids?
(For energy storage?)
• Two reasons:
1. The carbon in fatty acids (mostly -CH2groups) is reduced (so its oxidation yields
the most energy possible).
2. Fatty acids are not hydrated (as mono- and
polysaccharides are), so they can pack more
closely in storage tissues
1. Triacylglycerols represent the major energy
input in the modern American diet
•
Provides 30-60 % of calories
2. Triacylglycerols are also the major form of
stored energy in the body
3. Hormones (glucagon, epinephrine, ACTH)
trigger the release of fatty acids from
adipose tissue (Figure 23.2 & chapter 32)
Triacylglycerol lipase
Hormone-sensitive lipase
Figure 23.2 Liberation of fatty
acids from triacylglycerols in
adipose tissue is hormonedependent.
1. Triacylglycerols represent the major energy
input in the modern American diet
•
Provides 30-60 % of calories
2. Triacylglycerols are also the major form of
stored energy in the body
3. Hormones (glucagon, epinephrine, ACTH)
trigger the release of fatty acids from
adipose tissue (Figure 23.2 & chapter 32)
4. Degradation of dietary fatty acids occurs
primarily in the duodenum (Figure 23.3 &
chapter 24)
Figure 23.3
(a) The pancreatic duct
secretes digestive fluids into
the duodenum, the first portion
of the small intestine. (b)
Hydrolysis of triacylglycerols
by pancreatic and intestinal
lipases. Pancreatic lipases
cleave fatty acids at the C-1
and C-3 positions. Resulting
monoacylglycerols with fatty
acids at C-2 are hydrolyzed by
intestinal lipases. Fatty acids
and monoacylglycerols are
absorbed through the intestinal
wall and assembled into
lipoprotein aggregates termed
chylomicrons (discussed in
Chapter 24).
Figure 23.4 In the small intestine, fatty acids combine with bile salts in mixed micelles,
which deliver fatty acids to epithelial cells that cover the intestinal villi. Triacylglycerols
are formed within the epithelial cells.
23.2 – How Are Fatty Acids Broken
Down?
• Franz Knoop showed that fatty acids must be
degraded by removal of 2-C units (acetate)
• Albert Lehninger showed that this occurred
in the mitochondria
• F. Lynen and E. Reichart showed that the 2-C
unit released is acetyl-CoA, not free acetate
• The process begins with oxidation of the
carbon that is "b" to the carboxyl carbon, so
the process is called"b-oxidation"
Figure 23.5 Fatty acids are degraded by repeated cycles of oxidation at the β-carbon and
cleavage of the Cα-Cβ bond to yield acetate units, in the form of acetyl-CoA.
Coenzyme A activates Fatty Acids
for degradation
• The process of b-oxidation begins with the
formation of a thiol ester bond between the FA
and the thiol group of CoA
• Acyl-CoA synthetase condenses fatty acids with
CoA, with simultaneous hydrolysis of ATP to
AMP and PPi (acyl-CoA ligase or fatty acid thiokinase)
This reaction normally occurs at the outer mitochondrial
membrane or at the surface of the endoplasmic reticulum
Figure 23.7 The mechanism of the acyl-CoA synthetase reaction involves fatty acid
carboxylate attack on ATP to form an acyl-adenylate intermediate. The fatty acyl CoA
thioester product is formed by CoA attack on this intermediate.
Carnitine as a Carrier
Carnitine carries fatty acyl groups across the
inner mitochondrial membrane
• Short chain fatty acids are transported directly
into the mitochondrial matrix
• Long-chain fatty acids cannot be directly
transported into the matrix
– Long-chain FAs are converted to acyl-carnitines and
are then transported in the cell
• Acyl-CoA are formed inside the inner membrane
Figure 23.8 The formation of acylcarnitines and
their transport across the inner mitochondrial
membrane. The process involves the coordinated
actions of carnitine acyltransferases on both sides
of the membrane and of a translocase that shuttles
O-acylcarnitines across the membrane.
b-Oxidation of Fatty Acids
A Repeated Sequence of 4 Reactions
• First 3 reactions is to create a carbonyl group on
the b-C
• Fourth cleaves the "b-keto ester" in a reverse
Claisen condensation
• Products: an acetyl-CoA and a fatty acid (two
carbons shorter)
• The first three reactions are crucial and classic we will see them in other pathways (TCA cycle)
Figure 23.9
The b-oxidation of
saturated fatty
acids involves a
cycle of four
enzyme-catalyzed
reactions.
Figure 19.2
The tricarboxylic acid
cycle.
1. Acyl-CoA Dehydrogenase
Oxidation of the C-Cb bond
• A family of membrane-bound (VLCAD) and
three soluble matrix enzymes (LCAD,
MCAD, and SCAD)
• Mechanism involves proton abstraction,
followed by double bond formation and
hydride removal by FAD
• Electrons are passed to an electron transfer
flavoprotein, and then to the electron transport
chain (chapter20)
14C and longer
Figure 23.10 Very long-chain fatty acids proceed through several cycles of β-oxidation (left)
via membrane-bound enzymes in mitochondria, before becoming substrates for the separate
soluble enzymes of β-oxidation (right).
ETF: electron transfer flavoprotein
Figure 23.12 The acyl-CoA dehydrogenase reaction. The two electrons removed in this
oxidation reaction are delivered to the electron transport chain in the form of reduced
coenzyme Q (UQH2).
Figure 20.4
2. Enoyl-CoA Hydratase
Adds water across the double bond
• The reaction is catalyzed by enoyl-CoA
hydratase
– Also called crotonases
• Normal reaction converts trans-enoyl-CoA
to L-b-hydroxyacyl-CoA
3. L-Hydroxyacyl-CoA Dehydrogenase
Oxidizes the b-Hydroxyl Group
• This enzyme is completely specific for Lhydroxyacyl-CoA
• NADH produced in this reaction represents
metabolic energy
4. Thiolase (b-ketothiolase)
Cleavage of the b-ketoacyl-CoA
• Cysteine thiolate on enzyme attacks the bcarbonyl group of acyl-CoA
• Thiol group of a new CoA attacks the
shortened chain, forming a new, shorter acylCoA
• Formation of a new thioester, this reaction is
favorable and drives other three previous
reactions of the b-oxidation
Figure 23.15 The mechanism of the thiolase reaction.
Summary of b-Oxidation
Repetition of the cycle yields a succession of
acetate units
• Thus, palmitic acid yields eight acetyl-CoAs
• Complete b-oxidation of one palmitic acid
yields 106 molecules of ATP
• Large energy yield is consequence of the highly
reduced state of the carbon in fatty acids
• This makes fatty acids the fuel of choice for
migratory birds and many other animals (70%)
Migratory Birds Travel Long Distances on
Energy From Fatty Acid Oxidation
– Because they represent the most highly
concentrated form of stored biological
energy, fatty acids are the metabolic fuel of
choice for sustaining the long flights of
migratory birds
– These prodigious feats are accomplished by
storing large amounts of triacylglycerols prior
to flight
– These birds are often 70% fat by weight when
migration begins (compared with values of
30% and less for nonmigratory birds)
The ruby-throated hummingbird
flies nonstop across the Gulf of
Mexico
American golden plovers fly from
Alaska to Hawaii nonstop – 3300
km in 35 hours – more than
250,000 wing beats
Fatty Acid Oxidation is an Important Source
of Metabolic Water for Some Animals
• Large amounts of metabolic water are generated
by b-oxidation
• For certain animals, the oxidation of stored fatty
acids can be a significant source of dietary water
– Desert animals (such as gerbils)
– Killer whales (which do not drink seawater)
– Camels (whose hump is a large fat deposit)
• Metabolism of fatty acids from such stores
provides needed water, as well as metabolic
energy, during periods when drinking water is
not available
23.3 – How Are Odd-Carbon Fatty
Acids Oxidized?
Oxidation yields propionyl-CoA
• Odd-carbon fatty acids are metabolized
normally, until the last three-C fragment
(propionyl-CoA) is reached
• Three reactions convert propionyl-CoA to
succinyl-CoA (Figure 23.18)
– Propionyl-CoA carboxylase (biotin)
– Methylmalonyl-CoA epimerase
– Methylmalonyl-CoA mutase (B12)
• Succinyl-CoA can enter the TCA cycle
Figure 23.19 The methylmalonyl-CoA epimerase
mechanism involves a resonance-stabilized
carbanion at the -position.
Figure 23.18 The conversion of propionyl-CoA
(formed from b-oxidation of odd-carbon fatty acids)
to succinyl-CoA is carried out by a trio of enzymes as
shown. Succinyl-CoA can enter the TCA cycle.
Conversion of inactive vitamin B12 to active 5'deoxyadenosylcobalamin involves three steps
• Two flavoprotein reductases convert Co3+ to Co2+ and then to
Co+
• Co+ is a powerful nucleophile, which can attack the C-5’ of
ATP to form 5-deoxyadenosylcobalamin
23.4 – How Are Unsaturated Fatty
Acids Oxidized?
Monounsaturated fatty acids:
• Oleic acid, palmitoleic acid
• Normal b-oxidation for three cycles
• cis-3-acyl-CoA cannot be utilized by acylCoA dehydrogenase
• Enoyl-CoA isomerase converts this to trans2-acyl-CoA
b-oxidation continues from this point
Figure 23.21 b-Oxidation of unsaturated
fatty acids. In the case of oleoyl-CoA, three
b-oxidation cycles produce three molecules
of acetyl-CoA and leave cis-3-dodecenoylCoA. Rearrangement of enoyl-CoA
isomerase gives the trans-2 species, which
then proceeds normally through the boxidation pathway.
Polyunsaturated Fatty Acids
Slightly more complicated
• Same as for oleic acid, but only up to a
point:
– 3 cycles of b-oxidation
– enoyl-CoA isomerase
– 1 more round of b-oxidation
– trans-2, cis-4 structure is a problem
• 2,4-Dienoyl-CoA reductase to the rescue
Figure 23.22 The oxidation pathway for polyunsaturated fatty acids
23.5 – Are There Other Ways to
Oxidize Fatty Acids?
1. Peroxisomal & Glyoxysomal b-oxidation
2. Branched-chain -oxidation
3. -oxidation (dicarboxylic acids)
Peroxisomal b-Oxidation requires
FAD-dependent acyl-CoA oxidase
• Peroxisomes - organelles that carry out flavindependent oxidations, regenerating oxidized
flavins by reaction with O2 to produce H2O2
– Similar to mitochondrial b-oxidation, but initial
double bond formation is by acyl-CoA oxidase
– Electrons go to O2 rather than e- transport
– Fewer ATPs result
• Similar b-oxidation enzymes are also found in
glyoxysomes in plant
Figure 23.23 The acyl-CoA
oxidase reaction in peroxisomes.
Electrons captured as FADH2
are used to produce the
hydrogen peroxide required for
degradative processes in
peroxisomes and thus are not
available for eventual
generation of ATP.
Branched-Chain Fatty Acids
An alternative to b-oxidation is required
• Branched chain FAs with branches at oddnumber carbons are not good substrates for
b-oxidation
 -oxidation is an alternative
• Phytanic acid -oxidase decarboxylates
with oxidation at the alpha position
 b-oxidation occurs past the branch
Figure 23.24
Branched-chain fatty acids are
oxidized by -oxidation, as shown
for phytanic acid. The product of
the phytanic acid oxidase, pristanic
acid, is a suitable substrate for
normal b-oxidation. IsobutyrylCoA and propionyl-CoA can both
be converted to succinyl-CoA,
which can enter the TCA cycle.
-oxidation (dicarboxylic acids)
• In ER
• Cytochrome P-450
• O2
Figure 23.25 Dicarboxylic acids can be formed by
oxidation of the methyl group of fatty acids in a
cytochrome P-450-dependent reaction.
23.6 – What Are Ketone Bodies, and
What Role Do They Play in Metabolism?
A special source of fuel and energy for
certain tissues
• Some of the acetyl-CoA produced by fatty acid
oxidation in liver mitochondria is converted to
acetone, acetoacetate and b-hydroxybutyrate
–
–
–
–
These are called "ketone bodies"
Source of fuel for brain, heart and muscle
Major energy source for brain during starvation
They are transportable forms of fatty acids
Ketone Body synthesis
• In liver mitochondrial
matrix
1. First reaction is reverse
thiolase
2. Second reaction makes HMGCoA
3. Third reaction converts HMGCoA to acetoacetate and
acetyl-CoA by HMG-CoA
lyase
4. Acetoacetate is reduced to bhydroxybutyrate by bhydroxybutyrate
dehydrogenase
•
•
Acetoacetate and bhydroxybutyrate are
transported through the
blood from liver to target
organ
Acetoacetate and bhydroxybutyrate are
converted to acetyl-CoA
Figure 23.27 Reconversion of ketone bodies
to acetyl-CoA in the mitochondria of many
tissues (other than liver) provides significant
metabolic energy.