Lehninger Principles of Biochemistry 5/e

LIPID METABOLISM & LIPID
BIOSYNTHESIS
Reference: Lehninger’s Principles of Biochemistry 4&5th edition
Chapter 17 – Lipid catabolism
Chapter 21 – Lipid Biosynthesis
Sources of Fatty Acid Fuels
1. Diet
•About one third of our energy needs comes from dietary
triacylglycerols
Adipocytes
•~80% of energy needs of mammalian
heart and liver are met by oxidation of FA.
2. Lipid droplets stored in cells/seeds
(triacylglycerols - TAGs)
3. Synthesis and transport to another organ (TAGs)
(excess sugar in liver  fatty acids)
Many hibernating animals, such as grizzly bears rely almost
exclusively on fats as their source of energy
Fat stores in cells. Cross section of four guinea pig
adipocytes, showing huge fat droplets that virtually fill the
cells. Also visible are several capillaries in cross section.
Fats Provide Efficient - Fuel Storage
The advantage of fats over polysaccharides:
– Fatty acid carry more energy per carbon because they
are more reduced
– Fatty acids carry less water along because they are
nonpolar (unsolvated)
Glucose and glycogen are for short-term energy needs, quick
delivery
Fats are for long term (months) energy needs, good storage
(chemically inert), slow delivery
Glucose
Protein
Glycogen
CHO
Fats
Fats are first converted
digested, transported,
and delivered to cells as
free fatty acids
Fatty acids transported
to the mitochondria
where they are oxidized
Each two carbon segment
is converted to acetyl-CoA
that can be further
oxidized in the CAC
Significance
Fats store ~ 2 x energy/weight compared to protein and
carbohydrate, and triacylglycerols provide over half the
energy required for the liver, heart and resting muscle
Fuel Usage in Humans
Fat: 400 000 kJ Protein: 100 000 kJ
CHO: 2 500 kJ
Fat (fatty acid) catabolism provides:
1. Reducing equivalents that will ultimately produce ATP
2. Acetyl-CoA that can
a) be oxidized by the CAC  ATP
b) be converted to ketone bodies (alternate fuel
source for brain and other tissues) in the liver
c) be used for cholesterol biosynthesis
d) act as a biosynthetic precursor
Relationship to Other Pathways
Glycerol

glycerol 3phosphate

DHAP

G3P
Glucose
Glycolysis
Glycerol
FATS
Triacylglycerols
Pyruvate
PDH
Acetyl-CoA
Fatty acids
Ketone bodies
Cholesterol
Oxaloacetate
Citrate
Citric Acid Cycle
Digestion
Bile salts produced
in liver 
The Digestion and Absorption of Fats
Animation:
http://bcs.whfreeman.com/thelifewire/content/chp50/5002001.html
Digestion and Transport
1. Mixed micelle formation between dietary fats
and bile salts (amphipathic) increase
availability to lipases
2. Lipases break down triacylglycerols to
mono- and di-acyl glycerols, free fatty acids,
and glycerol that diffuse to epithelial cells of
intestine
3. Products taken up by epithelial cells of
intestine where they are reconverted to
triacylglycerols
4. TAGs incorporated into chylomicrons, made
fro
a. triacylglycerols
b. cholesterol
c. apolipoproteins
d. phospholipids for transport to other
tissues
Digestion and Transport
5. Chylomicron: hydrophobic TAGs at center, hydrophilic head groups
and proteins face out – chylomicrons are exocytosed and transported
by blood/lymphatic system
Chylomicron
6. Lipoprotein lipase in the
capillary of tissue
releases FAs + glycerol
(glycerol can enter glycolysis)
7. Cells uptake fatty acids:
a) adipose: storage as TAGs
b) muscle: oxidized for energy
8. Excess chylomicrons are taken
up by the liver and their
components recycled
Mobilization & Transport
• Hormones signal when
metabolic energy is required
– mobilization
• When glucose is low,
glucagon and epinephrine are
secreted
• Adenylyl cyclase is activated,
triggering a cascade
response – always involves a
second messenger, in this
case cAMP
• Lipase converts TAGs  FAs
that are transported by serum
albumin to skeletal muscle,
heart, and kidney where they
enter cells by a transporter for
fuel
Fatty Acid Transport to Mitochondria
• Fats are degraded into fatty acids and glycerol in
the cytoplasm
• -oxidation of fatty acids occurs in mitochondria
• Small (< 12 carbons) fatty acids diffuse freely
across mitochondrial membranes
• Larger fatty acids are transported via acylcarnitine / carnitine transporter
1. Activation
Fatty acid + CoA + ATP
 Fatty acyl-CoA + AMP + 2Pi
Transport to Mitochondria
2. Transfer to Carnitine  fatty acyl carnitine
3. Transport (IM space  Mitochondrial matrix)
4. Removal from Carnitine
*
* Acyl-carnitine/carnitine transporter – rate limiting step
5C. Fatty Acid Oxidation
Left: palmitate
Right:
Albert Lehninger
The Big Oxidative Picture
Where do all these reactions
take place?
The Mitochondria
n-ox = (nC/2) - 1
The cell stands to make a large
amount of energy in the form of
ATP by oxidizing only one fatty
acid
Stages of Fatty Acid Oxidation
 Stage 1 consists of oxidative conversion of two-carbon units
into acetyl-CoA with concomitant generation of NADH
 Stage 2 involves oxidation of acetyl-CoA into CO2 via citric
acid cycle with concomitant generation NADH and FADH2
 Stage 3 generates ATP from NADH and FADH2 via the
respiratory chain
The -Oxidation Pathway
• Every other carbon is converted
to a C=O
• Allows nucleophilic attack by
CoA-SH on remaining chain
• 1 CoA is used for every 2 carbon
segment to release acetyl-CoA
• Each round produces
1 FADH2, 1 NADH, 1 Acetyl-CoA
(2 in the last round)
Which three contiguous enzymes in either a pathway or cycle are
analogous to three enzymes in -oxidation, and why?
Step 1: Dehydrogenation of Alkane to Alkene
-Catalyzed by isoforms of acyl-CoA dehydrogenase (AD) on the inner
mitochondrial membrane
Step 2: Hydration of Alkene
-Catalyzed by two isoforms of enoyl-CoA hydratase:
Soluble short-chain hydratase (crotonase)
Membrane-bound long-chain hydratase, part of trifunctional complex
Water adds across the double bond yielding alcohol
Step 3: Dehydrogenation of Alcohol
-Catalyzed by -hydroxyacyl-CoA dehydrogenase
The enzyme uses NAD cofactor as the hydride acceptor
Only L-isomers of hydroxyacyl CoA act as substrates
Analogous to malate dehydrogenase reaction in the CAC
Step 4: Transfer of Fatty Acid Chain
-Catalyzed by acyl-CoA acetyltransferase (thiolase) via covalent
mechanism
The carbonyl carbon in -ketoacyl-CoA is electrophilic
Active site thiolate acts as nucleophile and releases acetyl-CoA
Terminal sulfur in CoA-SH acts as nucleophile and picks up the
fatty acid chain from the enzyme
The net reaction is thiolysis of carbon-carbon bond
The -Oxidation Series is Repeated Until Only CoA Remains
• 14 C  7 Acetyl-CoA
• 16 C ?
• How many ATP would be
produced by one round of
the citric acid cycle?
• CAC: 3 NADH, 1 FADH2,
1 GTP
 (7.5 + 1.5 + 1) ATP
 10 ATP/1 Acetyl-CoA
Figure 17-8
For a C14 lipid: 7 x 10 ATP
= 70 ATP
Monounsaturated Fatty Acids Require Enoyl-CoA Isomerase
• enoyl-CoA hydratase
can only use a trans
double bond as a
substrate
• enoyl-CoA isomerase
converts the cis double
bond to trans
• polyunsaturates need
yet another enzyme
Figure 17-9
Oxidation of Propionyl-CoA
 Most dietary fatty acids are even-numbered
 Many plants and some marine organisms also synthesize odd-
numbered fatty acids
 Propionyl-CoA forms from -oxidation of odd-numbered
fatty acids
 Bacterial metabolism in the rumen of ruminants also
produces propionyl-CoA
Odd Numbered Fatty Acids Require
Three More Enzymes
• Once proprionyl-CoA has been
formed, it can be converted
to succinyl-CoA as shown
• One ATP is used in the first
reaction to facilitate CO2
transfer by biotin
• Coenzyme B12 is required for
the mutase step – this is an
isomerization reaction with a
free radical mechanism (see
box 17-2)
Regulation
1. Fat mobilization is regulated by hormones and is related to
blood glucose levels
2. Regulation of Fatty Acid Oxidation - Considerations
• Tightly regulated – fat is a precious fuel
• Compartmentalization: Fatty acyl-CoA used for:
1. Synthesis of Triacylglycerols (cytosol)
2. Oxidation to Acetyl-CoA (mitochondria)
• The rate-limiting step is transport to the mitochondria
• Once in the mitochondria, FAs will be oxidized
• Malonyl CoA (first step in FA anabolism) shuts down
transport to the mitochondria, cutting off -oxidation
• Two enzymes are regulated by metabolites that signal
energy sufficiency ([NADH]/[NAD+] and acetyl-CoA)
Formation of Ketone Bodies in the Liver
 Entry of acetyl-CoA into citric acid cycle requires oxaloacetate
 When oxaloacetate is depleted, acetyl-CoA is converted into
ketone bodies
 The first step is reverse of the last step in the -oxidation:
thiolase reaction joins two acetate units
 Production of ketone bodies increases during starvation
 Ketone bodies are released by liver to bloodstream
 Organs other than liver can use ketone bodies as fuels
 Too high levels of acetoacetate and -hydroxybutyrate lower
blood pH dangerously
• Ketone bodies can be
synthesized from 2
Acetyl-CoA molecules
• Two acetyl groups are
joined and reduced
• Acetone is exhaled
• Ketone bodies are
water soluble, can be
transported in the
blood to other tissues
including the brain either it enters CAC or
is used to make myelin
Ketone Bodies as a Fuel Source
• Once transported,
D--Hydroxybutyrate
can produce 1 NADH
and 2 Acetyl-CoA
• If Acetyl-CoA is
required, will there be
enough Succinyl-CoA
to fuel this reaction?
• Where have we seen
succinyl-CoA before?
CAC
Brief
Fats are an important energy source in animals
Two-carbon units in fatty acids are oxidized in a four-step -
oxidation process into acetyl-CoA
In the process, lots of NADH and FADH2 forms; these can yield
lots of ATP in the electron-transport chain
Acetyl-CoA formed in the liver can be either oxidized via the
citric acid cycle or converted to ketone bodies that serve as fuels for
other tissues
http://www.wiley.com/college/grosvenor/0470197587/animatio
ns/Animation_Lipid_Metabolism/Energy/media/content/met/an
ima/met4a/frameset.htm