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Lecture 37
Lipid Metabolism 2
Fatty Acid Synthesis
Prostaglandins
Key Concepts
• Overview of fatty acid synthesis
• Synthesis of palmitate from malonyl CoA
• Regulation of fatty acid synthesis
• Eicosanoids: Prostaglandin (PGH2) synthesis
Overview of fatty acid synthesis
Fatty acids are synthesized and degraded by different pathways.
Difference
FA Synthesis
FA Degradation
subcellular location
cytosol
mitochondrial matrix
carrier protein
acyl carrier protein
(ACP)
coenzyme A (CoA)
enzymes
all activities on a single multiple enzymes
polypeptide chain
required
redox
reductant is NADPH
oxidants are NAD and
FAD
building block
malonyl-CoA (formed
by carboxylation of
acetyl-CoA)
Acetyl-CoA
The commitment step in fatty acid synthesis is the
carboxylation of acetyl-CoA to form malonyl-CoA by the
enzyme acetyl-CoA carboxylase.
Acetyl-CoA carboxylase uses a two-step mechanism that follows the
ping-pong mechanism of substrate addition and product release.
Two carbon units are sequentially added to the growing fatty
acid chain by a large protein complex called fatty acid synthase.
Palmitate (C16 ) is the end-product of the FA synthase reaction.
The net reaction of palmitate synthesis is:
8 acetyl-CoA + 7 ATP + 14 NADPH + 14 H+ →
palmitate + 14 NADP+ + 8 CoA + 7 ADP + 7 Pi + 6 H2O
Note that excess acetyl-CoA units in the mitochondria are transported to
the cytosol by the citrate shuttle. Moreover, an important component of this
shuttle is the generation of NADPH in the cytosol which can be used for FA
synthesis.
This shuttle will produce 8 NADPH in the synthesis of palmitate since 8
acetyl-CoA units need to be transported.
The synthesis of palmitate requires 14 NADPH as shown above in the
net reaction. Where do the additional 6 NADPH come?
Synthesis of palmitate from malonyl-CoA
ATP-dependent caboxylation of acetyl-CoA by the enzyme acetyl-CoA
carboxylase forms malonyl-CoA in the commitment step of the pathway.
Subsequent decarboxylation of malonyl-CoA by fatty acid synthase results
in the addition of C2 acetyl units to the growing fatty acid chain. Eight
rounds are required to form palmitate.
Note that the ATP-dependent carboxylation of acetyl CoA stores bond
energy in malonyl CoA that is then used to drive fatty acid synthesis
through the subsequent decarboxylation step. Indirectly, ATP hydrolysis
therefore drives the fatty acid synthase reaction through the addition and
removal of CO2.
Malonyl CoA must be covalently attached to the acyl carrier protein (ACP)
through a sulfhydral linkage at cysteine 163 prior to elongation:
malonyl-CoA + ACP ↔ malonyl-ACP + CoA
Once the enzyme is "primed" then
the first synthesis step condenses
C2 acetyl-ACP, with C3 malonylACP, and release of CO2:
1st round: C2 + C3 → CO2 + C4
2nd round: C4 + C3 → CO2 + C6.
Reduction, dehydration and
reduction steps then follow,
regenerating an acyl-ACP that is
ready for another cycle.
The growing fatty acid chain is translocated between the
condensing enzyme (CE) and acyl carrier protein (ACP).
Putting it all together:
Acetyl CoA Carboxylase Reactions:
7 acetyl-CoA + 7 CO2 + 7 ATP →
7 malonyl-CoA + 7 ADP + 7 Pi
Fatty Acid Synthase Reactions:
acetyl-CoA + 7 malonyl-CoA + 14 NADPH + 14 H+ →
palmitate + 7 CO2 + 14 NADP+ + 8 CoA + 6 H2O
The Combined Reactions
8 acetyl-CoA + 7 ATP + 14 NADPH + 14 H+ →
palmitate + 14 NADP+ + 8 CoA + 7 ADP + 7 Pi + 6 H2O
Regulation of fatty acid synthesis
Fatty acid metabolism is stringently
controlled to balance synthesis and
degradation in response to physiological
demands.
Acetyl-CoA carboxylase activity is
controlled by numerous signals. This
reaction is the key commitment step in
fatty acid synthesis (formation of malonylCoA).
Two mechanisms control acetyl-CoA carboxylase activity
phosphorylation and dephosphorylation
allosteric control by citrate binding
Just as we have seen before with glycogen phosphorylase, allosteric
control can partially activate the inactive form. In this case,
phosphorylated acetyl-CoA carboxylase can be allosterically activated by
citrate binding. Note that dephosphorylated acetyl-CoA carboxylase is
nearly fully active and not appreciably stimulated by citrate binding.
Explain the metabolic logic of citrate activation of acetyl-CoA carboxylase.
Does this take place in the mitochondrial matrix or the cytosol?
Why does it make sense that glucagon and epinephrine inhibit the activity
of acetyl-CoA carboxylase, whereas, insulin stimulates its activity?
Eicosanoids: Prostaglandin (PGH2) synthesis
Arachidonate is a C20 unsaturated fatty acid (four C=C bonds) that is the
major precursor to a class of signaling molecules collectively called
eicosanoids (eye-cose-anoids), which signifies the presence of 20
carbons from the Greek word "eikosi" which means twenty.
Eicosanoids are considered local hormones because they activate
signaling pathways in nearby cells and are short-lived molecules.
Prostaglandins are a type of
eicosanoids that stimulate the
inflammatory response.
Prostaglandin H2 (PGH2) is the product
of the enzymatic reaction catalyzed by
Prostaglandin Synthase.
This enzyme functions as a
cyclooxygenase and the two known
isoforms are called COX-1 and COX-2.
These enzymes are pharmacologic
targets of non-steroidal antiinflammatory drugs (NSAIDs) such as
aspirin and ibuprofen.
Aspirin and ibuprofen inhibit the production of PGH2 by irreversibly
blocking the cycloxygenase activity of prostaglandin synthase. It turns out
that aspirin and ibuprogen inhibit both COX-1 and COX-2, although
ibuprofen is more potent than aspirin.
The COX-2 isoform is considered the key proinflammatory enzyme in
many human pathophysiologies and was discovered by Donald Young,
M.D., a University of Rochester biochemist. Alternatively, COX-1 is an
important enzyme in the stomach that produces prostaglandins required
for regulation of gastric mucin.
Therefore, a major side effect of aspirin and ibuprofen is gastrointestinal
bleeding due to low amounts of mucin which are required to protect the
stomach against acids and pepsin. COX-1 is sometimes called the "good"
isoform and COX-2 the "bad" isoform. COX-2 is responsible for pain and
inflammation.
COX-2 specific inhibitors such as Celebrex and Vioxx have been shown to
very effective for the treatment of arthritis. However, they have also been
associated with an increased risk of death from cardiovascular disease
and are currently undergoing re-evaluation by the Federal Drug
Administration (FDA).
The selectivity of Cox-2 inhibitors is due to fact that they are larger than
aspirin and ibuprofen, both of which inhibit both Cox-1 and Cox-2.
Aspirin, also known as acetylsalicylate, is an acetylating agent that
irreversibly inhibits both COX-1 and COX-2 by acetylating a serine residue
in the active site of the enzyme. In contrast, drugs like Celbrex are large
and can only bind tightly to Cox-2, thus avoiding the side effects
associated with Cox-1 inhibition.
Aspirin, in small doses, dose have as a beneficial role as a anticoagulator. This is because PGH2 is the precursor of thrombaxane A2
(TXA2), a potent aggregator of blood platelets. Small amounts of aspirin
taken on a daily basis decrease TXA2 production by inhibiting PGH2
synthesis. Low doses of aspirin do not cause significant stomach
bleeding.
Do you know where aspirin was originally found?
In a plant, the willow tree!