Download Chem*3560 Lecture 20: Fatty acid biosynthesis

Chem*3560
Lecture 20:
Fatty acid biosynthesis
Fatty acid biosynthesis reverses the
β -oxidation sequence
β -Oxidation consists of a cycle of four reactions that
include two dehydrogenase steps. Three steps are
energetically favourable, and one complete cycle sums
to ∆Go ' = –12 kJ/mol, which makes the overall
process proceed in the direction of oxidation
(Lehninger p.604-606).
Fatty acid synthesis follows the same sequence in
reverse, and uses two strategies to drive reactions in
the opposite direction (Lehninger p. 770-771).
1) NADPH is used as reductant to reduce the
ketoacyl and enoyl intermediates. Since the ratio
[NADPH] / [NADP +] is high, this drives the reactions
in the direction of reduction
2) Decarboxylation drives the elongation step
that reverses the thiolase reaction. In order to add
two carbons to elongate the chain, the three-carbon
dicarboxylate malonyl-CoA – O2 C-CH2 -CO-SCoA
condenses with the growing acyl chain, and loses its
free carboxylate as CO2 . The energy made available
by decarboxylation provides the driving force to
maintain the synthesis direction.
Acetyl CoA Carboxylase makes the malonyl CoA for fatty acid biosynthesis
Acetyl CoA carboxylase is organized in three components, which like pyruvate carboxylase, includes
a central biotin carrier protein (BCP) which has a biotin molecule covalently bonded to a lysine side
chain. On one side is the ATP driven biotin carboxylase, which generates the active carboxylate donor
carboxybiotin from bicarbonate ion at the cost of ATP hydrolysis:
ATP + biotin + HCO3 – → ADP + Pi
+ biotin-CO2 –
On the other side is the substrate specific transcarboxylase (or carboxyltransferase), which uses acetyl
CoA as the carboxylate acceptor (Lehninger p.771).
biotin-CO2 – + acetyl-CoA →
biotin + malonyl-CoA.
Unlike pyruvate carboxylase, where the three components
are domains on a single polypeptide,
acetyl-CoA carboxylase consists of three separate proteins
held together in a single complex.
Regulation of fatty acid biosynthesis
Since fatty acid biosynthesis takes the opposite direction to
β-oxidation, it must be regulated to prevent the two
processes from operating as a futile cycle, in particular in
animals where significant amounts of energy are stored as
fats and fatty acids.
Bacteria make fatty acids for membrane lipids, but do not
use fatty acids and β-oxidation as a primary energy source,
so regulation less strict and is mainly correlated with cell
growth. In plants, fatty acid biosynthesis occurs in the
chloroplasts, and is dependent on photosynthesis to
provide substrate and NADPH (Lehninger p. 778-780)..
In animals, the bulk of stored fat is found in adipose
tissue . Fatty acids are released from triacylglycerol by
hormone sensitive lipase, which is actually activated by
protein kinase A in response to cyclic AMP induced by
the hormones epinephrine and glucagon. Fatty acid is
released from adipose tissue, and circulated in blood to
those muscle tissues (e.g. cardiac) which use β-oxidation of
fatty acid as an energy source. In contrast, fatty acid
biosynthesis occurs primarily in the liver, and involves the
following modes of regulation.
1) Compartmentation
β -oxidation occurs in mitochondria or in a specialized
organelle called the peroxisome, whereas fatty acid
biosynthesis is cytoplasmic.
2) Substrate availability
Fatty acid biosynthesis requires a cytoplasmic source
of acetyl CoA. Acetyl CoA produced by β-oxidation or
by oxidation of amino acids or pyruvate is located in mitochondria, and is directed towards the TCA
cycle. Cytoplasmic acetyl CoA is produced when more citrate is produced than is needed for the TCA
cycle to generate ATP (Lehninger Fig 21-11 p.779).
The TCA cycle enzyme isocitrate
dehydrogenase is under positive
allosteric regulation by ADP levels in
mitochondria.
If ATP is not being consumed quickly
enough, ADP levels drop, and
isocitrate dehydrogenase loses
activity.
This causes unused isocitrate to
accumulate, and because the aconitase
equilibrium is unfavourable (K eq =
0.075), citrate builds up to very high
levels.
A transporter allows excess citrate to
enter the cytoplasm, where it is broken
down by citrate lyase.
citrate + HSCoA + ATP
→
ADP + Pi
+ oxaloacetate + acetyl-CoA
Oxaloacetate is reduced to malate by cytoplasmic NADH. Malate returns the mitochondria to allow
the TCA cycle to continue, and enters by an exchange process via the same transporter that exports
citrate. Strict segregation of cytoplasmic and
mitochondrial compartments prevents citrate
lyase from developing a futile cycle with the
TCA cycle enzyme citrate synthase.
3) Enzyme regulation
Fatty acid biosynthesis is subject to
hormonal and allosteric regulation.
Insulin (secreted when blood glucose levels
are high) stimulates citrate lyase.
Glucagon (secreted when blood glucose
levels are low) and epinephrine (secreted
when demand for ATP is anticipated) induce
cyclic AMP, which stimulates protein kinase
A to phosphorylate and inactivate acetyl
CoA carboxylase (Lehninger p. 780).
Cytoplasmic citrate is a positive allosteric effector of acetyl CoA carboxylase.