Basic steps of lipid and cholesterol biosynthesis in eukaryotes.

1. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
pg. 1
Biosynthesis of Fatty Acids & Cholesterol
The biosynthesis of fatty acids is more or less the reverse of fatty acid oxidation, in the
case of both saturated and unsaturated fatty acid synthesis. The starter molecule is
acetyl-CoA, which is then subsequently converted into a fatty acid chain. In the case of
cholesterol, the process includes an additional and highly complex step of ring formation.
The details are as follows:
Biosynthesis of Saturated Fatty Acids
1. Malonyl-CoA Formation
Biosynthesis of fatty acids requires the participation of a three-carbon intermediate,
malonyl-CoA.
The formation of malonyl-CoA from acetyl-CoA is catalyzed by acetyl-CoA carboxylase.
The enzyme contains a biotin prosthetic group. The two-step reaction catalyzed by this
enzyme is as follows:
1) A carboxyl group, derived from bicarbonate (HCO-3), is first transferred to biotin in an
ATP-dependent reaction.
2) The biotinyl group serves as a temporary carrier of CO2, transferring it to acetyl-CoA
in the second step to yield malonyl-CoA.
Biosynthesis Process: The long carbon chains of fatty acids are assembled in a repeating
four-step sequence, catalyzed by fatty acid synthase. A saturated acyl group produced
by each four-step series of reactions becomes the substrate for subsequent condensation
with an activated malonyl group. With each passage through the cycle, the fatty acyl chain

2. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
pg. 2
is extended by two carbons. In β-oxidation, NAD+ and FAD serve as electron acceptors
and the activating group is the thiol (—SH) group of coenzyme A. By contrast, the
reducing agent in the synthetic sequence is NADPH and the activating groups are two
different enzyme-bound —SH groups.

3. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
pg. 3
Mechanism: Each malonyl group and acetyl (or longer acyl) group is activated by a
thioester that links it to fatty acid synthase. Step 1: Condensation of an activated acyl
group (an acetyl group from acetyl-CoA is the first acyl group) and two carbons derived
from malonyl-CoA, with the elimination of CO2 from the malonyl group, extends the acyl
chain by two carbons. The β-keto product of this condensation is then reduced in three
more steps nearly identical to the reactions of β-oxidation, but in the reverse sequence:
Step 2: the β-keto group is reduced to an alcohol. Step 3: elimination of H2O creates a
double bond, and Step 4: the double bond is reduced to form the corresponding saturated
fatty acyl group.
Overall Process: The fatty acyl chain grows by two-carbon units donated by activated
malonate, with loss of CO2 at each step. After each two-carbon addition, reductions
convert the growing chain to a saturated fatty acid of four, then six, then eight carbons,
and so on.

4. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
pg. 4
Biosynthesis of Unsaturated Fatty Acids
Unsaturated fatty acids are made from saturated fatty acids by their desaturation.
Palmitate and stearate serve as precursors of the two most common monounsaturated
fatty acids of animal tissues: palmitoleate, 16:1, and oleate, 18:1; both of these fatty acids
have a single cis double bond between C-9 and C-10. The double bond is introduced into
the fatty acid chain by an oxidative reaction catalyzed by fatty acyl–CoA desaturase. Two
different substrates, the fatty acid and NADH or NADPH, simultaneously undergo two-
electron oxidations. Mammalian hepatocytes can readily introduce double bonds at the
9th position of fatty acids but cannot introduce additional double bonds between C-10 and
the methyl-terminal end.
Fatty acyl desaturase introduces a double bond in the saturated fatty acid and makes it
mono-unsaturated. Two substrates: a fatty acyl–CoA and NADPH undergo oxidation by
molecular oxygen. These reactions take place in the smooth ER. The electrons are given
to cytochrome b5 and are then passed on to cytochrome b5 reductase (which contains
FAD) and then finally they are handed over to NADP+ (which is reduced to NADPH).
Cholesterol Biosynthesis
Cholesterol is a 27-carbon molecule with three 6-cornered rings and one 5-cornered ring.
Cholesterol plays a crucial role as a component of cellular membranes and as a precursor
of steroid hormones and bile acids. Its synthesis starts from acetate (a relatively much
simpler molecule).

5. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
pg. 5
Biosynthesis Mechanism: Cholesterol is made from
acetyl-CoA. Synthesis takes place in four stages:
(1) Condensation of three acetate units to form a
six-carbon intermediate, mevalonate. (2)
Conversion of mevalonate to activated isoprene
units. (3) Polymerization of six 5-carbon isoprene
units to form the 30-carbon linear squalene. (4)
Cyclization of squalene to form the four rings of the
steroid nucleus, with a further series of changes
(oxidations, removal or migration of methyl
groups) to produce cholesterol.
Regulation of Cholesterol Biosynthesis:
1) Hormonal Control: Cholesterol production is
regulated by intracellular cholesterol
concentration and by the hormones glucagon
and insulin. Hormones modify HMG-CoA,
glucagon stimulates phosphorylation of HMG-
CoA which inactivates it and insulin promotes dephosphorylates which activates it.

6. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
pg. 6
2) Transcriptional Control: Conversion of HMG-CoA to Mevalonate is the rate-limiting
reaction catalyzed by HMG-CoA reductase. Regulation in response to cholesterol
levels is mediated by transcriptional regulation of the gene encoding HMG-CoA
reductase.
3) Feedback Inhibition: HMG-CoA reductase is inhibited by mevalonate and cholesterol
(mevalonate is the immediate product of HMG-CoA, whereas cholesterol is the
ultimate product of the entire process).