Module 9 - Fatty Acid Synthesis

Question 1

How many stages are there to fatty acid synthesis?

Answer 1

Three

The first step is to shuttle the acetyl CoA from the mitochondria to the cytosol where fatty acid synthesis occurs

the second stage, acetyl CoA is activated to form malonyl CoA

the third and final stage, palmitate is synthesized in a five-step elongation cycle

Question 2

How is acetyl CoA shuttled out of the mitochondria?

Answer 2

First, acetyl CoA reacts with oxaloacetate to form citrate by the enzyme citrate synthase

the citrate can be shuttled out of the mitochondria through a transport protein that resides in the mitochondrial membrane

The net result is that acetyl CoA has now been brought into the cytosol at the expense of an ATP

Question 3

What happens once citrate is in the cytosol?

Answer 3

citrate is acted upon by the enzyme ATP-citrate lyase, which cleaves citrate into acetyl CoA and oxaloacetate as shown:

Citrate + ATP + CoA + H2O → Acetyl CoA + ADP + Pi + Oxaloacetate

The oxaloacetate is reduced to malate using NADH, followed by the conversion of malate to pyruvate by malic enzyme which produces a molecule of NADPH

Pyruvate is able to be transported back into the mitochondria and converted by a carboxylation reaction to oxaloacetate catalyzed by pyruvate carboxylase

Question 4

How is acetyl CoA activated?

Answer 4

by combining it with HCO3-, the form of CO2 in water

This carboxylation reaction, catalyzed by acetyl CoA carboxylase, is driven by the hydrolysis of ATP and is the committed step in fatty acid synthesis

Question 5

How does the carboxylation reaction occur?

Answer 5

In the first step, biotin is carboxylated and coupled with ATP hydrolysis which drives the reaction forward

Step 1: biotin-enzyme + ATP + HCO3- → CO2-biotin + ADP + Pi + H+

In step 2, the carboxyl group is transferred from biotin to acetyl CoA to form malonyl CoA, a 3-carbon molecule.

Step 2: CO2-biotin + acetyl CoA → malonyl CoA + biotin-enzyme

Question 6

What is palmitate?

Answer 6

a 16-carbon saturated fatty acid

Question 7

How is palmitate formed?

Answer 7

through a series of four reactions

Step 1: Condensation reaction catalyzed by β-ketoacyl synthase

Step 2: Reduction step catalyzed by β-ketoacyl reductase

Step 3: Dehydration step catalyzed by 3-hydroxyacyl dehydratase

Step 4: Reduction step catalyzed by enoyl reductase.

Question 8

A large enzyme system called fatty acid synthase performs the task of synthesizing fatty acids

Answer 8

In bacteria and plants, the fatty acid synthase complex is made up of individual proteins that each contain a separate catalytic activity;

in yeast, all of the catalytic activities are present in two polypeptides;

while in vertebrates, all of the enzyme activities are contained within one large polypeptide

Question 9

An important component of the fatty acid synthase complex is the acyl carrier protein or ACP

Answer 9

ACP has a phosphopantetheine prosthetic group linked to it.

The phosphopantetheine group has a free sulfhydryl group, and it is to this group that the intermediates of fatty acid synthesis are linked.

ACP thus acts as an arm, which moves the fatty acid intermediate from one enzyme to the other as it synthesizes palmitate.

Question 10

Step 1: Condensation reaction catalyzed by β-ketoacyl synthase

Answer 10

In this reaction, the acetyl group from acetyl ACP reacts with malonyl ACP to form acetoacetyl ACP (4-carbons), with the release of CO2 and the ACP from acetyl ACP

there is a substantial release of energy which drives this reaction forward and makes it energetically favorable

Question 11

Step 2: Reduction step catalyzed by β-ketoacyl reductase

Answer 11

In this reaction, acetoacetyl ACP is reduced to 3-hydroxybutyryl ACP using NADPH as the reducing agent.

Note that the keto group is now a hydroxyl group as a result of the reduction.

Question 12

Step 3: Dehydration step catalyzed by 3-hydroxyacyl dehydratase

Answer 12

a water molecule is removed (thus a dehydration), forming crotonyl ACP.

This removes the oxygen group and leaves a double bond.

Question 13

Step 4: Reduction step catalyzed by enoyl reductase.

Answer 13

The double bond in crotonyl ACP is reduced through the consumption of another molecule of NADPH, to form butyryl ACP.

At this point, a 4-carbon molecule has been formed.

The final product of fatty acid synthesis is palmitate, a 16-carbon molecule.

Thus, six more rounds of fatty acid synthesis (the four reactions above) have to occur.

Look at Figure 9-5 again and note the product of the first round, butyryl ACP.

If you take butyryl ACP and replace acetyl ACP with it at the top of the pathway, you see how this works.

Question 14

How is the synthesis of the fatty acid completed?

Answer 14

In order to complete the synthesis of a free fatty acid, palmitate has to be released from ACP.

This is accomplished by the action of an enzyme called thioesterase, which hydrolytically (using H2O) cleaves the link between the carboxyl group of the fatty acid and the sulfhydryl group of ACP

Question 15

What is the Overall Stoichiometry of Palmitate Synthesis?

Answer 15

8 Acetyl CoA + 7 ATP + 14 NADPH → palmitate + 14 NADP+ + 8 CoA + 6 H2O + 7 ADP + 7 Pi

Not surprising for a biosynthetic reaction, both ATP and NADPH are consumed to provide the energy and reducing equivalents, respectively.

Question 16

How many rounds does it take to create Palmitate? And how much NADPH is required?

Answer 16

Since it takes 7 rounds for the 16-carbon palmitate to be synthesized, there is a requirement for 14 molecules of NADPH for every fatty acid synthesized.

Question 17

Where does all of the NADPH that’s required for fatty acid synthesis come from?

Answer 17

Pentose Phosphate Pathway, -> one of the major roles of this pathway is to produce NADPH for synthetic reactions

AND for every acetyl CoA brought into the cytosol, a molecule of NADPH is also produced in the same cellular compartment

Since eight molecules of acetyl CoA are required to make palmitate, 8 molecules of NADPH required for palmitate synthesis come through the transport of acetyl CoA into the cytosol.

The other 6 molecules required come from the Pentose Phosphate Pathway.

Question 18

How do we make longer fatty acids, which are needed by the cell?

Answer 18

Longer fatty acids are made by adding 2-carbon units to the carboxyl end of both saturated and unsaturated fatty acids, using malonyl CoA as the substrate and reactions almost analogous with those carried out by the fatty acid synthase complex.

The specialized enzymes required to do this are located on the cytoplasmic side of the ER membrane.

Question 19

The two key essential fatty acids that our diets must provide are?

Answer 19

linoleate (18:2 cis-∆9, ∆12) and linolenate (18:3 cis-∆9, ∆12, ∆15).

These are ω-6 and ω-3 fatty acids, respectively, and you have probably heard of them referred to as omega fatty acids.

Question 20

What enzyme regulates Fatty Acid synthesis?

Answer 20

The reaction catalyzed by acetyl CoA carboxylase (the synthesis of malonyl CoA from acetyl CoA) is the first committed step in fatty acid synthesis and thus makes sense that this is the enzyme that is regulated.

Question 21

When is fatty acid synthesis the highest?

Answer 21

Fatty acid synthesis is highest when carbohydrates are plentiful and fatty acids are at low levels.

lots of carbohydrate translates into lots of acetyl CoA, the precursor for fatty acids

Question 22

How is acetyl CoA carboxylase regulated?

Answer 22

regulated by both covalent modification and by allostery.

Question 23

How does covalent modification affect acetyl CoA carboxylase?

Answer 23

A kinase called AMP-activated protein kinase (AMPK) phosphorylates acetyl CoA carboxylase which inhibits the enzyme.

Under low energy conditions, acetyl CoA carboxylase would be phosphorylated by AMPK and inhibited.

Under a high energy state, the high ATP would inhibit AMPK and reduce the rate of inactivation of acetyl CoA carboxylase.

The ever-present protein phosphatase 2A would dephosphorylate acetyl CoA carboxylase which puts it in a more active state

Question 24

Which allosteric means affects acetyl CoA carboxylase?

Answer 24

Acetyl CoA carboxylase is also allosterically activated by citrate.

high levels of citrate indicate lots of substrate for fatty acid synthesis.

Question 25

How does citrate activate the enzyme?

Answer 25

The inactive form of acetyl CoA carboxylase is a dimer of identical subunits

Citrate induces polymerization of the dimers into long, active filaments, through assistance by a protein called MIG12.

Conversely, palmitoyl CoA, a fatty acyl CoA, induces depolymerisation of acetyl CoA carboxylase into inactive dimers.

Question 26

What role does insulin play in fatty acid synthesis?

Answer 26

Acetyl CoA carboxylase and thus fatty acid synthesis is stimulated by insulin.

Insulin causes the activity of acetyl CoA carboxylase to increase by both inhibiting AMPK and stimulating phosphatase 2A

The net effect is to shift most of the acetyl CoA carboxylase into the active form.

Question 27

What role do glucagon and epinephrine play in fatty acid synthesis?

Answer 27

Glucagon and epinephrine both inhibit fatty acid synthesis.

these hormones both signal a need for energy production including fatty acid degradation.

Question 28

Desaturation

Answer 28

the removal of two hydrogen atoms with the creation of a double bond.

Question 29

Eicosanoids

Answer 29

these are hormones with diverse effects that are synthesized from unsaturated fatty acids. They are referred to as “local hormones” since they act on target cells close to their site of formation.

Question 30

Essential Fatty Acids

Answer 30

unsaturated fatty acids that our bodies cannot synthesize and thus are required in our diet. The two primary ones are linoleate (or linoleic acid) and α-linolenate (or α-linolenic acid).