Fatty Acid Biosynthesis

What are the four major differences between fatty acid synthesis and degradation?

1. Intermediates in synthesis are linked to -SH groups of acyl carrier proteins compared to -SH groups of CoA

2. Synthesis occurs in the cytosol while degradation occurs in the mitochondria

3. Synthesis uses NADPH/NADP+ while degradation uses NADH/NAD+

4. The stereochemistry of the hydration/dehydration reaction. Synthesis: 3-D-hydroxyacyl; Breakdown: 3-L-hydroxyacyl

What is the precursor to malonyl CoA?

Acetyl CoA.

What elongates fatty acids?

Malonyl-CoA.

What must happen first for fatty acid synthesis?

The acetate units must be activated.

What happens to Acetyl-CoA after activation?

It has been converted to malonyl-CoA.

What happens to drive chain growth?

Decarboxylation of malonyl-CoA and reducing power of NADPH.

How long does the chain grow to?

16 carbons only.

What are needed to add additional carbons or double bonds?

Additional enzymes.

Both pathways that make Acetyl-CoA are in the matrix, why does this cause an issue?

Fatty Acid synthesis takes place in the cytosol.

What are the sources of Acetyl-CoA?

Amino Acid degradation produces cytosolic Acetyl-CoA, Beta oxidation produces mitochondrial Acetyl-CoA, Glycolysis yields cytosolic pyruvate which is converted to Acetyl-CoA in the mitochondria, Citrate-malate-pyruvate shuttle.

What are the two main purposes of citrate-malate-pyrvuate shuttle (Tricarboxylate transport system)?

Provides cytosolic acetate units and reducing equivalents ie forms NADPH.

Carboxylation of Acetyl-CoA to Malonyl-CoA is done by what enzyme?

Acetyl-CoA Carboxylase.

The carboxylation of Acetyl-CoA to Malonyl-CoA is?

Irreversible and the first committed step.

What does Acetyl-CoA Carboxylase use?

Biotin, bicarbonate, and ATP.

What method is Acetyl-CoA Carboxylase regulated?

Allosterically.

Why are more enzymes required to add more carbons to a fatty acid chain?

Palmitoyl-CoA acts as an allosteric inhibitor, it favors protomers.

What is the protomer/monomer form?

Inactive.

What is the polymer form?

Active.

What is the allosteric inhibitor of Acetyl-CoA Carboxylase?

Palmitoyl-CoA

What is the allosteric activator of Acetyl-CoA carboxylase?

Citrate.

Explain why it makes sense that Citrate would be an allosteric activator of ACC.

An abundance of citrate indicates an abundance of Acetyl-CoA which means that ACC can work and form Malonyl-CoA.

When insulin is released what product is formed regarding ACC?

ACC polymerizes creating an active polymer.

When glucagon/epinephrine is released what product is formed regarding ACC?

The inactive monomer form.

When the monomers are phosphorylated, what form is the product in?

The inactive monomer form.

If the inactive monomer is exposed to citrate what will occur?

The result will be a partially active polymer because the monomer is still phosphorylated but the allosteric activator has a stronger pull on the result.

What are the monomers?

Acetyl-CoA Carboxylase.

When insulin is released, what enzyme catalyzes the reaction to form the active polymer?

Protein phosphatase 2A.

When glucagon/epinephrine is released what enzyme catalyzes the reaction to form the inactive monomer?

AMP-activated protein kinase.

AMPK is an important metabolic what?

Sensor.

Does insulin activate or inactivate AMPK?

Inactivate.

What enzyme catalyzes the reaction that inactivates AMPK?

Protein phosphatase 2C.

Does an increase in AMP activate or inactivate AMPK?

Activate.

What enzyme catalyzes the reaction that activates AMPK?

AMPK Kinase.

Is active AMPK phosphorylated or unphosphorylated?

Phosphorylated.

Is inactive AMPK phosphorylated or unphosphorylated?

Unphosphorylated.

What is the carrier of intermediates in fatty acid synthesis?

Acyl Carrier Protein.

In what organisms are fatty acid synthesis enzymes separate in a complex?

Bacteria and Plants.

What is the major enzyme in fatty acid synthesis in animals?

Fatty acid synthase.

Characteristics of fatty acid synthase.

A multienzyme complex, dimer, operates in concert, the subunits are arranged head to tail.

What is the difference between enzymes in beta oxidation and fatty acid synthesis?

Beta oxidation has four different enzymes compared to synthesis which has a single polypeptide with different functions.

What molecule is the priming group in the fatty acid synthesis cycle?

Acetyl-CoA.

After the first cycle of fatty acid synthesis, how many Acetyl-CoA molecules are used?

None, the first cycle is the only cycle that requires Acetyl-CoA as the priming group.

How many steps are in each cycle of fatty acid synthesis?

Four steps.

How many carbons are added to the chain every cycle?

Net addition of 2 carbons.

What are required inputs for each cycle of fatty acid synthesis?

One malonyl-CoA and the oxidation of 2 NADPH molecules (4 e-).

In the first three steps what other reaction also occurs?

Dehydration reaction.

In the final step, an enzyme catalyzes what reaction to release palmitate?

A hydrolysis reaction.

What enzyme catalyzes the hydrolysis reaction in the final step of fatty acid synthesis?

Palmitoyl thioesterase.

Palmitoyl thioesterase is a part of what?

Fatty acid synthase.

Acetyl-CoA is first added to which subunit?

ACP.

After Acetyl-CoA is added to the ACP subunit, it is then transferred to what subunit?

KS.

What protein subunit does all of the reactions take place on in fatty acid synthesis?

On the ACP protein.

There are seven dehydration steps required for palmitate synthesis, yet there is a net of only 6 H2O. Why?

One of the H2O molecules is used to hydrolyze the thioester linking the palmitate product to the enzyme.

Where does additional elongation of fatty acids primarily occur?

In the ER.

In most tissues the ER converts palmitate (16 C) to what?

Stearate (18 C).

The brain ER can synthesize what number fatty acids, which are used for brain lipids?

C 24.

In the ER where additional elongation occurs what are the intermediates?

CoA esters rather than ACP esters.

The addition of cis double bonds in eukaryotes does not occur until what?

The fatty acid has reached full length.

What is desaturation?

The formation of double bonds in fatty acids.

What enzymes catalyze desaturation reactions?

Desaturases.

Where does desaturation occur?

Endoplasmic reticulum.

Desaturases are “mixed function reactions” meaning what?

They oxidize two substrates (FA and NADH).

What do desaturases contain?

A nonheme iron site, NADH, and an oxygen binding site.

Three components are required for desaturation reactions to occur. What are they?

Desaturase, cytochrome b5, and NADH-cytochrome b5 reductase.

Desaturation reactions result in the loss of what?

2 hydrogens.

Mammals lack the enzyme that allows for the introduction of cis double bonds beyond what in the fatty acid chain?

Beyond C9.

Due to mammals lacking the enzyme that allows for cis double bonds beyond C9, mammals are unable to synthesize required unsaturated FAs. These are known as?

Essential fatty acids, such as omega 3.

Mammals can add additional double bonds to unsaturated FAs in?

Their diets.

Mammals can synthesize arachidonic acid, which is the precursors for what?

Molecules that cause fever and inflammation.

Can plants synthesize arachidonic acid?

No.

Arachidonic acid is made from essential FAs we get from our?

Diet.

Malonyl-CoA acts as an allosteric inhibitor of what molecule?

Carnitine acyltransferase.

Since malonyl-CoA inhibits carnitine acyltransferase, what process is inhibited?

Beta oxidation.

What is a major component of membranes?

Cholesterol.

Where is cholesterol synthesized in?

Liver cells.

What is cholesterol synthesized from?

Acetyl-CoA.

What are cholesterol derivatives that are important physiological ligands and pharmaceutical drugs?

Steroid hormones.

What are fatty acid derivatives that are precursors to prostaglandins, thromboxanes, and leukotrienes?

Eicosanoids.

What is important about the structure of cholesterol?

It has a hydroxy group and many CH groups, making it amphipathic.

Describe three important features of cholesterols structure.

Four fused rings, a carbon-hydrogen tail, and a carbon 3 hydroxy group.

When forming cholesterol, acetate is first converted to what?

C5 isoprene units.

The C5 isoprene units are then condensed to form what?

Squalene.

What is squalene?

A linear precursor of cholesterol.

What is the prevalent steroid in cells?

Cholesterol.

What is the precursor for bile salts and steroidal hormones?

Cholesterol.

What is the rate limiting step of cholesterol synthesis?

HMG-CoA reductase converts HMG-CoA to mevalonate.

What is the target of statin drugs?

HMG-CoA reductase.

What condition is required to turn on the cholesterol biosynthesis pathway?

Abundant glucose.

What reaction is the first committed step of cholesterol biosynthesis?

A reduction reaction that uses two molecules of NADPH and releases CoA.

What are the three fates of cholesterol synthesized in the liver?

Stored, exported, or converted to bile acids.

What would happen to cholesterol levels in the liver if bile acids were excreted rather than recycled back to the liver?

Cholesterol would be depleted through Le Chateliers principle.

How are cholesterol esters formed?

By adding fatty acids to the cholesterol molecule.

Once cholesterol esters are formed, they are no longer what?

Amphipathic, they are completely hydrophobic.

What enzyme converts cholesterol to cholesterol esters?

Acyl-CoA cholesterol acyl transferase (ACAT).

What is the principle site of regulation in cholesterol synthesis?

HMG-CoA reductase.

How is HMG-CoA reductase regulated?

During post translation, translation, and transcription.

What occurs post translationally that regulates HMG-CoA reductase?

Phosphorylation by cAMP-dependent kinases.

What does phosphorylation of HMG-CoA reductase by cAMP-dependent kinases results in?

The inactivation of the reductase.

What happens during translation that regulates HMG-CoA reductase?

Degradation of HMG-CoA reductase because the half life is 3 hours and depends on the cholesterol level.

What happens during transcription that regulates HMG-CoA reductase?

Gene expression (mRNA production) is controlled by cholesterol levels.