L14 - FA Synthesis

Overall lipid catabolism scheme

1. Acetyl-CoA turns into free fatty acids that can be used to make triacylglycerols, glycophospholipids, sphingolipids, or eicosanoids

2. Acetyl-CoA turns into HMG-CoA and enters keto body synthesis to allow acetyl-CoA to be distributed throughout the body

3. HMG-CoA is used to make cholesterol that can be turned into steroids, bile, acids, or vitamins (Ex. Vit D)

General information about FA synthesis

Makes long FA chains

Most cells can do this, but liver and adipose tissue create the most

Occurs in the cytosol

Has a three-carbon intermediate called malonyl-CoA

Energy-expensive/highly regulated

Requires 14 reducing equivalents (NADPH from PPP) and 7 ATPs

Formation of Malonyl-CoA; pathway within enzyme; similar to what enzyme seen previously

Acetyl-CoA + Bicarbonate + ATP → Malonyl-CoA + ATP + P_i via an enzyme complex consisting of: Biotin carboxylase domain + Biotin carrier protein domain + Transcarboxylase

1. HCO₃^- is attached to biotin via ATP hydrolysis at biotin carboxylase domain

2. Biotin carrier protein domain rotates 180^° to the transcarboxylase domain

3. Acetyl-CoA enters transcarboxylase domain and attaches to bicarb, which is then released from the biotin arm

Acetyl-CoA carboxylase (ACC); functions similarly to pyruvate carboxylase (Exam 2)

Since this is the first step in FA anabolism, it is rate-limiting (Exam 1) and therefore allosterically and covalently regulated

FA synthesis enzyme complex?

Fatty Acid Synthase

1. ACP - Acyl carrier protein (Swings around and brings substrate to each subunit)

2. KR - β-Ketoacyl-ACP reductase

3. ER - Enoyl-ACP reductase

4. DH - β-Hydroxyacyl-ACP dehydrogenase

5. MAT - Malonyl/acetyl-CoA-ACP transferase

6. KS - β-Ketoacyl-ACP synthase

Activation of FA Synthase requires what

1. Acetyl-CoA attaches to KS

2. Malonyl-CoA attaches to ACP

These steps are assisted by MAT

Step 1 of FA synthesis

Condensation (KS)

Produces CO₂

1. Fatty Acid Synthase Complex is charged with an acetyl and a malonyl group to make β-Ketobutyryl-ACP

Step 2 and 3 of FA synthesis

Reduction and Dehydration

2. β-Keto group goes to KR to be reduced to β-Hydroxybutyryl-ACP (Needs NADPH + H⁺)

3. β-Hydroxybutyryl-ACP goes to DH to be dehydrated to trans-Δ²-Butenoyl-ACP

Step 4 of FA synthesis

Reduction

4. trans-Δ²-Butenoyl-ACP goes to ER to be reduced to Butyryl-ACP (Needs NADPH + H⁺)

Step 5 of FA synthesis

Transfer

5. Butyryl-ACP is transferred to KS so the next malonyl-CoA can be loaded (Catalyzed by MAT)

Extension of FA chain

1. After FA chain is transferred to KS, ACP accepts another malonyl-CoA with the help of MAT

2. Condensation occurs again to form β-Keto group that can go through the steps of FA synthesis again

Overall Process of FA synthesis; How is the final product released from the enzyme

Thioesterase releases the finished FA from FA synthase

Notice how addition begins from the back end of the FA

The last two C comes from Acetyl-CoA, the rest is from Malonyl-CoA

Overall steps:

1. Acetyl-CoA attaches to KS

2. Malonyl-CoA attaches to ACP

   • These steps are assisted by MAT (Activation)

3. Fatty Acid Synthase Complex is charged with an acetyl and a malonyl group to make β-Ketobutyryl-ACP (KS)

4. β-Keto group goes to KR to be reduced to β-Hydroxybutyryl-ACP (Needs NADPH + H⁺)

5. β-Hydroxybutyryl-ACP goes to DH to be dehydrated to trans-Δ²-Butenoyl-ACP

6. trans-Δ²-Butenoyl-ACP goes to ER to be reduced to Butyryl-ACP (Needs NADPH + H⁺)

7. Butyryl-ACP is transferred to KS so the next malonyl-CoA can be loaded (Catalyzed by MAT)

8. After FA chain is transferred to KS, ACP accepts another malonyl-CoA with the help of MAT

9. Condensation occurs again to form β-Keto group that can go through the steps of FA synthesis again

10. Thioesterase comes in once FA chain is fully synthesized

Basic requirements of FA synthesis for Palmitate

Palmitate = 16C

1 Acetyl-CoA + 7 Malonyl-CoA + 7 ATP + 14 NADPH

Where does the body get the Acetyl-CoA required for FA synthesis; What processes are required to get this Acetyl-CoA

1. Via PDH complex (Pyruvate → Acetyl-CoA)

2. Acetyl-CoA is converted to Citrate (Citrate synthase + Oxaloacetate) in the mitochondria and transferred out via citrate transporter.

3. In the cytosol, citrate lyase reverses the synthase reaction.

4. Oxaloacetate is recovered either via pyruvate (PDH) or malate (Malate Dehydrogenase)

   • PHD may be preferred as malic enzyme produces reducing agent NADPH + H⁺, which is needed for FA synthesis

Allosteric control + Hormonal Control (FA Synthesis)

Palmitoyl-CoA inhibits acetyl-CoA carboxylase (ACC)

Glucagon, epinephrine, high [AMP] triggers phosphorylation/inactivation (Hormonal)

Acc is activated by citrate

Carnitine acyltransferase I is inhibited by malonyl-CoA (Stops β-oxidation)

Closer look at regulation of β-oxidation and FA synthesis

Hormonal regulation, coupled with β-oxidation being regulated by Malonyl-CoA, prevents futile cycle of coupled β-oxidation and FA synthesis

• Glucagon: Binding to receptor causes PKA activation which will inhibit ACC, activate HSL, and inhibit Pyruvate dehydrogenase (PDH)

  • Decreases FA synthesis and increases stored fat mobilization and gluconeogenesis

• Insulin: Dephosphorylates via phosphoprotein phosphatases which will activate ACC, inhibit HSL, and activate PDH

  • Increases FA synthesis and glycolysis while decreasing stored fat mobilization and gluconeogenesis