9.2 Fatty acid synthesis and degradation

Fatty acids are central to how your body stores and mobilizes energy. The pathways that build fatty acids (synthesis) and break them down (degradation) are tightly coordinated, and understanding both is essential for grasping lipid metabolism as a whole. This section covers the enzymatic machinery of fatty acid synthesis, the steps of beta-oxidation, the carnitine shuttle, lipolysis, and ketone body metabolism.

Fatty Acid Synthesis

Fatty Acid Synthase Complex

Fatty acid synthase (FAS) is a large, multi-enzyme complex that builds fatty acids from simple two-carbon precursors. Its primary product is palmitate, a 16-carbon saturated fatty acid.

FAS exists as a homodimer, with each monomer containing seven distinct enzymatic activities plus an acyl carrier protein (ACP). The ACP is modified post-translationally with a phosphopantetheine group, which acts as a flexible tether that swings the growing fatty acid chain between active sites on the complex.

The synthesis cycle works like this:

1. An acetyl group from acetyl-CoA is loaded onto the ACP (this is the "starter" unit).
2. A malonyl group from malonyl-CoA is loaded onto the ACP.
3. The malonyl group undergoes decarboxylation, and the released energy drives a carbon-carbon bond formation, extending the chain by two carbons.
4. The elongated chain undergoes reduction, dehydration, and a second reduction to produce a fully saturated intermediate.
5. Steps 2-4 repeat for a total of seven cycles, adding two carbons each time to the original two-carbon acetyl starter, yielding the 16-carbon palmitate.

Each elongation cycle consumes 2 NADPH as the reducing agent. The overall stoichiometry for palmitate synthesis is:

$\text{Acetyl-CoA} + 7\,\text{Malonyl-CoA} + 14\,\text{NADPH} + 14\,\text{H}^+ \rightarrow \text{Palmitate} + 7\,\text{CO}_2 + 8\,\text{CoA} + 14\,\text{NADP}^+ + 6\,\text{H}_2\text{O}$

Acetyl-CoA Carboxylase and Malonyl-CoA Formation

Acetyl-CoA carboxylase (ACC) catalyzes the rate-limiting and committed step of fatty acid synthesis: the irreversible carboxylation of acetyl-CoA to form malonyl-CoA. This reaction requires biotin as a coenzyme and consumes one ATP.

$\text{Acetyl-CoA} + \text{CO}_2 + \text{ATP} \xrightarrow{\text{ACC}} \text{Malonyl-CoA} + \text{ADP} + \text{P}_i$

Malonyl-CoA provides the two-carbon units for chain elongation (the third carbon from the carboxylation is lost as $\text{CO}_2$ during the condensation step on FAS, which is what makes the condensation thermodynamically favorable).

ACC is one of the most heavily regulated enzymes in metabolism:

• Allosteric activation by citrate (signals abundant building blocks)
• Allosteric inhibition by long-chain fatty acyl-CoA (product feedback)
• Hormonal regulation: insulin stimulates ACC (via dephosphorylation), while glucagon and epinephrine inhibit it (via phosphorylation by AMP-activated protein kinase, AMPK)
• Phosphorylation inactivates ACC; dephosphorylation activates it

Elongation and Desaturation of Fatty Acids

Palmitate released from FAS can be further modified in the endoplasmic reticulum (ER).

Elongation: Elongase enzymes in the ER membrane add two-carbon units (from malonyl-CoA) to the carboxyl end of the fatty acid chain. This produces longer-chain fatty acids of 18, 20, 22, or even 24 carbons.

Desaturation: Desaturase enzymes introduce cis double bonds at specific positions in the chain. These enzymes require molecular oxygen ($\text{O}_2$) and NADH or NADPH as electron donors. In humans, the available desaturases can introduce double bonds at the Δ9, Δ6, and Δ5 positions (numbered from the carboxyl end). For example:

• Stearoyl-CoA desaturase (SCD1) introduces a Δ9 double bond, converting stearate (18:0) to oleate (18:1)
• FADS2 introduces a Δ6 double bond
• FADS1 introduces a Δ5 double bond

Humans lack Δ12 and Δ15 desaturases, which is why linoleic acid (18:2, omega-6) and α-linolenic acid (18:3, omega-3) are classified as essential fatty acids. They must come from the diet.

Fatty Acid Degradation

Beta-Oxidation Pathway

Beta-oxidation is the major catabolic pathway for fatty acids. It takes place in the mitochondrial matrix and systematically shortens fatty acyl chains by removing two-carbon units as acetyl-CoA.

Before entering beta-oxidation, a fatty acid must first be activated in the cytosol. Acyl-CoA synthetase catalyzes the ATP-dependent formation of a fatty acyl-CoA thioester (this costs the equivalent of 2 ATP, since ATP is cleaved to AMP + PPi, and PPi is hydrolyzed).

Each cycle of beta-oxidation involves four steps:

1. Oxidation by acyl-CoA dehydrogenase (FAD-dependent), which introduces a trans-Δ2 double bond, producing $\text{FADH}_2$
2. Hydration by enoyl-CoA hydratase, which adds water across the double bond to form a 3-hydroxyacyl-CoA
3. Oxidation by 3-hydroxyacyl-CoA dehydrogenase ($\text{NAD}^+$-dependent), which oxidizes the hydroxyl group to a ketone, producing $\text{NADH}$
4. Thiolysis by β-ketoacyl-CoA thiolase, which cleaves the bond between the α and β carbons using a free CoA, releasing one acetyl-CoA

Per cycle yield: 1 acetyl-CoA, 1 $\text{FADH}_2$, 1 $\text{NADH}$

The shortened fatty acyl-CoA re-enters the cycle. For a 16-carbon palmitoyl-CoA, you get 7 rounds of beta-oxidation, producing 8 acetyl-CoA, 7 $\text{FADH}_2$, and 7 $\text{NADH}$.

Acetyl-CoA then enters the citric acid cycle for further oxidation, or it can be diverted to ketone body synthesis in the liver.

Odd-chain fatty acids are a special case. The final round of beta-oxidation produces propionyl-CoA (3 carbons) instead of acetyl-CoA. Propionyl-CoA is converted to succinyl-CoA (a citric acid cycle intermediate) through a three-step pathway requiring biotin and vitamin B12 as cofactors.

Carnitine Shuttle System

Long-chain fatty acyl-CoA molecules cannot cross the inner mitochondrial membrane on their own. The carnitine shuttle solves this transport problem.

1. CPT I (carnitine palmitoyltransferase I), on the outer mitochondrial membrane, transfers the fatty acyl group from CoA to carnitine, forming fatty acyl-carnitine.
2. CACT (carnitine-acylcarnitine translocase) carries fatty acyl-carnitine across the inner mitochondrial membrane into the matrix.
3. CPT II (carnitine palmitoyltransferase II), on the matrix side of the inner membrane, transfers the fatty acyl group back to a mitochondrial CoA, regenerating fatty acyl-CoA and releasing free carnitine.
4. Free carnitine is shuttled back to the cytosol by CACT, ready for another round.

A critical regulatory point: CPT I is inhibited by malonyl-CoA. Since malonyl-CoA is produced during fatty acid synthesis, this ensures that synthesis and degradation don't run simultaneously in the same cell. When you're building fatty acids, malonyl-CoA levels are high, and CPT I is shut down, preventing a futile cycle.

Lipolysis and Mobilization of Stored Triacylglycerols

Before fatty acids can be oxidized, they need to be released from storage. Lipolysis is the hydrolysis of triacylglycerols (TAGs) stored in adipose tissue, releasing free fatty acids and glycerol.

Three lipases work sequentially to fully hydrolyze a TAG:

• Adipose triglyceride lipase (ATGL) removes the first fatty acid, producing a diacylglycerol
• Hormone-sensitive lipase (HSL) removes the second fatty acid, producing a monoacylglycerol
• Monoacylglycerol lipase (MGL) removes the third fatty acid, releasing glycerol

HSL is the most tightly regulated of the three. Hormonal control works through the cAMP signaling cascade:

• Glucagon, epinephrine, and norepinephrine bind their receptors, activate adenylyl cyclase, raise cAMP levels, and activate protein kinase A (PKA). PKA phosphorylates and activates HSL, stimulating lipolysis.
• Insulin opposes this by activating a phosphodiesterase that degrades cAMP, keeping HSL in its inactive, dephosphorylated state.

Released free fatty acids travel through the bloodstream bound to serum albumin and are taken up by tissues like muscle and liver for beta-oxidation.

Ketone Body Metabolism

Synthesis and Utilization of Ketone Bodies

Ketone bodies are water-soluble fuel molecules produced in the liver when acetyl-CoA accumulates faster than the citric acid cycle can consume it. The three ketone bodies are acetoacetate, β-hydroxybutyrate, and acetone.

This happens during fasting, prolonged exercise, or uncontrolled diabetes mellitus, all situations where fatty acid oxidation is high but oxaloacetate (needed to feed acetyl-CoA into the citric acid cycle) is depleted or diverted to gluconeogenesis.

Ketone body synthesis (ketogenesis) occurs in liver mitochondria:

1. Thiolase condenses two acetyl-CoA molecules to form acetoacetyl-CoA.
2. HMG-CoA synthase adds a third acetyl-CoA to form β-hydroxy-β-methylglutaryl-CoA (HMG-CoA).
3. HMG-CoA lyase cleaves HMG-CoA to release acetoacetate and acetyl-CoA.
4. Acetoacetate can then be reduced to β-hydroxybutyrate by β-hydroxybutyrate dehydrogenase (NADH-dependent), or it can spontaneously decarboxylate to acetone (which is exhaled and largely a metabolic dead end).

Ketone body utilization (ketolysis) occurs in extrahepatic tissues such as the brain, heart, and skeletal muscle:

1. β-Hydroxybutyrate is oxidized back to acetoacetate (by β-hydroxybutyrate dehydrogenase, generating NADH).
2. Acetoacetate is activated to acetoacetyl-CoA by succinyl-CoA:3-ketoacid CoA transferase (SCOT). This enzyme transfers a CoA from succinyl-CoA to acetoacetate.
3. Thiolase cleaves acetoacetyl-CoA into two acetyl-CoA molecules, which enter the citric acid cycle.

The liver cannot use ketone bodies for fuel because it lacks SCOT. This makes sense: the liver produces ketone bodies as an export fuel for other tissues.

The brain normally depends on glucose, but during prolonged fasting (after several days), it adapts to derive a significant fraction of its energy from ketone bodies, since they can cross the blood-brain barrier. This adaptation reduces the body's need for gluconeogenesis and helps spare muscle protein.