Fatty Acid and Cholesterol Biosynthesis

Fatty acids are carboxylic acids with long hydrocarbon chains which serve as vital energy stores and structural components of cell membranes. Their physical and chemical properties are largely determined by the length of their carbon chain and the degree of saturation.

Key Structural Features:

  • Saturated Fatty Acids: These molecules contain no double bonds within the alkyl chain, meaning every carbon atom is fully "saturated" with hydrogen atoms.

  • Unsaturated Fatty Acids: These possess one or more double bonds in the alkyl chain. They are further classified as monounsaturated (one double bond) or polyunsaturated.

    • In naturally occurring unsaturated fatty acids, double bonds typically appear every three carbons and predominantly exist in the cis configuration, which introduces a "kink" in the chain, although trans configurations also exist.

Fatty Acid Synthesis (Lipogenesis)

Fatty acid synthesis primarily occurs in the cytosol of the liver and lactating mammary glands. The process involves the assembly of two-carbon units into a long-chain fatty acid, most commonly the 16-carbon palmitate.

Step 1: Transport of Acetyl CoA to the Cytosol:

  • Since the starting material, Acetyl CoA, is produced in the mitochondria but synthesis occurs in the cytosol, a transport mechanism is required because the mitochondrial membrane is impermeable to Acetyl CoA.

    • Acetyl CoA condenses with oxaloacetate to form citrate, which can be transported out of the mitochondria into the cytosol.

      • Once in the cytosol, citrate is cleaved back into oxaloacetate and Acetyl CoA, making the latter available for synthesis.

Step 2: Formation of Malonyl CoA (The Rate-Limiting Step):

  • The first committed step of lipogenesis is the carboxylation of Acetyl CoA to form malonyl CoA.

    • This reaction is catalysed by the enzyme acetyl CoA carboxylase (ACC).

    • The reaction requires CO2, ATP, and the cofactor biotin.

      • ACC is highly regulated; it is activated by citrate and inhibited by long-chain fatty acyl-CoAs, ensuring synthesis only occurs when energy and precursors are abundant.

Step 3: The Fatty Acid Synthase (FAS) Complex:

  • The actual assembly of the fatty acid chain is performed by the Fatty Acid Synthase multienzyme complex.

    • In eukaryotes, FAS is a large dimer where each monomer contains seven different enzymatic activities.

      • A critical component is the Acyl Carrier Protein (ACP), which contains a 4’-phosphopantetheine group that carries the growing fatty acid chain via a terminal thiol (-SH) group.

The Elongation Cycle:

The synthesis of palmitate involves repeated rounds of four key reactions:

  • Condensation: An acetyl group and a malonyl group are joined, releasing CO2. This decarboxylation drives the reaction forward.

  • Reduction: The keto group at carbon 3 is reduced to an alcohol using NADPH as the electron donor.

  • Dehydration: A molecule of water is removed, creating a double bond between carbons 2 and 3.

  • Reduction: The double bond is reduced to a single bond, again using NADPH, resulting in a saturated acyl chain that is two carbons longer.

This cycle repeats seven times until the 16-carbon palmitoyl-ACP is formed, which is then released as free palmitate by a thioesterase enzyme.

Degradation of Fatty Acids (Beta-Oxidation):

Fatty acids are stored as triacylglycerols (TAGs) in adipose tissue. When energy is needed (e.g., during starvation or stress), these stores are mobilised.

Mobilisation and Transport:

  • Hormones like glucagon or adrenaline activate hormone-sensitive lipase, which hydrolyses TAGs into free fatty acids and glycerol.

    • Fatty acids travel in the blood bound to albumin and enter target tissues like muscle or liver cells.

      • Activation: In the cytosol, fatty acids are activated by being attached to Coenzyme A (CoA) to form fatty acyl-CoA, a process that consumes the equivalent of 2 ATP.

The Carnitine Shuttle:

  • While short-chain fatty acids can enter the mitochondria directly, long-chain fatty acyl-CoAs cannot cross the inner mitochondrial membrane.

    • The acyl group is transferred to carnitine by carnitine acyltransferase 1 (CPT1).

      • The acyl-carnitine is then shuttled into the mitochondrial matrix, where it is converted back into fatty acyl-CoA.

The Beta-Oxidation Cycle:

  • In the mitochondrial matrix, the fatty acyl-CoA undergoes a four-step repeating cycle to remove two carbons at a time as Acetyl CoA:

    • 1. Oxidation: Dehydrogenation by Acyl-CoA dehydrogenase produces a trans double bond and FADH2.

      • 2. Hydration: Water is added across the double bond to form a hydroxyl group.

        • 3. Oxidation: The hydroxyl group is oxidised to a keto group, producing NADH.

          • 4. Thiolytic Cleavage: The bond between C2 and C3 is cleaved by a thiolase, releasing one Acetyl CoA and a shortened fatty acyl-CoA.

Energy Yield:

  • Beta-oxidation is highly efficient. For one molecule of palmitate (16C):

    • 7 rounds of oxidation produce: 7 FADH2, 7 NADH, and 8 Acetyl CoA.

      • When these products enter the Electron Transport Chain and TCA cycle, they yield a net total of 106 ATP (after accounting for the 2 ATP used for activation).

Ketone Body Metabolism:

Ketone bodies (acetoacetate and β-hydroxybutyrate) are water-soluble alternative fuels produced from Acetyl CoA when fatty acid oxidation is high, such as during starvation or uncontrolled diabetes.

Ketogenesis (Synthesis):

Ketogenesis occurs exclusively in the liver mitochondria.

  • Excess Acetyl CoA is converted into acetoacetate, which can then be reduced to β-hydroxybutyrate.

    • Acetone is also produced as a side product; it is volatile and gives a fruity smell to the breath of ketotic individuals.

Ketolysis (Utilisation):

Ketone bodies are released into the blood and used by extrahepatic tissues, most notably the brain, heart, and skeletal muscle.

  • In these tissues, β-hydroxybutyrate is oxidised back to acetoacetate, which is then converted into two molecules of Acetyl CoA to enter the TCA cycle for ATP production.

Ketoacidosis:

  • While ketosis is a normal physiological response to fasting, ketoacidosis is a life-threatening condition where ketone body concentrations rise excessively (up to 20mM in uncontrolled diabetes).

    • Because ketone bodies are acidic, they significantly lower blood pH, which can lead to coma or death if untreated.

Regulation of Fatty Acid Metabolism and Cholesterol Biosynthesis:

Fatty acid metabolism consists of two primary opposing pathways: synthesis (lipogenesis) and degradation (β-oxidation). Their regulation is dictated by the physiological state of the organism.

Regulation of β-Oxidation:

The degradation of fatty acids is regulated at multiple levels, from the mobilisation of fat stores to the entry of fatty acids into the mitochondria.

Mobilisation and Substrate Supply:

  • β-oxidation is largely regulated by the supply of fatty acids, which depends on the rate of lipolysis in adipose tissue.

    • During starvation or stress, hormones like glucagon and adrenaline activate hormone-sensitive lipase, which hydrolyses triacylglycerols (TAGs) into free fatty acids and glycerol.

The Carnitine Shuttle and Malonyl-CoA (Acute Control):

  • The entry of long-chain fatty acids (LCFAs) into the mitochondria is the primary point of acute regulation.

    • LCFAs must be converted to fatty acyl-carnitine by the enzyme Carnitine Palmitoyltransferase I (CPTI) to cross the inner mitochondrial membrane.

      • Reciprocal Regulation: Malonyl-CoA, the first intermediate of fatty acid synthesis, is a potent, rapid, and reversible inhibitor of CPTI.

        • When fatty acid synthesis is active, Malonyl-CoA levels rise and inhibit CPTI, thereby preventing newly synthesised fatty acids from being immediately degraded.

Transcriptional Regulation (Chronic Control):

  • Long-term regulation is achieved through the Peroxisome Proliferator-Activated Receptor alpha (PPARα).

    • Upon activation by fatty acid ligands, PPARα upregulates the expression of numerous genes involved in fatty acid oxidation, including CPTI, CPTII, carnitine translocase, and various mitochondrial dehydrogenases.

Regulation of Fatty Acid Synthesis:

Fatty acid synthesis is primarily regulated at the level of Acetyl-CoA Carboxylase (ACC), which catalyses the conversion of Acetyl-CoA to Malonyl-CoA.

Layers of ACC Regulation:

ACC is regulated through three main mechanisms to ensure it only operates when energy and precursors are abundant:

  • Allosteric Regulation (Polymerisation):

    • Activation by Citrate: High levels of citrate signal that the TCA cycle is "saturated" and energy is available for storage. Citrate causes inactive ACC dimers to polymerise into active filaments.

    • Inhibition by Palmitoyl-CoA: The end-product of the pathway, palmitoyl-CoA (a long-chain fatty acyl-CoA), causes the active polymer to depolymerise back into inactive dimers, providing feedback inhibition.

  • Covalent Modification (Phosphorylation):

    • AMP-activated Protein Kinase (AMPK): When cellular energy is low (high AMP, low ATP), AMPK phosphorylates and inactivates ACC. This effectively switches off energy-consuming fat synthesis and promotes energy-generating β-oxidation.

    • Protein Phosphatase 2A (PP2A): This enzyme dephosphorylates ACC, returning it to its active form.

  • Hormonal Control:

    • Insulin: Promotes the dephosphorylation (activation) of ACC, favouring fat synthesis following a meal.

    • Glucagon/Adrenaline: Promotes the phosphorylation (inactivation) of ACC via secondary messengers, inhibiting fat synthesis during fasting or stress.

Cholesterol Biosynthesis:

Cholesterol is a vital component of cell membranes, providing rigidity, and serves as a precursor for bile acids, steroid hormones, and Vitamin D.

The Synthesis Pathway:

Cholesterol is synthesised mainly in the liver cytosol through a complex pathway requiring 18 molecules of Acetyl-CoA, 36 ATP, and 16 NADPH. It proceeds in three stages:

  1. Synthesis of Isopentenyl Pyrophosphate (IPP): A 5-carbon isoprene unit formed from Acetyl-CoA via mevalonate.

  2. Condensation to Squalene: Six IPP units condense to form the 30-carbon squalene.

  3. Cyclisation to Cholesterol: Squalene undergoes cyclisation to form lanosterol, which is then converted into the 27-carbon cholesterol.

HMG-CoA Reductase: The Rate-Limiting Enzyme:

The conversion of HMG-CoA to mevalonate by HMG-CoA Reductase is the key regulatory step. Its activity is controlled by:

  • Covalent Modification: It is inhibited by phosphorylation.

  • Feedback Inhibition: High levels of free cholesterol reduce the activity and synthesis of the enzyme.

  • Pharmacological Inhibition: Statins are competitive inhibitors of HMG-CoA reductase. They reduce endogenous cholesterol synthesis and upregulate LDL receptors, lowering plasma cholesterol by 20–40%.

Protein Prenylation and Transport:

Protein Prenylation:

This is a post-translational modification where hydrophobic farnesyl (C15) or geranylgeranyl (C20) groups are added to proteins, typically via a thioether linkage to a cysteine residue.

  • This modification allows proteins to interact with cell membranes, which is essential for signal transduction.

    • Key substrates include the Ras and Rho superfamilies of small GTPases and nuclear laminins.

Lipoproteins and Transport:

Cholesterol is transported in the blood within lipoproteins because it has very low water solubility.

  • VLDL (Very Low-Density Lipoprotein): Transports endogenous TAGs and cholesterol from the liver to tissues.

  • LDL (Low-Density Lipoprotein): The primary carrier of cholesterol in the blood. High levels are associated with atherosclerotic plaques, which restrict blood flow.

  • HDL (High-Density Lipoprotein): Involved in "reverse cholesterol transport," moving excess cholesterol from tissues back to the liver.