Metabolism of Fatty Acids Flashcards

Structure and Component Chemistry of Co-enzyme A (CoA)

• Definition and Abbreviation: Co-enzyme A is a complex molecule essential for fatty acid metabolism, often abbreviated as CoA or CoA-SH to emphasize its functional sulfhydryl group.
• Constituent Molecules: The structure of CoA (as shown in Figure 11.7) consists of:
  • Beta-mercapto ethanolamine: Contains the functional SH group ($HS-CH_2-CH_2-NH$) that forms thioester bonds with acyl groups.
  • Beta-alanine: ($CO-CH_2-CH_2-NH$).
  • Pantoic acid: ($CO-CHOH-C(CH_3)_2-CH_2$). Together with beta-alanine, pantoic acid constitutes the B-complex vitamin pantothenic acid.
  • ADP (Adenosine Diphosphate): Specifically composed of $O-P-O-P-O-Ribose-Adenine$.
• Thioester Bond: The terminal SH (sulfhydryl) group of the beta-mercapto ethanolamine moiety is where the acyl groups attach via a thioester bond to form Acyl CoA.

Overview of Beta Oxidation of Fatty Acids

• Nomenclature: The process is termed "beta oxidation" because the chemical oxidation and subsequent splitting of two-carbon units occur at the beta-carbon atom of the fatty acid chain.
• Mechanism: The hydrocarbon chain undergoes sequential cleavage of two carbon atoms at a time.
• Historical Context: This mechanism was first proposed by Fray Knoop in 1904.
• Lipid Transport and Cellular Uptake:
  • Free fatty acids (FFA) are transported in the blood complexed with albumin.
  • Cells take up these free fatty acids from the blood to be oxidized for energy production.

Preparative Steps for Beta Oxidation

Preparative Step 1: Activation of Fatty Acids

• Location: Occurs in the cytoplasm.
• Chemical Reaction: Fatty acids are converted into their co-enzyme A (CoA) derivatives.
• Energy Requirement: ATP is hydrolyzed to AMP and inorganic pyrophosphate ($PPi$). The subsequent hydrolysis of $PPi$ drives the reaction forward. Effectively, two high-energy bonds are utilized in this activation.
• Enzymes involved:
  • Thiokinase (also known as Fatty acyl CoA synthetase).
  • Three distinct enzymes have been identified based on chain length: one for short-chain, one for medium-chain, and one for long-chain fatty acids.
  • Thiophorase: Small chain fatty acids may also be activated by this enzyme using succinyl CoA.
• Distinction Between Acetyl and Acyl Groups (Box 11.3):
  • Acetyl CoA: The combination of acetate/acetic acid (a 2-carbon unit) with Co-enzyme A.
  • Acyl CoA: The combination of any fatty acid (ranging from $C_4$ to $C_{26}$ in length) with Co-enzyme A.

Preparative Step 2: Role of Carnitine

• The Transport Problem: While fatty acids are activated in the cytoplasm, beta oxidation occurs in the mitochondria. Long-chain fatty acyl CoA molecules cannot pass through the inner mitochondrial membrane autonomously.
• Carnitine Definition: Chemically known as beta-hydroxy-gamma-trimethyl ammonium butyrate, with the formula $(CH_3)_3-N-CH_2-CHOH-CH_2-COOH$.
• Synthesis: Carnitine is synthesized from the amino acids lysine and methionine within the liver and kidney.

Preparative Step 3: Carnitine Acyl Transferase-I (CAT-I)

• Function: CAT-I transfers the fatty acyl group to the hydroxyl group of carnitine, forming acyl carnitine.
• Location: This reaction takes place on the cytosolic side of the inner mitochondrial membrane.

Preparative Step 4: Translocase and CAT-II

• Translocase: A protein carrier that moves the acyl carnitine across the inner mitochondrial membrane into the mitochondrial matrix.
• Carnitine Acyl Transferase-II (CAT-II): Once inside the matrix, CAT-II transfers the acyl group from carnitine back to a molecule of co-enzyme A.
• Recycling: The translocase then returns the free carnitine to the cytosolic side.

The Four Metabolic Steps of the Beta Oxidation Cycle

Once in the mitochondrial matrix, fatty acyl CoA undergoes four sequential reactions. Each cycle removes one two-carbon unit as acetyl CoA.

Step 1: FAD Linked Dehydrogenase

• Enzyme: Acyl CoA dehydrogenase.
• Action: Fatty acyl CoA is dehydrogenated to form trans enoyl CoA (specifically ̑̑-̑̑̑̑̑̑̑̑-unsaturated fatty acyl CoA).
• Electron Acceptor: FAD accepts the hydrogen atoms to become $FADH_2$.
• ATP Yield: When $FADH_2$ is oxidized in the electron transport chain (ETC), it produces $1.5$ ATP molecules.

Step 2: Hydration

• Enzyme: Enoyl CoA hydratase.
• Action: Water is added across the double bond to form beta-hydroxy fatty acyl CoA.
• Stereospecificity: Only the L-isomer is formed during the hydration of the trans double bond.

Step 3: NAD+ Dependent Dehydrogenase

• Enzyme: Beta-hydroxy fatty acyl CoA dehydrogenase.
• Action: The beta-hydroxy fatty acyl CoA is oxidized to form beta-keto fatty acyl CoA.
• Electron Acceptor: NAD+ is reduced to $NADH + H^+$. This enzyme is specific to the L-isomer.
• ATP Yield: When NADH is oxidized in the ETC, it generates $2.5$ ATP molecules.

Step 4: Cleavage (Thiolysis)

• Enzyme: Thiolase.
• Action: The beta-keto fatty acyl CoA is split by a molecule of CoA-SH (thiolytic cleavage).
• Products: One molecule of acetyl CoA and a new fatty acyl CoA that is shorter by two carbon atoms.

Energetics of Palmitic Acid (16 C) Oxidation

• Cycles Required: For a 16-carbon palmitic acid, $7$ cycles of beta oxidation are required (as illustrated in Figure 11.10).
• Total Products:
  • $8$ molecules of Acetyl CoA.
  • $7$ molecules of $FADH_2$.
  • $7$ molecules of NADH.
• ATP Calculation:
  • 8  \text{Acetyl CoA} \times 10  \text{ATP} = 80  \text{ATP}
  • 7  FADH_2 \times 1.5  \text{ATP} = 10.5  \text{ATP}
  • 7  \text{NADH} \times 2.5  \text{ATP} = 17.5  \text{ATP}
  • Gross Total: 108  \text{ATP}
  • Initial Activation Requirement: -2  \text{ATP} (equivalents of two high-energy bonds used by thiokinase).
  • Net Yield: 106  \text{ATP}.
• Efficiency: The efficiency of beta oxidation is approximately $33\%$.
• Revised ATP Estimations: Previous textbook editions used over-estimates (NADH = 3 ATP, FADH2 = 2 ATP), resulting in a net calculation of 129  \text{ATP}. Modern experiments indicate the net generation is actually 106  \text{ATP}.

Metabolism of Propionyl CoA

When odd-chain fatty acids are oxidized, the final three-carbon unit remains as Propionyl CoA. It is metabolized as follows (Fig 11.11):

1. Step 1: Propionyl CoA + $CO_2$ is catalyzed by Propionyl CoA carboxylase (utilizing Biotin and ATP) to form D-methyl malonyl CoA.
2. Step 2: Racemase converts D-methyl malonyl CoA to L-methyl malonyl CoA.
3. Step 3: Mutase (utilizing Adenosyl B12) converts L-methyl malonyl CoA to Succinyl CoA, which can then enter the TCA cycle.

Regulation and Clinical Applications

Regulation Factors

1. Free Fatty Acid Availability: The overall rate is regulated by the availability of FFA.
2. Hormonal Control: The glucagon:insulin ratio determines FFA levels. Glucagon increases FFA levels, while insulin decreases them.
3. Malonyl CoA inhibition: Malonyl CoA, produced during de novo fatty acid synthesis, inhibits CAT-I. This prevents simultaneous fatty acid synthesis and oxidation.

Clinical Applications and Deficiencies

1. Chain Length Variability: Medium-chain and short-chain fatty acids do not require carnitine for mitochondrial entry and are thus oxidized more easily.
2. Carnitine Deficiency: Often reported in preterm infants, leading to impaired oxidation and hypoglycemia as the body over-utilizes glucose.
3. Translocase Deficiency: Leads to defective metabolism of long-chain fatty acids. Symptoms include muscle cramps precipitated by fasting, exercise, or high-fat diets.
4. CPT-I and CPT-II Deficiencies:
   • CPT-I (Inherited): Affects the liver, resulting in reduced oxidation, reduced ketogenesis, and hypoglycemia.
   • CPT-II: Primarily affects skeletal muscle; if severe, it affects the liver.
   • Pharmacology: Sulfonylurea drugs (e.g., glibenclamide and tolbutamide) used for Type 2 Diabetes treatment inhibit CPT-I to reduce fatty acid oxidation and mitigate hyperglycemia.

Organic Acidurias

• Description: Disorders involving the metabolism of fatty acids, branched-chain/aromatic amino acids, or the citric acid cycle. They are characterized by organic acid accumulation in tissues and excretion in urine.
• MCAD Deficiency: "Medium chain acyl CoA dehydrogenase deficiency" affects approximately 1 in 2,500 live births (WHO, 2003). It is the second most common inborn error of metabolism.
• Symptoms: Acidosis, vomiting, convulsions, and coma. Often fatal in infancy; survivors face severe mental and physical retardation.
• Management: Dietary restriction, cofactor therapy, and substrate removal.