Beta oxidation of fatty acids-Lecture-2 (Activation and transportation of fatty acids)

Overview

• β-oxidation takes place in mitochondrion.
• Tissues in which β-Oxidation is carried out are liver, heart, kidney, muscle, brain, lungs, testes and adipose tissue. In cardiac muscle, 80% of energy derived from FA oxidation.
• A saturated acyl Co-A is degraded by a recurring sequence of four reactions:
  • Oxidation by flavin adenine dinucleotide (FAD)
  • Hydration,
  • Oxidation by NAD⁺, and
  • Thiolysis by Co A

The fatty acyl chain is shortened by two carbon atoms as a result of these reactions, and FADH2, NADH, and acetyl Co A are generated. Because oxidation is on the β carbon and the chain is broken between the α (2)- and β (3)-carbon atoms—hence the name – β oxidation.

Mechanism of fatty acid oxidation

1. Activation of fatty acid

Fatty acids must first be converted to an active intermediate before they can be catabolized. This is the only step in the complete degradation of a fatty acid that requires energy from ATP.  The activation of fatty acid is accomplished in two steps-

Figure-3- In the presence of ATP and coenzyme A, the enzyme acyl-CoA synthetase (thiokinase) catalyzes the conversion of fatty acid (or free fatty acid) to an “active fatty acid” or acyl-CoA, which uses one high-energy phosphate with the formation of AMP and PP_i . The PP_i is hydrolyzed by inorganic pyrophosphatase with the loss of a further high-energy phosphate, ensuring that the overall reaction goes to completion.

Acyl-CoA synthetases are found in the endoplasmic reticulum, peroxisomes, and on the inside of the outer membrane of mitochondria.

Transport of fatty acid into the mitochondrial matrix

• Fatty acids are activated on the outer mitochondrial membrane, whereas they are oxidized in the mitochondrial matrix.
• Activated long-chain fatty acids are transported across the membrane through “carnitine shuttle”.

Components of the carnitine shuttle

• Carnitine
• Carnitine Acyltransferase I
• Translocase
• Carnitine Acyl transferase-II

Carnitine

Carnitine (ß-hydroxy-Υ-trimethyl ammonium butyrate), (CH₃)₃N⁺—CH₂—CH(OH)—CH₂—COO^–, is widely distributed and is particularly abundant in muscle. Carnitine is obtained from foods, particularly animal-based foods, and via endogenous synthesis.

Carnitine acyltransferase I

The acyl group is transferred to the hydroxyl group of carnitine to form acylcarnitine. This reaction is catalyzed by carnitine acyltransferase I.

Translocase

Acyl carnitine is shuttled across the inner mitochondrial membrane by a translocase.

Carnitine acyltransferase II

The acyl group is transferred back to CoA on the matrix side of the membrane. This reaction is catalyzed by carnitine acyltransferase II. Finally, the translocase returns carnitine to the cytosolic side in exchange for an incoming acylcarnitine (figure 3)

Figure-3- Role of carnitine in the transport of long-chain fatty acids through the inner mitochondrial membrane. Long-chain acyl-CoA cannot pass through the inner mitochondrial membrane, but its metabolic product, acylcarnitine, can cross through with the help of a carnitine shuttle. (AS- Acyl co A synthetase; CPTI-Carnitine palmitoyltransferase I; CT-Carnitine Translocase; CPTII- Carnitine palmitoyltransferase II).

Significance of fatty acid transport through carnitine shuttle

1) Biological significance- This counter-transport system provides regulation of the uptake of fatty acids into the mitochondrion for oxidation. As long as there is free CoA available in the mitochondrial matrix, fatty acids can be taken up and the carnitine returned to the outer membrane for the uptake of more fatty acids. However, if most of the CoA in the mitochondrion is acylated, then the fatty acid uptake is inhibited.

This carnitine shuttle also serves to prevent uptake into the mitochondrion (and hence oxidation) of fatty acids synthesized in the cytosol in the fed state; malonyl CoA (the precursor for fatty acid synthesis) is a potent inhibitor of carnitine palmitoyltransferase I in the outer mitochondrial membrane.

Short and medium-chain fatty acids do not require carnitine for their transportation across the inner mitochondrial membrane.

2) Clinical significance

Carnitine deficiency

Causes of carnitine deficiency include the following:

• Inadequate intake (e.g., due to fad diets, lack of access, or long-term TPN)
• Inability to use carnitine due to enzyme deficiencies (e.g., carnitine palmitoyl Transferase deficiency)
• Decreased endogenous synthesis of carnitine due to a severe liver disorder
• Excess loss of carnitine due to diarrhea, diuresis, or hemodialysis
• A hereditary disorder in which carnitine leaks from renal tubules (Primary carnitine deficiency)
• Increased requirements for carnitine when ketosis is present or demand for fat oxidation is high (eg, during a critical illness such as sepsis or major burns; after major surgery of the GI tract)

Clinical manifestations

 Symptoms and the age at which symptoms appear depend on the cause.

Carnitine deficiency may cause-

• muscle aches and fatigue
• muscle necrosis, myoglobinuria
• fasting hypoketotic hypoglycemia
• fatty liver and hyperammonemia,
• cardiomyopathy, heart failure
• coma, and sudden unexpected death 

Hypoglycemia in carnitine deficiency is a consequence of impaired fatty acid oxidation with the resultant imbalance between demand and supply of glucose which is the sole of the source of energy in such individuals.

Deficiencies in the carnitine acyl Transferase enzymes I and II can cause similar symptoms. Inherited CAT-I deficiency affects only the liver, resulting in reduced fatty acid oxidation and ketogenesis, with hypoglycemia. CAT-II deficiency affects primarily skeletal muscle and, when severe, the liver.

Diagnosis

• Extremely reduced carnitine levels in plasma and muscle(1–2% of normal).
• Fasting ketogenesis may be normal if liver carnitine transport is normal, but it may be impaired if dietary carnitine intake is interrupted and there is associated liver disorder.
• The fasting urinary organic acid profile may show a hypoketotic dicarboxylic aciduria pattern if hepatic fatty acid oxidation is impaired, but it is otherwise unremarkable.

Treatment

• Treatment of this disorder with pharmacologic doses of oral carnitine is highly effective in correcting the cardiomyopathy and muscle weakness as well as any impairment in fasting ketogenesis.
• All patients must avoid fasting and strenuous exercise.
• Some patients require supplementation with medium-chain triglycerides and essential fatty acids (e.g., Linoleic acid, Linolenic acid).
• Patients with a fatty acid oxidation disorder require a high-carbohydrate, low-fat diet.