Fatty Acid Catabolism Overview

Fatty Acid Catabolism Learning Objectives

• Fatty acid catabolism is essential for energy production.
• β-oxidation is a crucial step in fatty acid breakdown. It involves enzymes, metabolites, chemical changes, ATP utilization, and ATP generation.
• Unsaturated and "odd" fatty acids also undergo oxidation, leading to a specific yield of ATP.
• Ketone body formation is important, although its significance is context-dependent.
• Plants can convert lipids into carbohydrates, but this is not possible in humans.

Fatty Acid Catabolism - Energy Source

• Fatty acids are a significant energy source.
• They are stored as triacylglycerols.
• Fatty acid oxidation releases substantial energy, exemplified by palmitic acid (C16 saturated fatty acid).
• The oxidation of palmitic acid yields -2340 kcal/mol and is coupled to the formation of 106 ATPs.
• Fatty acid oxidation creates reduced electron carriers and acetyl-CoAs. This process fuels activities in the heart and liver.

Fatty Acid Catabolism - Digestion and Transport

• Bile salts emulsify dietary fats.
• Intestinal lipases break down triacylglycerols, and fatty acids, glycerol, and other products are absorbed.
• These components are converted to triacylglycerols inside intestinal cells.
• In the cytoplasm, triacylglycerols are combined with cholesterol and apolipoproteins to form chylomicrons.
• Chylomicrons travel through the lymphatic system and bloodstream to tissues.
• Lipoprotein lipase, activated by apoC-II, converts triacylglycerols into fatty acids and glycerol.

Degradation of Triacylglycerol

• Triacylglycerols are broken down into glycerol and fatty acids by the enzyme lipase.

Glycerol to Glycolytic Metabolites

• Glycerol is converted to glycolytic metabolites through a series of enzymatic reactions, including phosphorylation and dehydrogenation, ultimately leading to the production of dihydroxyacetone phosphate (DHAP), which can enter glycolysis.

Fatty Acid Activation

• Fatty acids must be converted to acyl-CoA derivatives to become substrates for β-oxidation in mitochondria.
• Enzyme: acyl-CoA synthetase
• This process requires ATP and produces AMP and pyrophosphate

Transport of Fatty Acyl CoA into Mitochondria

• Fatty acyl-CoA cannot directly traverse the inner mitochondrial membrane.
• Carnitine is required to transport fatty acyl chains across the membrane.

β-Oxidation of Saturated Fatty Acids - Spiral Pathway

• A series of sequential reactions (β-oxidation) progressively shortens the fatty acyl chain.
• The end product of each cycle is acetyl-CoA.

β-Oxidation Steps

• Oxidation: removal of hydrogens to produce FADH2
• Hydration—addition of water
• Oxidation: removal of hydrogens to form NADH
• Thiolysis—addition of coenzyme A.

ATP Formation from β-Oxidation of C16:0

• Each cycle of β-oxidation generates 1 FADH2 and 1 NADH.
• Palmitic acid (C16) requires 7 β-oxidation cycles to produce 8 acetyl-CoA molecules.
• Total ATP from β-oxidation of palmitic acid is about 106.

Oxidation of "Odd" Fatty Acids

• Odd-numbered fatty acids produce propionyl-CoA.
• Propionyl-CoA is further processed to succinyl-CoA.

How Succinyl-CoA is Oxidized

• Succinyl-CoA is an intermediate in the citric acid cycle.
• Succinyl-CoA undergoes multiple steps to release its energy and be fully oxidized.

Formation of Ketone Bodies

• Ketone bodies are generated when acetyl-CoA from fatty acid oxidation are in excess and the citric acid cycle intermediates are reduced by gluconeogenesis.
• Ketone bodies are critical fuel sources in metabolic conditions like starvation.

Ketone Body Formation and Export from the Liver

• Ketone bodies are synthesized in the liver.
• They are exported to other tissues.
• They are crucial energy sources in times of low glucose.

Utilization of Ketone Bodies

• Non-hepatic tissues (brain, heart, kidney, skeletal muscle) utilize ketone bodies as fuel.

β-Oxidation and Glyoxylate Cycle in Plant Seeds

• Plant seeds process fatty acids for energy and synthesis of carbohydrates.
• The glyoxylate cycle helps plants convert fatty acids into carbohydrates in specific metabolic circumstances.

The Glyoxylate Cycle

• The glyoxylate cycle is a unique metabolic pathway in plants.
• It enables the conversion of acetyl-CoA into succinate.
• The cycle bypasses the CO2-producing steps of the citric acid cycle.

Conversion of Lipids to Carbohydrates in Plants

• Plants can utilize the glyoxylate cycle to convert lipids into carbohydrates.
• This process is significant for seed development and germination.

This quiz covers the essential concepts of fatty acid catabolism, focusing on β-oxidation and its role in energy production. You'll explore different types of fatty acids, their oxidation processes, and the significance of ketone body formation. Understand how fatty acids serve as a vital energy source and their biochemical implications.