Easy - S12 Fatty Acid Catabolism

Fats and Triglycerides

• Dietary triglycerides, regardless of plant or animal source, are composed of three fatty acid acyl chains and one glycerol molecule.
• Fats are an important source of energy, providing about one-third of our energy needs.

Advantages of Fats over Polysaccharides

• Fatty acids carry more energy per carbon due to their higher reduction state, resulting in more electrons available for ATP production.
• Fatty acids carry less water along because they are nonpolar.

Storage Locations of Fats

• In vertebrates, fats are stored in specialized cells called adipocytes, mainly located under the skin.
• In plants, fats are stored in seeds.

Fat Biosynthesis and Degradation Pathways

• The pathways for fat biosynthesis and degradation are different and compartmentalized.

Digestion, Mobilization, and Transport of Fats

• Dietary fatty acids are absorbed in the small intestine.
• Fats are transported and absorbed quickly.

Oxidation of Fatty Acids

• Fatty acids are oxidized through β-oxidation, producing energy.
• Mono-unsaturated fatty acids require an isomerase to bypass the first enzyme of β-oxidation.

Regulation of Fatty Acid Metabolism

• High glucose levels inhibit fatty acid breakdown by preventing fatty acid entry into mitochondria.
• Malonyl-CoA, an intermediate of fatty acid synthesis, inhibits carnitine acyltransferase I, blocking fatty acid breakdown.

β-Oxidation in Plants

• Seeds are rich in fatty acids, which are used as an energy source during germination.

Ketone Bodies

• Ketone bodies are formed when acetyl-CoA is converted to acetone, producing energy.
• This occurs in diabetic patients with high glucose levels.

Glycerol from Fats

• Glycerol from fats enters glycolysis through glycerol kinase, which activates glycerol at the expense of ATP.

Fatty Acids Conversion and Activation

• Fatty acids are converted to fatty acyl-CoA in the cytoplasm through a series of reactions.
• Fatty acyl-CoA is then transported into the mitochondria for β-oxidation.

Fatty Acid Transport into the Mitochondria

• Fatty acyl-CoA is converted to fatty acyl carnitine, which is transported into the mitochondria through a series of reactions.
• Only carnitine is transported back out of the mitochondria.

Glycerol from Fats

• Glycerol gets cleaved and enters glycolysis through the action of glycerol kinase, which activates glycerol at the expense of ATP.
• Glycerol is converted into an intermediate of glycolysis (Step 4).

Fatty Acids

• Fatty acids are converted to fatty acyl-CoA in the cytosol.
• Fatty acyl-CoA is then cleaved and undergoes beta oxidation in the mitochondria.
• Palmitic acid (C16) is a dominant carbon chain fatty acid.

Fatty Acid Transport into the Mitochondria

• Fatty acid transport into the mitochondria requires the conversion of fatty acyl-CoA to fatty acyl carnitine.
• The carnitine transporter is an integral protein that replaces CoA with carnitine, allowing the fatty acid to be transported into the mitochondria.
• Only carnitine can transport fatty acids across the mitochondrial membrane.
• Small fatty acids (< 12 carbons) can diffuse freely across the mitochondrial membrane.

Beta Oxidation of Fatty Acids

• Beta oxidation of fatty acids occurs inside the mitochondria.
• Fatty acid oxidation occurs in three stages:
• Stage 1: oxidative conversion of fatty acids into acetyl-CoA, generating NADH.
• Stage 2: oxidation of acetyl-CoA into CO2 via the citric acid cycle, generating NADH and FADH2.
• Stage 3: ATP generation from NADH and FADH2 via the respiratory chain.

Hydrolysis of Fats

• Hydrolysis of triacylglycerols is catalyzed by lipases.
• Hormone-sensitive lipase is regulated by hormones glucagon and epinephrine.
• Glucagon and epinephrine increase lipolysis by releasing the second messenger cAMP, which activates hormone-sensitive lipase.
• Lipase breaks the ester bond, releasing fatty acids and glycerol from stored triacylglycerols.