Lipid Metabolism Overview

Lipid Metabolism

• Lipids are essential for cell structure and function
• Lipids are hydrophobic and nonpolar
• Lipids make up 15-20% of human body weight
• Triglycerides (TAGs) are the most abundant lipid (85-90%), stored in adipose tissue
• Lipids serve as energy reserves and insulation for maintaining body temperature

Why Fat as a Fuel Reserve?

• TAGs are highly concentrated energy stores (9 Cal/g)
• Fatty acids in TAGs are in a reduced form
• TAGs are nonpolar and hydrophobic
• Glycogen (a polar carbohydrate) requires more water for storage (1g glycogen + 2g water)
• Long-chain fatty acids are the ideal storage fuel
• Fats support energy needs during food deprivation

Metabolic Profile of Organs

• Table showing fuel reserves (glucose/glycogen, TAGs, proteins) per organ in a typical 70kg human
  • Blood: 60 kcal glucose/glycogen, 45 kcal TAGs, 0 kcal protein
  • Liver: 400 kcal glucose/glycogen, 450 kcal TAGs, 400 kcal protein
  • Brain: 8 kcal glucose/glycogen, 0 kcal TAGs, 0 kcal protein
  • Muscle: 1200 kcal glucose/glycogen, 450 kcal TAGs, 24,000 kcal protein
  • Adipose Tissue: 80 kcal glucose/glycogen, 135,000 kcal TAGs, 40 kcal protein

Lipid Transport

• Lipids are insoluble
• Lipoproteins (lipids associated with proteins) transport lipids in the bloodstream
• Free lipids are not detectable in the blood because they are not soluble in water

Dynamic State of Body Lipids

• Lipids are not inert storage compounds
• Lipids are continuously degraded and resynthesized
• Adipose tissue stores and releases lipids (energy source) for the body as needed

Fat Mobilization

• Hormone-sensitive TAG lipase breaks down TAGs
• Lipases break down DAG and MAG
• Lipolysis is the complete breakdown of TAGs into glycerol and free fatty acids

Fate of Glycerol and Fatty Acids

• Glycerol can't be phosphorylated in adipose tissue
• Glycerol is transported to the liver and converted
• Free fatty acids enter circulation, bound to albumin, and transported to tissues
• Tissues use fatty acids for energy

Ketone Bodies

• Water-soluble and energy-yielding
• ẞ-hydroxybutyrate is not a true ketone as it does not have a keto group
• Acetone is an exception to ketone metabolic processes
• They are synthesized in the liver
• Enzymes for synthesis in the mitochondrial matrix

Ketogenesis

• Synthesis of ketone bodies occurs in the liver
• Acetyl CoA, fatty acids, and amino acids create ketones
• Ketone bodies are used as an alternative energy source for cells when glucose is insufficient

Utilization of Ketone Bodies

• Water-soluble (easily transported)
• Acetoacetate and β-hydroxybutyrate serve as an energy source to peripheral tissues (muscle, cardiac muscle, and renal cortex)
• Tissues lacking mitochondria cannot use ketone bodies
• Ketone bodies are important fuel in starvation and diabetes mellitus.

Reactions of Ketone Bodies

• Reversal of synthesis: β-hydroxybutyrate is converted into acetoacetate by β-hydroxybutyrate dehydrogenase
• Acetoacetate is activated to acetoacetyl CoA, then cleaved by thiolase into acetyl CoA
• Thiophorase is absent in the liver

Starvation

• Increased fatty acid degradation to meet the body's energy needs
• Overproduction of acetyl CoA when TCA cycle is impaired by oxaloacetate deficiency
• Excess acetyl CoA and its diversion to ketone body overproduction

Diabetes Mellitus

• Associated with insulin deficiency
• Increased lipolysis
• Accumulation of acetyl CoA leading to ketone body formation
• High ketone body concentration in the blood and urine
• Severely high levels of ketone bodies can be fatal

Biosynthesis of Fatty Acids

• Excess carbohydrates and amino acids are converted to fatty acids
• De novo fatty acid synthesis predominantly occurs in the liver, kidney, adipose tissue, and lactating mammary glands
• Enzyme machinery in the cytosol

Explore the fundamental concepts of lipid metabolism, including the roles and types of lipids in the human body. Understand how triglycerides serve as concentrated energy reserves and learn about the metabolic profiles of different organs. This quiz will enhance your knowledge of how lipids contribute to cellular structure and energy storage.