Lipid Metabolism Overview

Lipid Metabolism

• Lipids are a diverse group of compounds, mainly characterized by their insolubility in water.
• Lipids have crucial roles in energy storage, forming biological membranes, and hormone production.

Biological Functions

• Lipids are hydrophobic, meaning they repel water.
• Stored forms of energy include fatty acids (9 kcal/g), glycerol, and triglycerides.
• Structural components of biological membranes include phospholipids and sterols like cholesterol.
• Lipids are also crucial for signaling and function as hormones (steroid hormones).
• Vitamins (A, D, E, and K) are liposoluble, meaning they are dissolved in lipids.
• Specific lipids are essential for various physiological processes, including eye function and blood clotting.

Classification of Lipids

• Simple lipids cannot be divided into simpler structural units. These include fatty acids, terpenes, sterols (e.g., cholesterol), and eicosanoids.
• Complex lipids can be divided into several structural units. Examples include acylglycerides (mono-, di-, and triglycerides), phosphoacylglycerides, sphingolipids, and waxes.

Fatty Acids

• Fatty acids are carboxylic acids with hydrocarbon chains ranging from 4 to 36 carbons.
• They are stored forms of energy, but poorly soluble in water.
• Classification is based on the presence or absence of double bonds in the hydrocarbon chain. Saturated fatty acids have no double bonds; unsaturated fatty acids have one or more double bonds.

Acylglycerides (Neutral Fats/Triglycerides)

• Formed by the combination of glycerol and fatty acids.
• Categories include monoacylglycerides (1 fatty acid), diacylglycerides (2 fatty acids), and triacylglycerides (3 fatty acids).
• The most common storage form of lipids.
• Nonpolar and hydrophobic, making them insoluble in water.
• Crucial for energy storage.

Sterols (Cholesterol)

• Structural lipids found in cell membranes.
• Amphipathic molecules, possessing both polar and nonpolar components.
• Critical precursors for steroid hormones like estrogen, testosterone, and cortisol.
• Bile acids are crucial for emulsifying dietary fats, aiding digestion and absorption.
• Essential for vitamin D synthesis.

Digestion and Absorption of Lipids

• Bile salts emulsify dietary fats into smaller droplets for increased accessibility to lipid-digesting enzymes.
• Lipases (pancreatic enzymes) break down triglycerides into fatty acids and monoglycerides.
• The resulting products are absorbed by intestinal cells and reformed into triglycerides.
• Chylomicrons transport the newly formed triglycerides to the liver.

Transport of Lipids in the Blood

• Lipids are transported in the blood as lipoproteins, macromolecular complexes of specific carrier proteins and various lipid combinations.
• Lipoproteins have different types (e.g., chylomicrons, VLDL, LDL, HDL) with differing lipid compositions and functions related to target tissues.
• Apoproteins are the protein components of lipoproteins, crucial for directing lipids to the appropriate tissues (cellular targets).

Lipid Storage

• Under conditions of excess, fatty acids are stored as triglycerides in adipose tissue.
• Triglycerides are stored in lipid droplets within adipocytes (fat cells).
• When energy is needed, these stored triglycerides are mobilized and transported to tissues where fatty acids are oxidized for energy production.

Lipid Removal

• Mobilization of stored triglycerides (lipolysis) is triggered by hormones (e.g., epinephrine, glucagon).
• Lipases break down triglycerides into fatty acids and glycerol
• Fatty acids are transported into cells, activated, and transported into mitochondria for oxidation.
• L-carnitine is required for fatty acid transport into the mitochondria.
• Fatty acids are subsequently oxidized to acetyl-CoA, entering the citric acid cycle (Krebs Cycle) to generate ATP (energy).

Fatty Acid Synthesis

• Synthesis of fatty acids from acetyl-CoA, occurs in the cytosol of the liver.
• Fatty acid synthase is the key enzyme involved.
• Fatty acid synthesis is essentially the reverse pathway of fatty acid oxidation. This is often regulated differently to fatty acid breakdown.

Overall Regulation of Lipid Metabolism

• Lipid metabolism is tightly regulated to maintain energy homeostasis.
• Hormone regulation (e.g., insulin, glucagon, epinephrine) is crucial in adjusting between lipolysis (breakdown) and lipogenesis (synthesis).
• Dietary intake of carbohydrates influences fatty acid synthesis.