Biochemistry: Introduction to Lipids

Lipids: Introduction

• Lipids provide 9 calories per gram.
• They are important constituents of cell membranes.
• Lipids aid in the absorption of fat-soluble vitamins (A, D, E, and K).
• Cholesterol is a precursor for vitamin D synthesis.
• Lipids are a crucial component of nervous tissue.
• They act as a thermal insulator in subcutaneous tissues.
• Lipids maintain cellular integrity.
• Lipids act as metabolic regulators (steroid hormones) and emulsifying agents (amphipathic lipids).

Lipids Vs. Carbohydrates

• Lipids, specifically triacylglycerol (TAG), are chosen as a fuel reserve over carbohydrates (glycogen) during growth.
• TAG stores allow survival for several weeks.
• Glycogen sustains physiological function for about 24 hours.

Fatty Acids

• Fatty acids are the building blocks of lipids.
• They are composed of a carboxylic group (-COOH) and a hydrocarbon chain (-R).

Classification of Fatty Acids

• Fatty acids are categorized into:
  • Saturated fatty acids (with only single bonds).
  • Unsaturated fatty acids (with at least one double bond).
    • Further classified into mono- and polyunsaturated fatty acids based on the number of double bonds.

Double Bonds in Fatty Acids

• The position of a double bond is indicated by Δn, where n represents the lower-numbered carbon in the double-bonded pair.
• Double bonds in polyunsaturated fatty acids are separated by a CH2 (methylene) group, indicating they are not conjugated.
• The double bonds in fatty acids have a cis- configuration (same direction).

Melting Point of Fatty Acids

• The length of the hydrocarbon chain and the degree of unsaturation affect the melting point of fatty acids.
  • As the number of carbons increases, the melting point increases (solid at room temperature).
  • As the number of double bonds increases, the melting point decreases (liquid at room temperature).

Digestion of Lipids - Stage 1: Mouth

• Lingual lipase, present in infants, begins digestion in the stomach as it is an acid-stable enzyme.

Digestion of Lipids - Stage 2: Stomach

• Gastric lipase is secreted by the stomach.
• Lipids require a slightly acidic pH (pH 6) for digestion.
• Gastric lipase hydrolyzes TAG molecules, primarily those found in milk fat.
• This is particularly important for infants, as milk fat is their primary calorie source.
• Note: Stomach pH in adults ranges from 1.5 to 2.5, while it is 4 to 6 in infants.

Digestion of Lipids - Stage 3: Small Intestine

• Acidic chyme containing dietary fat moves from the stomach to the duodenum, stimulating the secretion of pancreatic juice.
• Digestion of lipids in the upper small intestine (duodenum) occurs in two main steps:
  • Emulsification: Bile salts serve as effective emulsifying agents for fats.
  • Hydrolysis: Pancreatic juice contains lipolytic enzymes (lipases) for breaking down different lipids.

Emulsification of Lipids

• Bile salts play a crucial role in the effective functioning of pancreatic lipase.
• They aid in the combination of lipase with co-lipase (a small protein) in the intestinal lumen.
• Bile salts emulsify fats into fine droplets by lowering surface tension. This increases the surface area for pancreatic lipase activity.

Hydrolysis of Lipids

• Pancreatic enzymes from pancreatic juice act on dietary lipids in the small intestine.
• Pancreatic lipase breaks down TAGs into 2-monoacylglycerol (MAG) and two free fatty acids (F.F.As).
• Phospholipase A2 breaks down phospholipids into lysophospholipids.
• Cholesterolesterase breaks down cholesterol ester into cholesterol and a free fatty acid.

Absorption of Lipids

• The products of fat digestion are absorbed along with the help of bile salts and acids.
• Micelles are formed.
• Micelles reach the brush border of intestinal mucosal cells.
• Lipids from micelles passively diffuse through mucosal cells and are absorbed.

Re-esterification and Assembly of Chylomicrons

• Resynthesis of triglycerides occurs using partial glycerides and liberated free acids.
• The resynthesized triglycerides enter the lymphatics of the abdominal cavity.
• They travel through the thoracic duct and ultimately appear in the blood as lipoprotein particles called chylomicrons.
• Chylomicrons have a triglyceride core surrounded by a phospholipid and protein outer layer.

Lipid Malabsorption (Steatorrhea)

• Steatorrhea is a clinical condition characterized by increased fat content in feces (daily excretion exceeding 6 gm/day).
• This is caused by defective digestion and absorption of fat.

Lipoproteins

• Lipoproteins are large, water-soluble spherical complexes.
• They transport lipids from various tissues for storage and utilization.
• Lipoproteins have a hydrophobic core containing TAGs and cholesterol esters.
• Their hydrophilic surface is composed of phospholipids, unesterified cholesterol, and apoproteins.

Exogenous & Endogenous Pathways of Lipid Transport

• All lipids in plasma are transported as lipoproteins.
• The transport process is generally divided into two pathways:
  • Exogenous Pathway: Transports dietary lipids from the intestine to the liver.
    • Involves chylomicrons.
  • Endogenous Pathway: Transports lipids synthesized in the liver to peripheral tissues.
    • Involves very low-density lipoproteins (VLDL).

Transport Function of Lipoproteins & Their Fates:

• Chylomicrons (CM)
  • Source: Small intestine
  • Transported Lipids: Dietary exogenous TAGs
  • Fate: Chylomicron remnants containing cholesterol and fat are removed from the blood by the liver.
• Very Low Density Lipoprotein (VLDL)
  • Source: Liver
  • Transported Lipids: Endogenously synthesized TAGs in the liver
  • Fate: Circulate and deposit fats in all tissues and become LDL.
• Low Density Lipoprotein (LDL)
  • Source: Derived from VLDL in circulation
  • Transported Lipids: Cholesterol synthesized in the liver to peripheral tissues
  • Fate: Eventually removed from the blood by the liver and other tissues.
• High Density Lipoprotein (HDL)
  • Source: Liver
  • Transported Lipids: Excess tissue cholesterol to the liver for disposal in feces
  • Fate: Eventually removed from the blood primarily by the liver.

Clinical Significance of High Cholesterol

• Elevated cholesterol levels in the body can be associated with pathogenic conditions:
  • Heart diseases
  • Atherosclerosis
  • Gallstones

Lipid Profile

• A lipid profile is a blood test that measures cholesterol and other fats like TAGs in the blood.
• It helps assess the risk of heart disease and stroke.
• Typically, a fast of 8 to 12 hours is required before the test.
• A lipid profile measures:
  • Total cholesterol
  • LDL ("bad") cholesterol
  • HDL ("good") cholesterol
  • TAGs
• The atherogenic index of plasma (AIP) can be calculated to predict atherosclerosis and cardiovascular disease:
  • Atherogenic index = LDL-cholesterol / HDL-cholesterol
  • Atherogenic index = TAG concentration / HDL-cholesterol

Explore the essential roles of lipids in biochemistry, from energy provision to cell membrane structure. This quiz covers the classifications, functionalities, and comparisons of lipids and carbohydrates, as well as the significance of fatty acids. Perfect for anyone studying biochemistry or health sciences.