BIOC 4131 – Biochemistry II

Lipids

Overview

• Lipids are waxy, greasy, or oily compounds produced by living organisms (plants and animals) that are soluble only in nonpolar solvents.
• The group of compounds called lipids includes a variety of substances with different compositions and structures.

Textbook Resources

• Fundamentals of Biochemistry:
  • Chapter 10: Lipids
  • 10.1: Introduction to lipids
  • 10.2: Lipid Aggregates in Water - Micelles and Liposomes
  • 10.3: Membrane Bilayer and Monolayer Assemblies - Structures and Dynamics

Lipid Types

• Major types of lipids include:
  • Fatty acids
  • Triacylglycerols (triglycerides)
  • Phospholipids (phosphoacylglycerols)
  • Sphingolipids
  • Sphingomyelins
  • Cerebrosides
  • Gangliosides
  • Glycolipids
  • Cholesterol
  • Waxes

Fatty Acids

Structure

• Fatty acids are long hydrocarbon chains with a carboxylic acid group, generally containing 12 to 20 carbons.
• They are major components of natural fats and oils but are primarily present in a bound form rather than as free entities.

Characteristics of Fatty Acids:

• Usually straight-chain carboxylic acids (no branching)
• Typically contain an even number of carbons
• Can be saturated (no double bonds) or unsaturated (one or more double bonds):
  • Monounsaturated (one double bond)
  • Polyunsaturated (multiple double bonds)

Unsaturation and Molecular Shape

• Unsaturated fatty acids typically possess cis double bonds, which create kinks in the carbon chain that affect how closely fatty acids can pack together.
• Trans double bonds do not induce a kink and retain a straight chain similar to saturated fatty acids.

Physical Properties

• Fluidity and Melting Point:
  • The presence of cis double bonds reduces intermolecular interactions and increases fluidity, resulting in a lower melting point compared to saturated fatty acids.
• Fatty acids can be identified systematically:
  • Changing the suffix of the parent hydrocarbon from "-e" to "-oic acid" for carboxylic acids.
  • Denoting double bonds as:
  • One = enoic
  • Two = dienoic
  • Three = trienoic
  • Four = tetraenoic
• Example: Linoleic acid is systematically named "cis,cis-Δ9,Δ12-octadecadienoic acid" indicating its structure.

Amphipathic Nature

• Fatty acids consist of a hydrophilic carboxylic acid head and a hydrophobic tail, forming micelles in aqueous solutions to maximize interactions with water and hydrophobic interactions among chains.
• Micelle: A spherical aggregate of fatty acids in water, useful for delivering lipophilic drugs.
• Essential fatty acids in the diet include linoleic acid (18 carbons) and α-linolenic acid (also 18 carbons), as well as arachidonic acid (20 carbons).

Carbon Numbering

• Numbering begins from the carboxylic acid. The first two carbons of the chain are termed α and β, respectively, while the terminal carbon is called ω (omega).
• Omega-3 Fatty Acids: Have a double bond between the 3rd and 4th last carbons.
• Common sources of fatty acids include:
  • Saturated and monounsaturated fatty acids: butter, ghee, animal fats.
  • Polyunsaturated fatty acids: corn, linseed, olive, and fish oils.

Health Implications

• Omega-3 fatty acids (α-linolenic acid sources include linseed and fish oils) reduce the risk of heart disease and lower blood pressure.
• Omega-6 fatty acids (linoleic acid sources include sunflower, safflower, corn, soybean oils) are important for growth, skin health, and normal reproductive function.

Triacylglycerols

Structure

• Triacylglycerols (triglycerides) consist of a glycerol molecule esterified to three fatty acids, serving primarily as energy storage in organisms.

Types of Adipose Tissue

1. White Adipose Cells:
   • Store lipids, release fatty acids and glycerol into the bloodstream upon hormonal signal.
2. Brown Adipose Cells:
   • Contain numerous mitochondria and can oxidize lipids to generate heat (thermogenesis) instead of ATP when stimulated by hormones during cold exposure.

Properties and Function

• Triacylglycerols do not dissolve in water, forming a separate phase when mixed with water.
• Saponification: The hydrolysis of triacylglycerols in basic conditions yields glycerol and carboxylic acid salts (soaps).
• Example Reaction:
  $R<em>C + R'OH ightarrow R</em>C + R' + NaOH$

Lipids Overview

Phospholipids

• Phospholipids constitute a major type of membrane lipid, composed of:
  • A glycerol backbone
  • Two ester-linked fatty acids
  • An ester-linked phosphate group
  • An alcohol attached to the phosphate group.
• Phosphatidic acid is the foundational molecule from which other phospholipids are derived.
• Phosphoglycerides commonly include:
  • Phosphatidylcholine (lecithin)
  • Phosphatidylethanolamine
  • Phosphatidylglycerol
  • Phosphatidylserine

Sphingolipids

• Constructed on sphingosine (an amino alcohol), distinguished from glycerol-based lipids.
• Sphingomyelins:
  • Formed by adding a fatty acid to sphingosine, linked by amide bonds, and a phosphocholine or phosphoethanolamine by ester bonds.
  • Found in plasma membranes, especially in nerve fibers, providing insulation and facilitating signal transduction.

Glycolipids

• Cerebrosides: Contain sphingosine, a fatty acid, and a monosaccharide (galactose or glucose).
• Gangliosides: Consist of acylated sphingosine and a polysaccharide, playing a vital role in cell recognition and signaling.

Cholesterol

• A type of steroid with a structure comprising four hydrocarbon rings; provides structure in cell membranes and is a precursor for hormones, vitamin D, and bile salts.

Waxes

• Esters of long-chain alcohols with long-chain fatty acids, making them insoluble and water-repellent.

Lipid-Soluble Vitamins

Vitamins and Their Functions

• Vitamin A (retinol): Derived from β-carotene; crucial for vision and cell function; deficiency can lead to night blindness.
• Vitamin D3 (cholecalciferol): Made from cholesterol; regulates calcium and phosphate metabolism; deficiency leads to rickets.
• Vitamin E (α-tocopherol): An antioxidant protecting cells from damage; deficiency causes muscle and nerve damage.
• Vitamin K (phylloquinone): Essential for blood clotting; deficiency results in bleeding disorders.

Lipid Messengers

Hormones

• Hormones are chemical messengers produced by endocrine glands, affecting target cells with receptors to induce biological effects.
• Examples include insulin, which regulates glucose levels, and vitamin D, which affects calcium and phosphate metabolism.

Steroid Hormones

• Derived from cholesterol; include:
  • Adrenocorticoids:
  • Mineralocorticoids (e.g., aldosterone) that regulate ion concentration.
  • Glucocorticoids (e.g., cortisol) influencing metabolism and inflammation.
  • Sex Hormones:
  • Androgens (e.g., testosterone) promoting muscle growth.
  • Estrogens (e.g., estrone, estradiol) and progesterone regulating female reproductive functions.

Prostaglandins and Leukotrienes

• Prostaglandins: Synthesized from arachidonic acid, involved in reproductive processes, blood pressure regulation, and inflammation.
• Leukotrienes: Also derived from arachidonic acid, notable for smooth muscle constriction in the lungs, often impacted during asthma attacks.