Lipids: Introduction & Classification

Lipids: Introduction & Classification

• Lipids are organic substances that are generally insoluble in water but soluble in organic solvents like chloroform, ether, and benzene.
• Lipids have various functions, including being a source of energy, providing insulation, contributing to flavor and taste, and playing roles in metabolism and cell signaling.
• They are crucial for protecting internal organs.
• Lipids have functional roles in vitamin absorption and electrical/thermal insulation.

Lipid Classification

• Lipids are classified into three main groups: simple, complex, and derived lipids.
• Simple lipids include natural fats and waxes.
  • Neutral fats/oils (alcohol is glycerol)
  • Waxes (alcohol is other than glycerol)
• Complex lipids are esters of fatty acids with alcohols containing additional prosthetic groups.
  • Phospholipids
  • Glycolipids
  • Lipoproteins
• Derived lipids are formed from the hydrolysis of simple and complex lipids and share lipid characteristics
  • Fatty acids
  • Sterols
  • Cholesterol
  • Lipid soluble vitamins
  • Hormones
  • Ketone bodies

Simple Lipids

• Simple lipids are esters of fatty acids (FAs) with various alcohols.
• Neutral fats or oils are esters of fatty acids with glycerol.
• Waxes are esters of fatty acids with alcohols other than glycerol, offering characteristics like water repellency for skin and hair. Examples include lanolin, beeswax, and whale sperm oil.

Functions of Lipids

• Neutral fats/oils are important for energy storage in adipose tissues and contribute to body contouring and insulation.
• Simple lipids like waxes help keep skin and hair pliable and water-repellent. Earwax protects the tympanic membrane from foreign bodies.
• Complex lipids like phospholipids and glycolipids are vital components of cell membranes, influencing their fluidity, permeability, and signal transduction.
• Derived lipids are crucial in numerous biological processes including acting as precursors for hormones and vitamins.

Lipid Peroxidation

• Lipid peroxidation is the oxidative breakdown of lipids.
• It's a free radical chain reaction, primarily affecting polyunsaturated fatty acids (PUFAs) in cell membranes, causing damage.
• End products from peroxidation are mutagenic and carcinogenic.

Free Radicals

• A free radical is an atom or molecule with an unpaired electron.
• Free radicals are highly reactive, damaging cell membranes, proteins, and DNA.
• Free radical reactions occur when weak chemical bonds break, creating molecules with unpaired electrons.
• Excessive free radicals can lead to oxidative stress.

Sources of Free Radicals

• Exogenous: Foods, air pollutants, radiation, cigarette smoking
• Endogenous: Metabolic processes (mitochondria, peroxisomes), detoxification (cytochrome P450), and immune cells.

Mechanisms of Lipid Peroxidation

• Photooxidation: Involves singlet oxygen and sensitizers (like porphyrins, myoglobin, riboflavin, and bilirubin).
• Enzymatic oxidation: Reactions involving cyclooxygenase and lipoxygenase.
• Autoxidation: Free radical chain reaction.

Reactive Oxygen Species (ROS)

• ROS are free radicals and other molecules containing oxygen that are highly reactive.
• Examples include superoxide, hydroxyl radical, hydroperoxyl radical, peroxyl, alkoxyl, hydrogen peroxide, hypochlorous acid, singlet oxygen, ozone, and triplet oxygen.

Oxidative Stress

• Oxidative stress is an imbalance in the body, where prooxidants (factors promoting the production of free radicals) outnumber antioxidants (chemicals that neutralize free radicals).
• This imbalance can cause cell and tissue damage, impacting the aging process.

Consequences of Lipid Peroxidation

• Lipid peroxidation can alter membrane fluidity, impair signaling proteins, increasing membrane permeability. Damage leads to loss of integrity and function.

Pathological Conditions Involving Oxidative Stress

• Oxidative stress underlies several pathological conditions including inflammation, atherosclerosis, ischemia/reperfusion injury, cancer, and aging.

Antioxidants

• Antioxidants are chemicals that neutralize free radicals, slowing or preventing oxidation.
• They impede free radical chain reactions by removing unpaired electrons.
• Ideal antioxidants are effective at low concentrations, have no harmful effects, don't alter food taste or color, have stable structure, and readily available.
• Examples of antioxidants include vitamins A, C, and E, selenium, and various phytonutrients.

Antioxidant Defenses

• Fat-soluble antioxidants include vitamins E, beta-carotene, and coenzyme Q10, protecting cell membranes.
• Water-soluble antioxidants include vitamins C, glutathione peroxidase, superoxide dismutase, and catalase to neutralize free radicals in the body's water compartments.