Lipid Chemistry

Mr. Arvind Kumar

Definition of Lipids

Lipids may be defined as organic substances insoluble in water but soluble in organic solvents like chloroform, ether and benzene.

They are esters of fatty acids with alcohol esters and are utilizable by the living organism.

Properties of Lipids

  1. Lipids may be either liquids or non-crystalline solids at room temperature.
  2. Pure fats and oils are colourless, odourless, and tasteless.
  3. They are energy-rich organic molecules.
  4. Insoluble in water but in organic solvents like alcohol, chloroform, acetone, benzene, etc.
  5. No ionic charges.

Biological Importance of Lipids

  1. More palatable food
  2. Storable to unlimited amounts compared to carbohydrates.
  3. High-energy value
  4. Supply Essential fatty acids.
  5. Supply fat-soluble vitamins (A, D, E and K).
  6. Cholesterol – a. In membrane structure,
  7. Synthesis of some hormones,
  8. Synthesis of vitamin D3 and bile acids.

Classification of Lipids

Bloor’s (1943) Classification

Simple Lipids

They are esters of fatty acid with alcohol

They are divided into two groups –

  1. Fats and oils – Fatty Acid + Glycerol
  2. Waxes – Fatty Acid + High molecular weight alcohol other than glycerol

Compound lipids

Fatty Acid + Alcohol (Glycerol/Sphingosine) + additional groups

They are divided into two groups –

  1. Phospholipids 

Fatty Acid + Alcohol (Glycerol/Sphingosine) +        Phosphate + Nitrogenous base

Phospholipids are divided into two groups –

  • Glycerophospholipids: The alcohol in this group is Glycerol, e.g., Lecithin, Cephalin.
  • Sphingophospholipids: The alcohol in this group is an amino alcohol Sphingosine, e.g. Sphingomyelin.
  1. Glycolipids or Glycosphingolipids

Fatty Acid + Alcohol (Sphingosine) + Carbohydrate,

e.g. Cerebroside, Ganglioside.

  1. Lipoproteins

Lipids complexed with proteins,

    • Chylomicrons
    • VLDL
    • LDL
    • HDL

Derived Lipids or Precursor Lipids

Compounds that are derived from the above group of Lipids,

    • Fatty acids
    • Steroids
    • Cholesterol
    • Ketone bodies.
    • Lipid soluble vitamins
    • Hormones

Functions of Lipids

  1. Storage form of energy: Fats and oils are used almost universally as stored forms of energy in living organisms.
  2. Structural Lipids: Lipids are major structural components of membranes, e.g. phospholipids, glycolipids and sterols.
  3. Cholesterol, a sterol, is a precursor of many steroid hormones, vitamin D, and an important component of the plasma membrane.
  4. Lipid is a thermal insulator in the subcutaneous tissues and around certain organs.
  5. Nonpolar lipids act as electrical insulators in neurons.
  6. Lipids help absorb fat-soluble vitamins (A, D, E and K). They act as a solvent for the transport of fat-soluble vitamins.

Fatty acids

  1. Fatty acids are organic molecules that are long-chained carboxylic acids with 4-36 carbon atoms.
  2. Naturally occurring fatty acids are mostly unbranched, occurring in three main classes of lipids: triglycerides, phospholipids, and cholesteryl esters.
  3. Fatty acids are not found in the free state but remain associated with alcohol to form triglycerides.
  4. Fatty acids are stored as an energy reserve (fat) through an ester linkage to glycerol to form triglycerides.

Classification of Fatty Acid

Fatty acids are classified into four major classes

  1. Straight-chain fatty acids
  2. Branched-chain fatty acids
  3. Substituted fatty acids
  4. Cyclic fatty acids

1  Straight Chain Fatty

Acids Fatty acids, in which the carbons are arranged linearly, are subclassified into two classes:

    1. Saturated fatty acids
    2. Unsaturated fatty acids

Saturated fatty acids

There is no double bond in the hydrocarbon chain of these fatty acids. Saturated fatty acids are subclassified into two classes:

      • Even carbon acids carry an even number of carbons,

e.g. palmitic acid and stearic acid.

      • Odd carbon acids carry an odd number of carbons,

e.g. propionic acid.

Unsaturated fatty acids

These contain double bonds in their hydrocarbon chains.

These are subclassified according to the number of double bonds present in the structure as follows:

    1. Monoenoic or monounsaturated fatty acid
    2. Polyenoic or polyunsaturated fatty acid.
    3. Monoenoic or monounsaturated fatty acids, e.g. oleic acid.
    4. Polyenoic or polyunsaturated fatty acids, for example:
      • Dienoic acids
      • Trienoic acids,
      • Tetraenoic acid,

2. Branched Chain Fatty Acids

These are less abundant than straight-chain acids in animals and plants, e.g.        

    1. Isovaleric acid
    2. Isobutyric acid.

3. Substituted Fatty Acids

In substituted fatty acids, one or more hydrogen atoms have been replaced by another group, e.g.

    1. Lactic acid of blood.
    2. Cerebronic acid and oxynervonic acids of brain glycolipids.
    3. Ricinoleic acid of castor oil.

4. Cyclic Fatty Acids

Fatty acids bearing cyclic groups are present in some bacteria and seed lipids.

Functions of Fatty Acids

Fatty acids have three major physiological functions.

  1. They serve as building blocks of phospholipids and glycolipids. These amphipathic molecules are important components of biological membranes.
  2. Fatty acid derivatives serve as hormones, e.g. prostaglandins.
  3. Fatty acids serve as a major fuel for most cells.

Essential Fatty Acid

The fatty acids that humans require but are not synthesized in the body and hence need to be supplied in the diet are known as essential fatty acids (EFA). Humans lack the enzymes that can introduce double bonds beyond 9th Carbon.

Functions of Essential Fatty Acid

  1. They are integral components of membrane structure, often in the 2 positions of phospholipids
  2. Eicosanoids are synthesized from Arachidonic Acid. Essential fatty acids are needed for the synthesis of Arachidonic Acid
  3. Lower the risk of cardiovascular diseases
  4. Lower the risk of Fatty liver.

Deficiency of Essential Fatty Acid

  1. Skin: Acanthosis and Hyperkeratosis
  2. Fatty liver
  3. Swelling of mitochondrial membrane and reduction in efficiency of oxidative phosphorylation
  4. Decrease in fibrinolytic activities.

Significance of ω3 Fatty Acid

  1. Decrease the risk of cardiovascular disease.
  2. Appear to replace arachidonic acid in platelet membranes.
  3. Lower the production of Thromboxane and tendency of the platelet aggregation.
  4. Decrease Serum Triglycerides.
  5. Important for Infant Development.
  6. Lower the risk of various mental illnesses (Depression, ADHD)
  7. Lower the risk of chronic degenerative diseases such as Cancer, Rheumatoid Arthritis, and Alzheimer’s Disease.

Functions of Triglycerides

  1. Triglycerides are important macromolecules as they store most energy in the body.
  2. These are stored in fat cells and then released into the bloodstream by different hormones whenever necessary.
  3. The fat stored in the body forms a layer of insulation beneath the skin, which helps to maintain the body temperature.
  4. Triglycerides also aid in the absorption and transport of fat-soluble vitamins.

Phospholipids

Compound lipids comprise fatty acid, alcohol, phosphoric acid and a nitrogenous base. Based on the alcohol present, phospholipids are divided into two groups

  1. Glycerophospholipid: They contain alcohol, Glycerol
  2. Sphingophospholipids: They contain alcohol and sphingosine.

Glycerophospholipid Contain

    • Glycerol
    • Fatty acid esterified to the first two carbon atoms
    • Nitrogenous base
    • Phosphoric Acid.

Sphingophospholipid

The second group of phospholipids contains sphingosine as the backbone alcohol.

Functions of Phospholipids

  • Phospholipids are the major lipid constituents of cell membranes.
  • Phospholipid (lecithin) acts as a lung surfactant, which prevents alveolar collapse.
  • They regulate the permeability of membranes and the activation of some membrane-bound enzymes.
  • Plasmalogens (platelet-activating factors) are involved in platelet aggregation and degranulation.

Glycolipids

Classification of Glycolipids Four classes of glycolipids have been distinguished:

  1. Cerebrosides – (Ceramide + Monosaccharides)
  2. Sulfatides – (Ceramide + Monosaccharide + Sulfate)
  3. Globosides – (Ceramide + Oligosaccharide)
  4. Gangliosides – (Cerebroside + Oligosaccharides + N-acetylneuraminic acid, NANA)

Functions of Glycolipids

  • Glycolipids are important constituents of the nervous tissue, such as the brain and outer leaflet of all cell membranes.
  • They play a role in regulating cellular interactions, growth and development.

Cholesterol

Functions of Cholesterol

  • It is a major structural constituent of the cell membranes and plasma lipoproteins.
  • Cholesterol serves as the precursor for a variety of biologically important products, including:
    1. Steroid hormones: Cholesterol is the precursor of the five steroid hormones, e.g.
      1. Progesterones
      2. Glucocorticoids
  • Mineralocorticoids iv. Androgens (male sex hormones)
  1. Estrogen (female sex hormones).
  1. Bile acids: Bile acids, derived from cholesterol, act as a detergent in the intestine, emulsifying dietary fats to make them readily accessible to the digestive enzyme lipase.
  2. Vitamin D is derived from cholesterol and is essential in calcium and phosphate metabolism.

Lipoproteins

Classes of Lipoproteins

  1. Chylomicrons

Site of synthesis – Intestine

Function – Transport of dietary lipids from intestine to peripheral tissues

  1. Very low-density lipoproteins (VLDL)

Site of synthesis – Liver

Function – Transport of triacylglycerol from liver to peripheral tissues

  1. Low-density lipoprotein (LDL)

Site of synthesis – Plasma VLDL

Function – Transport of cholesterol from the liver to peripheral tissues

  1. High-density lipoprotein (HDL).

Site of synthesis – Liver and intestine

Function – Transport of free cholesterol from peripheral tissues to the liver

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