Lipids - Structure and Biological Functions

Lipids - Chemical Structure and Function

• Lipids are a diverse group of compounds, including fats, oils, steroids, waxes, and related compounds.
• They are characterized by being relatively insoluble in water but soluble in nonpolar solvents like ether and chloroform.

Biological Functions of Lipids

• Lipids are important dietary components due to their high energy value.
• Fat-soluble vitamins and essential fatty acids found in fats are crucial.
• Fat is stored in adipose tissue, acting as insulation and thermoregulation.
• Nonpolar lipids insulate electrically, enabling rapid nerve signal propagation.
• Lipoproteins, combinations of lipids and proteins, transport lipids in the blood.

Fat as an Energy Storage

• Triacylglycerols (TGs) are the most efficient form of energy storage in adipose tissue.
• For a 70 kg man, TG stores are 15 kg; for a 140 kg man, they are 80 kg.
• Protein stores are 6 kg for a 70 kg man and 8 kg for a 140 kg man.
• Glycogen stores are minimal and do not significantly contribute to energy storage.
• Estimated starvation survival is 3 months for a healthy-weight individual and 14 months for an obese one.

Fatty Acids

• Fatty acids are hydrocarbon chains terminating with carboxylic acid groups.
• Chain lengths range from C4 to C36.
• Unsaturation degree varies.
• Systematic names are derived from the parent hydrocarbon, replacing the final 'e' with 'oic'.
• Example: C18 saturated fatty acid = octadecanoic acid (stearic acid).
• C16 fatty acid = hexadecanoic acid (palmitic acid).
• Different numbers of double bonds will have different names (monounsaturated, polyunsaturated).
• 18:0 denotes an 18-carbon fatty acid with no double bonds; 18:2 signifies two double bonds.
• Common fatty acids have even numbers of carbon atoms and an unbranched chain length of 12-24 carbons.

Nomenclature of Fatty Acids

• The location of double bonds in unsaturated fatty acids follows specific patterns: (9,10), (A12), (A15)

• Polyunsaturated fatty acid double bonds are not conjugated. They have methylene group separators.

• Example: Arachidonic acid (all-cis 5,8,11,14-eicosatetraenoic acid and all-cis 6,9,12,15-eicosatetraenoic acid)

Common Fatty Acids of Physiological Importance

• Various fatty acids with chain lengths 2-24 have varying roles in the human body.
• The table lists some examples of fatty acids, their names, and chain lengths

Saturated vs Unsaturated Fatty Acids

• Saturated fatty acids have no double bonds, and their hydrophobic non-polar chains lead to solid fats at room temperature.
• Unsaturated fatty acids have one or more double bonds and typically result in liquid fats.

Classification of Lipids

• Simple lipids: Fatty acids, waxes, and neutral lipids (Storage lipids).
• Complex lipids: Phospholipids (Glycerophospholipids like phosphatidylcholine, sphingophospholipids, plasmalogens), Glycolipids (Sphingoglycolipids like Galctosylceramide, Galactitol lipids, and Sulfolipids).
• Steroids and other isoprenoid lipids: Steroids (Cholesterol, steroid hormones), Terpenes, Carotenoids.

Triacylglycerols (TAGs)

• TAGs are the simplest lipids.
• They are constructed from fatty acids attached to glycerol.
• They are the primary storage form of energy.
• They are nonpolar and hydrophobic, making them insoluble in water.
• Important examples: Tripalmitin, tristearin, triolein.

Advantages of Using Triacylglycerols (TAGs) vs. Polysaccharides

• Fatty acids have a higher reduction state than sugars, yielding more energy.
• Carrying fat stored as fuel is more efficient as it doesn't carry extra water weight.

Triacylglycerols (TAGs) Functions

• Insulation of low temperatures
• Energy storage

Waxes

• These are complex esters of long-chain fatty acids and long-chain alcohols.
• They are water-repellent and serve as energy reserves.
• Often found on skin, hair, and feathers.
• Protect plants from water loss and parasites.

Glycerophospholipids

• Major components of biological membranes.
• Consist of glycerol-3-phosphate esterified to fatty acids at C1 or C2 positions; linked with phosphoryl group to a substituent X.
• Amphipathic: hydrophobic tails and hydrophilic heads.
• Important examples: Phosphatidic acid, phosphatidylethanolamine, phosphatidylcholine, phosphatidylserine, phosphatidylglycerol, Cardiolipin Phosphatidylinositol.

Plasmalogens

• A type of glycerophospholipid.
• Have an ether-linked alkyl chain at the sn-1 position instead of an ester.
• Primarily in brain and muscle tissues.

Platelet Activating Factor

• An ether-linked phospholipid.
• Released from leukocytes and stimulates platelet aggregation and the release of serotonin.
• Involved in inflammation and allergic responses.

Chloroplast Lipids

• Chloroplasts contain significant amounts of monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) — galactolipids.
• They are critical components of thylakoid membranes.
• Sulfolipids are also found in chloroplasts.

Sphingophospholipids

• Derived from sphingosine.
• Major component of myelin (insulating sheath around nerve fibers).
• Example: Sphingomyelin.

Sphingoglycolipids

• Sphingosine backbone with sugar moieties
• Examples: Cerebrosides, Sulfatides, and Gangliosides
• Important for cell signaling and structure.
• Involved in blood type determination.

Cholesterol — Steroids

• Dominant sterol in animal tissues.
• Amphipathic: polar head and non-polar hydrocarbon tail.
• Essential component of animal cell membranes.
• Precursor for numerous biologically active substances like steroid hormones and bile acids.

Degradation of Phospholipids and Sphingolipids.

• Gangliosides are broken down by lysosomal enzymes.
• Genetic defects in lysosomal enzymes can lead to accumulation of these gangliosides and neurological disorders.