Lipids in Medical Biochemistry

Lipids

• Lipids are a heterogeneous group of compounds formed by mainly carbon and hydrogen atoms.
• They are insoluble in water but soluble in non-polar solvents such as ether, chloroform, and benzene.
• Lipids are important dietary constituents due to their high energy value, fat-soluble vitamins, and essential fatty acids.

Properties of Lipids

• Lipids are characterized by low solubility in water, making them relatively hydrophobic.
• They have a high energy value, making them a good source of energy.

Biological Functions of Lipids

• Storage of energy: lipids provide a lot of available energy due to their good packing and low thermal conductivity.
• Insulation from the environment: lipids provide mechanical protection, prevent excessive wetting, and prevent loss of water via evaporation.
• Membrane structure: lipids are the main structure of cell membranes.
• Cofactors for enzymes: lipids act as cofactors for enzymes, such as vitamin K and coenzyme Q.
• Signaling molecules: lipids act as signaling molecules, such as paracrine hormones, steroid hormones, growth factors, and vitamins A and D.
• Pigments: lipids are responsible for the color of certain fruits and vegetables.
• Antioxidants: lipids, such as vitamin E, act as antioxidants.

Classification of Lipids

• Lipids can be classified into three categories: simple lipids, complex lipids, and isoprene-derived lipids.
• Simple lipids include fatty acids, triglycerides, and waxes.
• Complex lipids include phospholipids, glycolipids, and lipoproteins.
• Isoprene-derived lipids include terpenes and steroids.

Fatty Acids

• Fatty acids are carboxylic acids with hydrocarbon chains containing between 4 to 36 carbons.
• Almost all natural fatty acids have an even number of carbons.
• Most natural fatty acids are unbranched.
• Fatty acids can be classified as saturated, monounsaturated, or polyunsaturated based on the number of double bonds in the chain.

Important Fatty Acids

• Palmitic acid: 16 C, saturated
• Palmitoleic acid: 16 C, a single double-bond, mono-unsaturated
• Stearic acid: 18 C, saturated
• Oleic acid: 18 C, a single double-bond, mono-unsaturated
• Linoleic acid: 18 C, two double-bond, polyunsaturated
• Linolenic acid: 18 C, three double-bond, polyunsaturated
• Arachidonic acid: 20 C, four double-bond, polyunsaturated

Essential Fatty Acids

• Essential fatty acids cannot be synthesized in the body and must be obtained through the diet.
• Linoleic acid, linolenic acid, and arachidonic acid are essential fatty acids.

Solubility and Melting Point of Fatty Acids

• Solubility of fatty acids decreases as the chain length increases.
• Melting point of fatty acids decreases as the chain length decreases and as the number of double bonds increases.

Conformation of Fatty Acids

• Saturated fatty acids tend to adopt extended conformations.
• The double bonds in natural unsaturated fatty acids are commonly in cis configuration, which kinks the chain.