General Veterinary Biochemistry

SIMPLE LIPIDS - TRIACYLGLYCEROLS

• Glycerol is a compound with three-carbon atom each of which contains an alcohol side chain. In the three dimensional configuration of glycerol, carbon 1 and 3 are not identical. For numbering the carbon atom of glycerol steriochemical numbering system is used (-sn). Enzymes in the system are more specific for C-1 or for C-3. Glycerol is generally phosphorylated on –sn –3 by the enzyme glycerol kinase to produce glycerol 3-phosphate and not glycerol 1-phosphate.
• Triacylglycerols are also referred to as triglycerides, fats or neutral fats. Fatty acid moiety in lipid ester is known as acyl group.
• Triglycerides are compounds that contain glycerol, to which 3 fatty acids are attached. The hydroxyl group of glycerol can react with fatty acids to form monoacylglycerols, diacylglycerols and triacylglycerols.
• In simple triacylglycerol, glycerol is attached to same kind of fatty acids in all the 3 positions. Eg: tristearin (made up of stearic acid) and tripalmitin (made up of palmitic acid).
• Mixed triacylglyerols contain 2 or more different types of fatty acids attached to the glycerol.
• Triacylglycerols are non polar and insoluble in water. They exist in solid (fats) or liquid form (oils) depending on the nature of the constituent fatty acids (saturated or unsaturated).
• Most plant triacylglycerols have low melting point and are liquid at room temperature, because they contain large proportion of unsaturated fatty acids such as oleic, linoleic and linolenic acids.
• The unsaturated fatty acids may be converted to saturated fatty acids by a process known as hydrogenation.
• In contrast, animal triacylglycerols contain higher proportion of saturated fatty acid such as palmitic and stearic acids resulting in higher melting points and are semisolid or solid at room temperature.
• Most naturally occurring fats such as butter and other food fats are made up of complex mixtures of simple and mixed triacylglycerols containing a variety of fatty acids differing in chain length and degree of saturation.
• Triacylglycerols undergo hydrolysis, when boiled with acids or bases or when acted upon by enzyme, lipase. Hydrolysis of triacylglycerols by KOH or NaOH is called as saponification (meaning soap formation) which yields a mixture of K⁺ or Na⁺ soaps and glycerol.
• Triacylglycerols with largely unsaturated fatty acids, which are thus liquid at room temperature, can be converted chemically into solid fats by hydrogenation of their double bonds.
• When exposed to air, TG containing highly unsaturated fatty acids tend to undergo a complex process called auto oxidation. Rancid fats contain appreciable amount of peroxidized fat.
• Large amount of TG can be stored in adipose tissue without the weight gain due to water as these molecules are hydrophobic in nature.