Triacylglycerols (TAGs), also known as triglycerides, are the primary form of dietary fat and energy storage in animals and plants, consisting of a glycerol backbone esterified with three fatty acyl chains. These neutral lipids dominate adipose tissue and oils, exhibiting vast structural diversity that dictates physical properties and metabolic roles.
Chemical Structure
TAGs feature a glycerol moiety with fatty acids at sn-1, sn-2, and sn-3 positions, yielding regioisomers and stereoisomers when chains differ. Saturated chains (e.g., palmitic C16:0) favor sn-1/sn-3; unsaturated (e.g., oleic C18:1) prefer sn-2, as in human milk where ~70% palmitate occupies sn-2 for optimal digestion. Diversity arises from chain length (C4-C60+), unsaturation degree, and positional specificity.
Physical Properties
Nonpolar and hydrophobic, TAGs are insoluble in water but dissolve in chloroform and hexane, with specific gravity <1 causing flotation. Melting point inversely correlates with unsaturation—saturated TAGs (e.g., tripalmitin ~65°C) solidify easily, while polyunsaturated (e.g., trilinolein <-10°C) remain oils; polymorphism (α, β', β crystals) influences texture in foods.
Biosynthesis and Metabolism
Synthesized via the glycerol-3-phosphate or monoacylglycerol pathways in liver/adipose, TAGs store excess energy in droplets and hydrolyze by lipases to DAGs, MAGs, and fatty acids for β-oxidation. Interesterification rearranges acyl chains, reducing trisaturated (S3) species while boosting monounsaturated (S2U), altering melting profiles for margarines.
Biological Functions
TAGs provide 9 kcal/g energy, insulate organs, and cushion impacts; elevated plasma levels (>150 mg/dL) signal cardiovascular risk via atherosclerosis. In infants, sn-2 palmitate enhances fat absorption; dietary TAG structure modulates postprandial lipemia.
