Triacylglycerols

Triacylglycerols (TAGs), commonly known as triglycerides, are a fundamental type of lipid molecule found abundantly in living organisms. Comprising three fatty acid chains esterified to a glycerol backbone, triacylglycerols play pivotal roles in energy storage, thermal insulation, and structural support in cells. Their multifaceted functions make them indispensable components of biological systems, impacting various physiological processes from metabolism to membrane integrity.

These myriad molecules are naturally contained in oils and fats.

Chemistry

Triacylglycerols consist of three fatty acid chains, which are long hydrocarbon tails, linked via ester bonds to a glycerol molecule, forming a triester. These fatty acids can vary in length and saturation, contributing to the diverse properties of triacylglycerols.

 Triacylglycerols show characteristic solid-phase behaviour as well as crystalline structure. These two aspects determine the melting point, texture, and shelf-life of a host of products from food to pharmaceuticals and cosmetics.

Packing of Triacylglycerols

The lateral packing of TAGs during crystallization is determined by the formation of  α, β′, and β polymorphs. These polymorphs correspond to the hexagonal (H), orthorhombic perpendicular (O_┴), and triclinic parallel (T_//) subcells, respectively (Timms, 2003). This in turn means the longitudinal packing of TAGs defines the chain-length structure (CLS), which is typically double (2L) or triple (3L). In this
connection, β′ crystals are preferred in spreadable foods like margarine, whereas β-3L crystals confer their unique melting behavior and texture to chocolate.

Three mixing states can occur in TAG blends. These are complete solid solution
(ss), phase separation or eutectic interaction, and molecular compound (MC) formation (Zhang et al., 2018).

Eutectic formation results in a depression in melting point and a lower hardness, which may be advantageous to provide melt-away properties to candy centers or fat fillings (Hartel et al., 2018).

Biochemical Synthesis

The synthesis of triacylglycerols, known as triacylglycerol biosynthesis, occurs primarily in the endoplasmic reticulum of cells, involving the acylation of glycerol-3-phosphate with fatty acyl-CoA molecules.

The significance of triacylglycerols lies primarily in their role as energy reservoirs. When energy is abundant, such as after a meal, excess nutrients are converted into triacylglycerols through a process called lipogenesis and stored in adipose tissue. During times of energy deficit, such as fasting or exercise, these stored triacylglycerols are broken down through lipolysis, releasing fatty acids and glycerol to be used as fuel by various tissues, particularly muscles and the liver.

Moreover, the structure of triacylglycerols allows them to serve as efficient thermal insulators in organisms. Adipose tissue, where triacylglycerols are predominantly stored, acts as a thermal insulator, helping to maintain body temperature by reducing heat loss. Additionally, the subcutaneous layer of adipose tissue provides mechanical cushioning and protection against physical trauma.

Triacylglycerols also play vital roles in membrane structure and function. While phospholipids are the primary constituents of cell membranes, triacylglycerols are present in cellular membranes, particularly in lipid droplets, which serve as dynamic organelles involved in lipid metabolism. These lipid droplets can store and release triacylglycerols based on cellular energy demands and signaling cues.

Beyond their physiological functions, triacylglycerols have significant implications for human health. Elevated levels of circulating triacylglycerols, often associated with conditions like obesity, insulin resistance, and metabolic syndrome, are recognized risk factors for cardiovascular diseases. Excessive intake of dietary fats, especially saturated and trans fats, can lead to increased synthesis and storage of triacylglycerols in adipose tissue, contributing to obesity and related metabolic disorders.

Conversely, certain dietary fats, such as polyunsaturated fatty acids (PUFAs) and monounsaturated fatty acids (MUFAs), have been shown to have beneficial effects on health by reducing circulating triacylglycerol levels and improving lipid profiles. Omega-3 fatty acids, a type of PUFA found in fatty fish and flaxseeds, have been particularly associated with cardiovascular benefits due to their anti-inflammatory and lipid-lowering properties.

Triacylglycerol metabolism is tightly regulated by various enzymes, hormones, and signaling pathways to maintain energy homeostasis in the body. Hormones like insulin promote triacylglycerol synthesis and storage in adipose tissue, while hormones like glucagon and adrenaline stimulate lipolysis to release stored fatty acids during times of energy demand. Enzymes such as lipoprotein lipase (LPL) facilitate the hydrolysis of circulating triacylglycerols in lipoprotein particles, allowing tissues to take up fatty acids for energy or storage.

Applications in Industry

In addition to their physiological and health implications, triacylglycerols have numerous industrial applications. They serve as essential components in the food industry, contributing to the texture, flavor, and shelf life of various food products. Triacylglycerols are also utilized in the production of biodiesel, where they undergo transesterification to yield fatty acid methyl esters, which can be used as renewable alternatives to petroleum-based diesel fuel.

Triacylglycerols are versatile lipid molecules with crucial roles in energy metabolism, thermal regulation, membrane structure, and human health. Their synthesis, storage, and metabolism are tightly regulated processes essential for maintaining energy homeostasis and physiological functions in organisms. Understanding the biology of triacylglycerols is not only essential for advancing our knowledge of lipid metabolism but also for developing strategies to prevent and treat metabolic disorders and optimize industrial applications in various fields.

References

Hartel RW, von Elbe JH, Hofberger R. (2018) Confectionery science and technology. Gewerbestrasse: Springer International Publishing.

Timms, R.E. (2003) Confectionery Fats Handbook. In: Timms RE, editor. Properties, Production and Application. Bridgwater: The Oily Press.

Zhang, L., Ueno, S., Sato, K. (2018) Binary phase behavior of saturated-unsaturated mixed-acid triacylglycerols—a review. J. Oleo Sci. 67 pp. 679–87  .