Definition

A lipid is any of various organic compounds that are insoluble in water. They include fats, waxes, oils, hormones, and certain components of membranes and function as energy-storage molecules and chemical messengers. Together with proteins and carbohydrates, lipids are one of the principal structural components of living cells.

Classification

According to LIPIDMAPS classification, lipids are further divided into eight major categories, including Fatty Acyls, Glycerolipids, Glycerophospholipids, Sterol Lipids, Prenol lipids, Sphingolipids, Saccharolipids, Polyketides. There are thousands of distinct lipids. Each of them is chemically unique and they have many different biological functions.

Fig1. Categorization of lipids (Wu, Zhuojun.; et al, 2020)

Biological Functions of Lipids

The majority of lipids in biological systems function either as a source of stored metabolic energy or as structural matrices and permeability barriers in biological membranes. Very small amounts of special lipids act as both intracellular messengers and extracellular messengers such as hormones and pheromones.

• Cellular energy source

• Storage of triglyceride in adipose cells

The fat stored in adipose tissue arises from the dietary intake of fat or carbohydrate in excess of the energy requirements of the body. Excess triglyceride is delivered to the adipose tissue by lipoproteins in the blood. There the triglycerides are hydrolyzed to free fatty acids and glycerol through the action of the enzyme lipoprotein lipase, which is bound to the external surface of adipose cells. The liberated free fatty acids are then taken up by the adipose cells and resynthesized into triglycerides, which accumulate in a fat droplet in each cell.

• Mobilization of fatty acids

When hormones signal the need for energy, fatty acids and glycerol are released from triglycerides stored in fat cells (adipocytes) and are delivered to organs and tissues in the body.

Fig2. Hormone signaling; Adipose tissue (Thompson, Thomas E, 2020)

• Oxidation of fatty acids

Fatty acids provide highly efficient energy storage, delivering more energy per gram than carbohydrates like glucose. In tissues with high energy requirement, such as heart, up to 50–70% of energy, in the form of ATP production, comes from fatty acid (FA) beta-oxidation.

Fig3. Fatty acid beta-oxidation pathway (Creative Proteomics)

• Lipids in biological membranes

Biological membranes consist of a double sheet (known as a bilayer) of lipid molecules. Most biological membranes contain a variety of lipids, including the various glycerophospholipids such as phosphatidyl-choline, -ethanolamine, -serine, -inositol, and -glycerol as well as sphingomyelin and, in some membranes, glycosphingolipids. Cholesterol, ergosterol, and sitosterol are sterols found in many membranes.

• Intracellular and extracellular messengers

In multicellular organisms, the internal mechanisms that control and coordinate basic biochemical reactions are connected to other cells by means of nerves and chemical "messengers." The overall process of receiving these messages and converting the information they contain into metabolic and physiological effects is known as signal transduction.

Many of the chemical messengers are lipids. There are several types of external messengers. The first of these are hormones such as insulin and glucagon and the lipids known collectively as steroid hormones.

A second class of lipid molecules is eicosanoids, which are produced in tissues and elicit cellular responses close to their site of origin. They are produced in very low levels and are turned over very rapidly. Hormones have sites of action that are remote from their cells of origin and remain in the circulation for long periods.

Fig4. Biological functions of lipids (Bou Khalil, Maroun.; et al, 2010)

Since lipids play a crucial role in many biological processes, any imbalance in their homeostasis can lead to serious conditions in living organisms, such as chronic inflammation, cardiovascular diseases, diabetes, and neurodegenerative diseases, to name just a few. Hence, the importance of identifying and quantifying lipids in biomedical research should not be underestimated. The method of choice for the analysis of lipid molecules or huge assemblies of them (known as the lipidome) is undoubtedly mass spectrometry (MS), due to its sensitivity and specificity. With extensive experience and state-of-the-art technologies, Creative Proteomics offers cutting-edge mass spectrometry (MS)-based lipidomics services for biomedical research institutions, biotechnology and pharmaceutical companies.

References:

1. Thompson, Thomas E. Lipid. Encyclopedia Britannica, 2020, 21 Feb.
2. Wu, Zhuojun.; et al. "Lipidomics": Mass spectrometric and chemometric analyses of lipids." Advanced drug delivery reviews. 2020.
3. Bou Khalil, Maroun.; et al. Lipidomics era: accomplishments and challenges. Mass spectrometry reviews. 2010, 29.6: 877-929.