A Quick Look at Biochemistry: Lipid Metabolism

Lipids are a group of naturally occurring compounds and hydrophobic or amphiphilic molecules that form structures such as vesicles, liposomes or membranes in an aqueous environment. They are a large and diverse group of organic macromolecules; hence, it is always difficult for scientists to provide a specific definition for the word “lipid”. Therefore, due to the absence of a widely-accepted definition in this regard, lipids are categorized based on their different chemical properties including their ability for saponification, their chemical compositions and their building blocks. Saponification is the ability of lipids to be hydrolyzed by basic solutions into compounds such as glycerol and fatty acids (FAs). Based on their chemical composition, lipids are classified into simple and complex lipids. Simple lipids contain one or two different types of compounds. However, complex lipids frequently consist of three or more chemical identities (e.g., glycerol, FAs, and sugar) and they are usually amphipathic. Based on their building-blocks, lipids are categorized into two ketoacyl and isoprene groups. In this categorization, lipids with ketoacyl subunits are divided into eight categories including FAs, glycerolipids, glycerophospholipids, sphingolipids, saccharolipids, and polyketides while sterol lipids and prenol lipids derived from isoprene subunits. Regardless of all diversities in lipid classification, overlapping between different classifications still exist. This section simply introduces definitions of each category that is described in general books [1]. Nomenclatures, abbreviations and formulas are explained in the text and figures 1 and 2.