Structure and metabolic pathways for some common lipids Structure and metabolic pathways for some common lipids (A) Structures of cholesterol and cholesterol ester. In cholesterol ester, the R group is a fatty acid as shown in (D). (B) Hydrolysis of triglyceride to glycerol and fatty acids by a lipase. There are several different lipases (e.g. lipoprotein lipase of endothelial cells and hormone-sensitive lipase in adipocytes). (C) Key steps in the multistep synthetic pathway of cholesterol. HMG CoA, 3-hydroxy-3-methylglutaryl-CoA. HMG CoA reductase is the rate-limiting step. (D) Fatty acids are carbon chains (most commonly 12–22 carbons) with a methyl group at one end and a carboxyl group at the other. Saturated fatty acids are ‘filled’ (saturated) with hydrogen and have no double bonds. Monounsaturated fatty acids (MUFAs) have one carbon–carbon double bond which can occur in different positions. These MUFAs may have a double bond with hydrogens in the cis configuration (i.e. hydrogens at either side of the double bond are orientated in the same direction) or the trans configuration (i.e. hydrogens are orientated in different orientations). The cis configuration introduces a kink in the molecular shape of the carbon chain altering physical properties. Polyunsaturated fatty acids (PUFAs) have more than one double bond. The letter n or Greek symbol ω, is used to indicate the position of the bond closest to the methyl end. For example, n−6 PUFAs are characterised by the presence of at least two double bonds with the first between the sixth and seventh carbon from the methyl end. Alastair J. Barr Essays Biochem. 2018;62:619-642 ©2018 by Portland Press Ltd