BIOCHEMISTRY AND HUMAN NUTRITION

Module 1. Bio-molecules

Lesson 4
LIPIDS

4.1 Introduction

• Lipids are a broad group of naturally occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoacylglycerols, di acylglycerols, phospholipids, and others.
• Lipids consist of a wide group of compounds that are generally soluble in organic solvents and largely insoluble in water.
• The main biological functions of lipids include energy storage, as structural components of cell membranes, vitamins, hormones and as important signaling molecules.
• Lipid is sometimes used as a synonym for fats, fats are a subgroup of lipids called tri acylglycerols.
• Lipids also encompass molecules such as fatty acids and their derivatives (including tri-, di-, and monoacylglycerols and phospholipids), as well as other sterol-containing metabolites such as cholesterol.
• Humans and other mammals use various biosynthetic pathways to both degrade and synthesize lipids, some essential lipids cannot be made this way and must be obtained from the diet.

4.2 Lipid Classification

• Fatty Acids
• Acylglycerols
• Glycerophospholipids
• Sphingolipids
• Sterols
• Prenol lipids
• Saccharolipids

4.2.1 Fatty acids

• A fatty acid consists of a hydrocarbon chain and a terminal carboxylic acid group.
• This arrangement confers the molecule with a polar, hydrophilic end, and a nonpolar, hydrophobic end that is insoluble in water.
• The fatty acid structure is one of the most fundamental categories of biological lipids, and is commonly used as a building block of more structurally complex lipids.
• The carbon chain, typically between 4 to 24 carbons long, may be saturated or unsaturated. A saturated fatty acid has all of the carbon atoms in its chain saturated with hydrogen atoms with general formula CH₃(CH₂)nCOOH where n is an even number.
• Mono-unsaturated fatty acids have one double bond in their structure while polyunsaturated fatty acids have two or more double bonds.
• The double bonds in polyunsaturated fatty acids are generally separated by at least one methylene group.
• Where a double bond exists, there is the possibility of either a cis or trans geometric isomerism, which significantly affects the molecule's molecular configuration.
• Cis-double bonds cause the fatty acid chain to bend, an effect that is more pronounced when more double bonds are there in a chain. This in turn plays an important role in the structure and function of cell membranes.
• Most naturally occurring fatty acids are of the cis configuration, although the trans form does exist in some natural and partially hydrogenated fats and oils.
• Shorter the chain of fatty acids lower is the melting temperature than those with longer chains.
• Unsaturated fatty acids have lower melting temperatures than saturated fatty acids of same chain length. (PPT. 4.1)
  

4.2.2 Glycerolipids

• Glycerolipids are composed mainly of mono-, di- and tri-substituted glycerols, the most well-known being the fatty acid esters of glycerol (triacylglycerols), also known as triglycerides or fats. In these compounds, all three hydroxyl groups of glycerol are esterified, usually by different fatty acids (Mixed Lipids).
• They function as a food store, these lipids comprise the bulk of storage fat in animal tissue and oil seeds.
• Triglycerides or fats may be either solid or liquid at room temperature, depending on their structure and composition.
• Oils" is usually used to refer to fats that are liquids at normal room temperature, while "fats" is usually used to refer to fats that are solids at normal room temperature. "Lipids" is used to refer to both liquid and solid fats, along with other related substances.

Fig. 4.2 Triacylglycerols

4.2.3 Glycerophospholipids

• Glycerophospholipids, also referred to as phospholipids, are key components of the lipid bilayer of cells, as well as being involved in metabolism and cell signaling.
• Neural tissue (including the brain) contains relatively high amounts of glycerophospholipids, and alterations in their composition has been implicated in various neurological disorders.
• Examples of glycerophospholipids found in biological membranes are phosphatidylcholine (also known as PC, or lecithin), phosphatidylethanolamine (PE ) and phosphatidylserine (PS ).
• Plasmalogens are also a type of glycerolipids that contain a fatty alcohol at C-1 of Sn glycerol with double bond instead of a fatty acid.

Fig. 4.3 Glycerophospholipids

4.2.4 Sphingolipids

• Sphingolipids are a complex family of compounds that share a common structural feature, a sphingoid base backbone that is synthesized de novo from the amino acid serine and a long-chain fatty acyl CoA, then converted into ceramides, phosphosphingolipids, glycosphingolipids and other compounds.
• The major sphingoid base of mammals is commonly referred to as sphingosine. Ceramides (N-acyl-sphingoid bases) are a major subclass of sphingoid base derivatives with an amide-linked fatty acid. The fatty acids are typically saturated or mono-unsaturated with chain lengths from 16 to 26 carbon atoms.

Fig. 4.4 Sphingolipids

4.2.5 Sterols

• Sterol lipids, such as cholesterol and its derivatives, are an important component of membrane lipids, along with the glycerophospholipids and sphingomyelins.
• The steroids, all derived from the same fused four-ring core structure, have different biological roles as hormones and signaling molecules. The eighteen-carbon (C18) steroids include the estrogen family whereas the C19 steroids comprise the androgens such as testosterone and androsterone. The C21 subclass includes the progestogens as well as the glucocorticoids and mineralocorticoids. The secosteroids, comprising various forms of vitamin D, are characterized by cleavage of the B ring of the core structure.
• Other examples of sterols are the bile acids and their conjugates, which in mammals are oxidized derivatives of cholesterol and are synthesized in the liver.

Fig. 4.5 Cholesterol

4.2.6 Prenol lipids

• Prenol, or 3-methyl-2-buten-1-ol, is a natural alcohol. It is one of the most simple terpenes.
• Prenol lipids are synthesized from the 5-carbon precursors isopentenyl diphosphate and dimethylallyl diphosphate that are produced mainly via the mevalonic acid (MVA) pathway.
• The simple isoprenoids (linear alcohols, diphosphates, etc.) are formed by the successive addition of C5 units, and are classified according to number of these terpene units.
• Structures containing greater than 40 carbons are known as polyterpenes.
• Carotenoids are important simple isoprenoids that function as antioxidants and as precursors of vitamin A.
• Another biologically important class of molecules is exemplified by the quinones and hydroquinones, which contain an isoprenoid tail attached to a quinonoid core of non-isoprenoid origin.
• Vitamin E and vitamin K, as well as the ubiquinones, are examples of this class.

4.2.7 Saccharolipids

• Saccharolipids describe compounds in which fatty acids are linked directly to a sugar backbone, forming structures that are compatible with membrane bilayers.
• In the saccharolipids, a monosaccharide substitutes for the glycerol backbone present in glycerolipids and glycerophospholipids.
• The most familiar saccharolipids are the acylated glucosamine precursors of the Lipid A component of the lipopolysaccharides in Gram-negative bacteria

Table 1.1 Common Fatty Acids