Chapter 10: Lipids

A brief introduction to lipids

Adapted from Wikipedia:

• Lipids are a broad group of naturally-occurring molecules that include: (i.) fatty acids, (ii.) triacylglycerides, (iii.) phospholipids, (iv.) sphingolipids, (v.) waxes, (vi.) sterols, (vii.) terpenes, and more.

• The main biological functions of lipids include energy storage, as structural components of cell membranes, and as important signaling molecules.

• Lipids may be broadly defined as hydrophobic or amphiphilic small molecules; the amphiphilic nature of some lipids allows them to form structures such as vesicles, liposomes, or membranes in an aqueous environment.

• Although the term lipid is sometimes used as a synonym for fats, fats are a subgroup of lipids called triacylglycerides.

• Although humans and other mammals use various biosynthetic pathways to both break down and synthesize lipids, some essential lipids cannot be made this way and must be obtained from the diet.

• Of the classes of biological molecules that we have covered in this class (proteins, carbohydrates, nucleic acids, and now lipids), lipids are the most structurally diverse…yet in some ways (i.e., they all contain nonpolar moieties), they are also quite uniform.

(i.) Fatty acids

Adapted from Wikipedia:

• A fatty acid is a carboxylic acid often with a long unbranched aliphatic tail (chain), which is either saturated or unsaturated.

• The most abundant natural fatty acids have an even number of carbon atoms because their biosynthesis involves acetyl-CoA, a coenzyme carrying a two-carbon-atom group (see fatty acid synthesis).

• Fatty acids are produced by the hydrolysis of the ester linkages in a fat or biological oil (both of which are triglycerides), with the removal of glycerol.

• It is proposed that the blend of fatty acids exuded by mammalian skin, together with lactate and pyruvate, are distinctive and enable animals with a keen sense of smell to differentiate individuals.

Fatty acid structure

Saturated vs. unsaturated fatty acids

Fatty acid nomenclature

Some common fatty acids

Cis vs. trans unsaturated fatty acids

(ii.) Triacylglycerides (TAGs)

Adapted from Wikipedia:

• Glycerol + 3 FA = TAG.

• TAGs are the main constituent of vegetable oil and animal fats.

• Chain lengths of the fatty acids in naturally occurring triglycerides can be of varying lengths, but 16, 18 and 20 carbons are the most common.

• Most natural fats contain a complex mixture of individual TAGs; consequently, they typically melt over a broad range of temperatures.

• Cocoa butter is unusual in that it is composed of only a few TAGs, which gives rise to a fairly sharp melting point, causing chocolate to melt in the mouth without feeling greasy.

Fat is stored as TAGs in lipocytes,which are also known as adipocytes

Saponification = alkali hydrolysis of esters

(iii.) Phospholipids

Adapted from Wikipedia:

• Glycerol + 2 FA + phosphate + alcohol = phospholipid.

• Phospholipids are a class of lipids, which are the major lipid component of all cell membranes as they can form bilayers.

• The 'head' of a phospholipid is hydrophilic (attracted to water), while the hydrophobic 'tails' repel water.

o The hydrophillic head contains the negatively charged phosphate group, and may contain other polar groups.

o The hydrophobic tail usually consists of long fatty acid hydrocarbon chains.

• When placed in water, phospholipids form a variety of structures depending on the specific properties of the phospholipid.

Phospholipid structure and aggregation

Phospholipids are classified basedon their alcohol head group

Phospholipase is an enzyme that hydrolyzes phospholipids into fatty acids and other lipophilic

substances

(iv.) Sphingolipids

Adapted from Wikipedia:

• Sphingolipids are a class of lipids derived from the aliphatic amino alcohol sphingosine.

• Ceramide is the fundamental structural unit common to all sphingolipids…it consists of a fatty acid chain attached through an amide linkage to sphingosine.

• There are three main types of sphingolipids, differing in their head groups:

o Sphingomyelins have a phosphorylcholine or phosphoroethanolamine head group.

o Glycosphingolipids are ceramides with one or more sugar residues joined in a β-glycosidic linkage at the 1-hydroxyl position.

Cerebrosides have a single glucose or galactose at the 1-hydroxy position.

Gangliosides have at least three sugars, one of which must be sialic acid.

Sphingolipid structure

Phosphatidylcholine vs. sphingomyelin

Myelin sheath

Dendrite

Soma

Nucleus

Myelin sheath

Node of Ranvier

Axon terminal

Schwann cell

Axon

Myelin sheath

Adapted from Wikipedia:

• Myelin is a dielectric (electrically insulating) material that forms a layer, the myelin sheath, usually around only the axon of a neuron.

• It is essential for the proper functioning of the nervous system. Myelin is an outgrowth of a glial cell…Schwann cells supply the myelin for peripheral neurons.

• Myelin is composed of about 80% lipid and about 20% protein.

o Myelin is made up primarily of a sphingolipid called galactocerebroside.

o The intertwining of the hydrocarbon chains of sphingomyelin serve to strengthen the myelin sheath.

• Demyelination is the loss of the myelin sheath insulating the nerves, and is the hallmark of some neurodegenerative autoimmune diseases, including multiple sclerosis.

(v.) Waxes

Adapted from Wikipedia:

• Wax refers to beeswax or another substance with similar properties...the traditional meaning, beeswax, refers to a substance secreted by bees and used by them in constructing their honeycombs.

• The term has come to refer more generally to a class of substances with properties similar to beeswax: (a) plastic (malleable) at normal ambient temperatures, (b) a melting point above approximately 45 °C (113 °F) (which differentiates waxes from fats and oils), (c) a relatively low viscosity when melted (unlike many plastics), (d) insoluble in water, and (e) hydrophobic.

• Chemically, a wax is a type of lipid that may contain a wide variety of long-chain alkanes, esters, polyesters, etc.

o They are usually distinguished from fats by the lack of triglyceride esters.

o Many biological waxes are mixtures of several different types of lipids.

Beeswax is an ester ofpalmitic acid with the alcohol triacontanol

(vi.) sterols

Adapted from Wikipedia:

• Sterols are an important class of organic molecules.

• They occur naturally in plants, animals and fungi, with the most familiar type of animal sterol being cholesterol.

• Cholesterol is vital to cellular function, and a precursor to fat-soluble vitamins and steroid hormones.

Cholesterol

Lipoprotein(aka chylomicron)

Steroids derived from cholesterol

Steroids derived from cholesterol

(vii.) Terpenes = molecules constructed of isoprene

Adapted from Wikipedia:

• Terpenes are a large and varied class of hydrocarbons, produced primarily by a wide variety of plants, particularly conifers.

• They are the major components of resin and of turpentine, which is produced from resin. In fact, the name "terpene" is derived from the word "turpentine".

• In addition to their roles as end-products in many organisms, terpenes are major biosynthetic building blocks within nearly every living creature. Steroids, for example, are derivatives of the triterpene squalene.

• When terpenes are modified chemically, such as by oxidation or rearrangement of the carbon skeleton, the resulting compounds are generally referred to as terpenoids. Some authors will use the term terpene to include all terpenoids. Terpenoids are also known as Isoprenoids.

Some common terpenes

Conjugated terpenes are commonly usedpigments in plants and bird feathers

Many vitamins are terpenes