General features of lipids:

• High content of hydrocarbons
• Low solubility in water
• Hydrocarbon is a highly-reduced form of Carbon. Oxidation to an alcohol or keto group releases energy (oxygen likes to grab two electrons and will take these from carbon, releasing energy as it does)

• Great choice for an "energy-storage" molecule:
• Lots of energy released upon oxidation
• Physical properties (insolubility) provide an easy way to compartmentalize for storage in aqueous systems

• Notice there is no N or P listed in this energy diagram. These atoms are required for polymers of proteins and nucleic acids (i.e. information containing molecules). There is something essential about group 5 elements for the construction of information containing polymers.

Fatty Acids

Have a molecular organization consisting of a hydrocarbon "tail" and carboxyl group "head"

• The carboxyl group is ionized under normal conditions, giving a negative charge to one end of the molecule.
• Hydrocarbon tail usually contains an even number of carbons, with typical lengths of 14-24 carbons. Odd numbered hydrocarbon tails are found in some marine organisms.
• If the hydrocarbon tail is composed exclusively with carbons having single bonds, it is saturated.
• Single bonds are free to rotate, and therefore, saturated fatty acids are quite flexible. Although they are free to adopt a variety of conformations, the extended (trans-) conformation is the lowest energy.
• The trans conformation can pack side by side efficiently. This results in close-packing that is solid-like (i.e. not much space for movement)
• If there are double bonds present, then the tail is unsaturated. A mono-unsaturated fatty acid means there is one double bond present. Polyunsaturated means there are several double bonds in the fatty acid tail.
• Double bonds are not free to rotate. Thus, the introduction of a double bond limits the conformational freedom of the fatty acid
• Double bond in unsaturated fatty acids nearly always in the cis (kinked) configuration. This precludes efficient side by side packing. Thus, there is space for molecules to move around and the arrangement of unsaturated fatty acids is more fluid than with saturated fatty acids (even though they are less-flexible molecules overall compared to saturated fatty acids!).

Naming of fatty acids can be done in one of three ways:

1. The systematic name
2. The common name
3. Shorthand notation

• The number of carbons (including carboxyl carbon) followed by a colon and then the number of double bonds. Sometimes the location of the double bond(s) is specified in parentheses after this (referencing the carbonyl as carbon #1).

"Essential" fatty acids. Some fatty acids are essential for animal growth, but only plants can make them. So animals must consume plants that contain these fatty acids (e.g. linoleic, and g-linoleic acid)

Cardiovascular disease. The incidence of cardiovascular disease is associated with diets high in saturated fatty acids (e.g. palmitic acid)

Triacylglycerols (triglycerides)

• These compounds represent one of the major forms of fatty acids in the body
• Are a major energy reserve in the body
• Consist of glycerol esterified with three fatty acids
• If all three fatty acids are the same, it is a simple triglyceride
• If they are not all the same, it is a mixed triglyceride

• Triglycerides are stored in body fat in animals
• The fatty acid tail of triglycerides is rich in reduced carbon. These can be oxidized with the liberation of large amounts of energy (about twice as much per weight in comparison to carbohydrates or proteins)
• Triglycerides are also a good insulator. Arctic animals rely on body fat for both energy and insulation.

Glycerophospholipids (also known as phosphoglycerides, or glycerol phosphatides)

• A 1,2-diacylglycerol molecule that has a phosphate group esterified at the glycerol carbon number 3.

• Phosphoglycerides are one of the largest class of lipids in the body, and an essential component of cell membranes
• Glycerophospholipids are a subclass of the general class of phospholipids
• The C2 carbon of the glycerol moiety is chiral. Thus, there can be chiral forms of a phosphoglyceride.

Prochiral centers and sn- system of naming chiral centers

• The C2 of glycerol is not chiral. The mirror image can be rotated to overlay the original:

• However, if either of the CH₂OH groups is modified, then the C2 atom in glycerol becomes chiral. Thus, the C2 carbon is said to be prochiral.
• The positions of the CH₂OH groups therefore do have uniquely different stereochemical identies should the prochiral C2 carbon become chiral through some kind of chemical modification of one of the CH₂OH groups.

• A prochiral center is indicated by the prefix sn-

Common Phospholipids

The parent compound for the glycerol-based phospholipids is phosphatidic acid which is composed of:

• sn-glycerol-3-phosphate (phosphate at pro-R position, which will be called C3).
• C18 saturated fatty acids esterified at the C1 and C2 positions

• A variety of polar groups are esterified to the phosophoric acid group of phosphatidic acid. This phosphate ester structure is referred to as the "head group" of the phospholipid

Ether Glycerophospholipids

This class of phospholipids has an ether linkage to the hydrophobic tail at the C1 position of the glycerol, instead of an acyl linkage:

• Often the group bonded to the C2 is a small acetate (2-carbon) unit. This results in an increase in the aqueous solubility of the phospholipid
• Often the group bonded to the C1 contains a cis-double bond in the aliphatic chain (between C1 and C2). These molecules are a class known as plasmalogens.

Sphingolipids

Sphingolipids are are built up starting with an 18-carbon amino alcohol molecule called sphingosine:

• Sphingomyelins are a subclass of sphingolipids that contain a phosphate group. They are an important element in the insulating myelin sheath surrounding nerve cells
• Glycosphingolipids contain one or more carbohydrate groups in a b-glycosidic linkage to the hydroxyl group of the C3 carbon in the glycerol backbone.
• If the carbohydrate is a single glucose or galactose then the molecule is a cerebroside.
• More complex carbohydrate head groups on a glycosphingolipid produces a ganglioside.
• Glycosphingolipids are present in low concentrations in cell membranes, but appear to play a key role in cell-cell recognition.
• Glycosphingolipid accumulation is associated with several genetic diseases.

Waxes

These are esters of long-chain alcohols with long chain fatty acids:

• The small polar ester group provides minimal solubility, and waxes are essentially insoluble. Because of this, however, they are useful in the "waterproofing" of biological structures (e.g. feathers)

Terpenes

These are a class of lipids built up from a 5-carbon basic building block known as an isoprene unit (2-methyl-1,3-butadiene)

• Two isoprene units bonded together form a monoterpene (C₁₀ molecule)
• There are various orientations that the isoprene units can adopt when forming such bonds

• Monoterpenes occur in all higher plants, and are the building blocks of cholesterol and other steroids, and carotenoids (photosynthetic pigments).
• Long-chain polymers of isoprene units with a terminal alcohol are called polyprenols. These have functionalities ranging from carbohydrate transfer molecules to hydrophobic anchor moieties in conjugated lipoproteins.
• Aldehydes and alcohols of monoterpenes are aromatic compounds in plants (e.g. menthol and citronella)
• Rubber is a terpene polymer

Steroids

The steroids are an important class of terpene-based lipids. The fundamental structural unit of steroids is described by three six-membered rings, and one five-membered ring fused together. The functional properties of this basic class of molecules is extremely diverse.

• Cholesterol. The principle component of animal cell plasma membranes. Also a component of lipoprotein complexes in blood, and of plaques that form on arterial walls in atherosclerosis.

• Steroid Hormones. Steroids derived from cholesterol in animals include five families:
• Androgens. Mediate development of sexual characteristics and function (e.g. testosterone, estradiol)
• Progestins. Regulate menstrual cycle (e.g. progesterone)
• Glucocorticoids. Regulate carbohydrate, protein and lipid metabolism (e.g. cortisol)
• Mineralocorticoids. Regulate salt balance in tissues
• Bile acids. Detergents secreted by gallblader that help solubilize lipids in diet (e.g. deoxycholic acid)

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