index
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Chapter 9: Lipids and Membranes
Section 9.1: Diversity of Lipids
Functional Roles of Lipids
Section 9.2: Fatty Acids
Fatty Acid Chain Length and Saturation
Fatty Acid Chain Length and Saturation, 2
Structures of 3 Fatty Acids
Ball-and-Stick Models of 3 Fatty Acids
Fatty Acid Nomenclature
Atom Numbering in Fatty Acids
Double-Bond Nomenclature
Common Fatty Acids
Origins of Some Common Names of Fatty Acids
Trans Fats
Questions
References
Next Lecture: Sections 9.3 - 9.7

Fundamentals of Biochemistry

Biochem 380 - Fall 2006

Lecture 025

Outline

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Questions from previous lecture
Section 9.1: Diversity of Lipids
Section 9.2: Fatty Acids

Announcements

Lecture 24 notes are online

Questions

Any questions on the material from the previous lecture?

Chapter 9: Lipids and Membranes

Lipids are the third major class of molecules in organisms. Because they are mostly hydrocarbons, lipids are distinct from proteins and carbohydrates in being very hydrophobic and insoluble
This property of lipids is associated with their use in cell membranes, which are hydrophobic layers that surround and separate the aqueous regions of cells
Cell membranes form unique 2-dimensional environments in which hydrobphobic molecules can diffuse laterally. The lipid bilayer also provides a framework for embedded proteins that function as pumps, channels, and receptors, allowing certain materials to pass through while blocking others (selective permeability)
Consequently, lipids and membranes play many important roles in the communication systems of cells and multicellular organisms. The actions and effects of natural signals such as hormones, and synthetic molecules such as drugs, are crucially dependent upon detailed interactions that begin at the membranes and receptors that are the 'doorways' to the interiors of living cells

Section 9.1: Diversity of Lipids

Lipids have a greater structural diversity than proteins and carbohydrates, and consequently the classification of different types of lipids is more complex
However, all lipids are built from one of two simple building blocks: fatty acids and isoprenoids

Functional Roles of Lipids

Cell membranes are among the most significant lipid structures. Membranes are composed of many different types of lipids, including glycerophospholipids and sphingolipids. In animals, cholesterol is also an major lipid component in cell membranes
Another important role of lipids is their use as energy storage molecules. Because they are more reduced than other energy molecules such as carbohydrates, the energy content of lipids is about twice as great. In animals, fats and oils also provide thermal insulation and padding
Waxes are esters of long, saturated hydrocarbon chains that form highly insoluble solids. They act as waterproof and protective coatings on plant cells, on exoskeletons in insects, and on fur and feathers in animals
Many hormones are composed of steroids, which help to regulate and integrate metabolic activities in animals. Eicosanoids are derivatives of polyunsaturated fatty acids, and help to regulate blood pressure, body temperature and smooth-muscle contraction

Section 9.2: Fatty Acids

Fatty acids are one of the main building blocks for larger lipid molecules
A fatty acid consists of a long hydrocarbon tail attached to a carboxylate group:

The structure of fatty acids can vary in the length of the hydrocarbon chain, and in the amount, position and type of saturation of the carbon bonds
A fully saturated hydrocarbon is one without any double bonds
One or more double bonds in the chain results in an unsaturated fatty acid

Fatty Acid Chain Length and Saturation

Variation in chain length and amount of saturation in fatty acid chains will affect its chemical properties such as shape and melting point
There are usually an even number of carbons in the chain of fatty acids, typically between 14 and 24 in length
Lengths between 16 and 18 are most common
The melting points of fatty acids decrease with decreasing chain length. This is because shorter chains have fewer numbers of van der Waals contacts and do not pack together as tightly at higher temperatures, relative to longer chains
For example, the melting point of palmitic acid (C₁₆) is 6.5° less than stearic acid (C₁₈)

Fatty Acid Chain Length and Saturation

The degree of unsaturation (number of double bonds) will also affect the shape and melting point
The presence of a single double bond will cause a bend in the linear chain:

Like shorter chain lengths, unsaturation will also decrease the melting point of a fatty acid. This is because the double bonds in a chain will produce bends, so that the chains do not pack together as tightly as straight chains
For example, the melting point of stearic acid (C₁₈) is 69.6°C compared to the melting point of oleic acid (C_18:1), which is 13.4°C

Structures of 3 Fatty Acids

Ball-and-Stick Models of 3 Fatty Acids

Fatty Acid Nomenclature

The name of a fatty acid is derived from the name of its parent hydrocarbon
For example, the hydrocarbon octadecane, with 18 carbons, is the parent of the fatty acid octadecanoic acid
Double bonds in the chain are indicated with the notation N:D, where N is the number of carbons and D is the number of double bonds
For example, 18:1 refers to a chain with 18 carbons and 1 double bond, or octadecenoic acid
18:2 refers to a chain with 18 carbons and 2 double bonds, or octadecadienoic acid
18:3 refers to a chain with 18 carbons and 3 double bonds, or octadecatrienoic acid, etc.

Atom Numbering in Fatty Acids

Atoms in the fatty acid chain are numbered starting from the carboxylate end:

The carboxylate carbon is number 1
The next carbon is number 2, and is referred to as the α carbon
The next carbon is number 3, the β carbon, and so on down the chain
The final carbon at the distal end of the chain is referred to as the ω carbon (ω is the final letter in the greek alphabet)

Double-Bond Nomenclature

Double bonds in the fatty acid chain can be in either the cis or trans configuration
The position of a double bond in the chain is indicated by the symbol Δ and a superscript number
For example, cis-Δ⁹ refers to a cis double bond between carbon atoms 9 and 10 in a chain
trans-Δ² refers to a trans double bond between carbon atoms 2 and 3
Double bonds are also referred to relative to the ω carbon at the distal end of the chain
For example, an ω-3 fatty acid is one with a double bond between the third and fourth carbons from the end of the chain:

Common Fatty Acids

Some naturally occurring fatty acids have a common name in addition to their systematic name
A number of common fatty acids are listed in table 9.1 in the text:

Origins of Some Common Names of Fatty Acids

Number of Carbons	Number of Double Bonds	Fatty Acid	Origin
12	0	Laurate	present in oil from the laurel plant (Laurus nobilis) (1873)
14	0	Myristate	oil from nutmeg (Myristica fragrans) (1848)
16	0	Palmitate	from palm oil (1857)
18	0	Stearate	from French stearique, referring to fat from steers, or tallow (1831)
18	1	Oleate	from Latin oleum (oil) (1899)
18	2	Linoleate	found in linseed oil lin + oleate (1857)

Trans Fats

Differences in metabolism of saturated and unsaturated fatty acids may explain some health risks associated with consumption of different types of fat. Traditionally, solid fats used in food preparation contained saturated fats such as lard and butter, obtained from animal sources
In the early 20th century, a method called hydrogenation was developed to produce trans fats from cheaper sources such as vegetable oils, creating products such as margarine and partially hydrogenated vegetable oil. The metal ions that catalyze partial hydrogenation in this process also convert some cis unsaturated bonds to the lower energy trans configuration
Excessive consumption of foods high in saturated and trans fats may lead to elevated blood levels of cholesterol and low-density lipoprotein (LDL), which are associated with an increased risk of cornary heart disease (CHD), a leading cause of death in developed countries
Consequently, FDA guidelines recommend reduced intake of saturated fats and avoidance of trans fats. A new labeling law went into effect on Jan 1, 2006 that requires labeling of packaged foods that contain trans fats

Questions

Questions about the material covered today?

References

Trans Fats

Revealing Trans Fats
http://www.fda.gov/fdac/features/2003/503_fats.html
FDA Consumer Magazine article on Trans Fats, Sep 2003
(Recommends reduction or removal of trans fats from the diet)

Next Lecture: Sections 9.3 - 9.7

Read Sections 9.3 - 9.7

Table of Contents

Fundamentals of Biochemistry

Biochem 380 - Fall 2006

Lecture 025

Outline

Announcements

Questions

Chapter 9: Lipids and Membranes

Section 9.1: Diversity of Lipids

Functional Roles of Lipids

Section 9.2: Fatty Acids

Fatty Acid Chain Length and Saturation

Fatty Acid Chain Length and Saturation

Structures of 3 Fatty Acids

Ball-and-Stick Models of 3 Fatty Acids

Fatty Acid Nomenclature

Atom Numbering in Fatty Acids

Double-Bond Nomenclature

Common Fatty Acids

Origins of Some Common Names of Fatty Acids

Trans Fats

Questions

References

Trans Fats

Next Lecture: Sections 9.3 - 9.7