Unit 3 - Lipids

Lipids

naturally occurring organic compounds that are grouped together due to their insolubility in water and other polar solvents. They are, however, soluble in non-polar solvents, such as ether, chloroform, and carbon tetrachloride.

Functions of Lipids

1\. Storage form of energy (9 kcal/g)

2\. Structural components of biological membranes

3\. Metabolic regulators

4\. Act as surfactants, detergents and emulsifying agents

5\. Act as electric insulators in neurons

6\. Provide insulation against changes in external temperature

7\. Give shape and contour to the body

8\. Protect internal organs by providing a cushioning effect

9\. Help in absorption of fat soluble vitamins (A, D, E and K)

10\. Improve taste and palatability of food

Fatty acids

are often regarded as the building block of lipids by some textbooks. But it is not, because there are other lipids that do not contain fatty acids. Accurately, they are considered to be the commonly encountered component of lipids. They considered as derived lipids. making it amphipathic.

cis fatty acid

same orientation. is the most common form of naturally occurring fatty acids.

trans fatty acid

different orientation. They are not common, but they exist. They are present in dairy products and in hydrogenated edible oils because they increase the shelf life of the fried food.

Omega-3 (ω-3) Fatty Acids

These are unsaturated fatty acids where their endmost double bonds are 3 carbons away from their ω carbon. have positive roles in infant development, cancer, cardiovascular diseases, and in various mental illnesses, including depression, attention-deficit hyperactivity disorder (ADHD), and dementia.

Omega-6 (ω-6) Fatty Acids

These are unsaturated fatty acids where their endmost double bonds are 6 carbons away from their ω carbon

Linoleic acid

starting material for the biosynthesis of arachidonic acid. Arachidonic acid is the major starting material for eicosanoids, substances that help regulate blood pressure, clotting, and several other important body functions.

Linolenic acid

starting material for the biosynthesis of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). EPA and DHA are necessary for normal brain development.

unsaturated fatty acids nonpolar tails

30o kink on the structure.

The hydrophobic interaction

is the type of intermolecular force present in the nonpolar tails of the fatty acids.

Water solubility

The solubility of fatty acids in water is a direct function of carbon chain length, i.e. solubility decreases as carbon chain length increases.

Melting Point

for fatty acids are strongly influenced by both carbon chain length and degree of unsaturation (number of double bonds present).

triacylglycerols (TAG)

The type of lipids present in fats and oils Other sources still refer them as triglycerides, which is a much older name

The main function of TAG

r long-term energy storage.

What special cell TAG is concentrated in?

adipocytes

Where is adipocytes located?

located in the subcutaneous layer of the skin. Adipose tissue is abundant in the lower abdomen, and in mammary gland in females.

Structural Definition of Triacylglycerols

is defined as a triester of glycerol and 3 fatty acids Each fatty acid is attached to the glycerol backbone through an ester bond, hence they are also referred to as acyl groups

Formation of Triacylglycerols

Triacylglycerols are formed by dehydration synthesis reaction between a glycerol molecule and 3 fatty acids.

for every molecule of triacylglycerol produced

3 water molecules are formed

formation of trilauroylglycerol

a TAG abundantly found in coconut oil

Simple triacylglycerols

comprise 3 identical fatty acids. The example above, trilauroylglycerol, is an example of simple triacylglycerol.

Mixed triacylglycero

comprise 1 or more of 3 fatty acids that are not identical. Biologically important TAGs are usually mixed. When a PUFA is present in a TAG, it is usually located at the 2nd fatty acid.

Where does polyunsaturrated fatty acid usually present in a TAG?

it is usually located at the 2nd fatty acid.

General rules in nomenclature for tryglycerol

1\. The name of triacylglycerol takes glycerol as the parent name.

2\. The names of the fatty acids are converted by replacing the suffix –ic acid into –oyl.

For simple triacylglycerols (nomenclature)

\
a. Convert the names of the fatty acid present according to the convention (replace the suffix –ic acid to –oyl).

b. Put the word tri- at the beginning of the name of the fatty acid.

c. Attached the parent name glycerol as a suffix.

\

For mixed triacylglycerols: (nomenclature)

a. The fatty acids are designated with locants 1, 2, and 3, where 1 is given for the fatty acid drawn on top, 2 is given for the one in the middle, and 3 for the one at the bottom. The names of these fatty acids are converted by replacing the suffix –ic acid to –oyl.

b. If a fatty acid occurs twice in the molecule, use the prefix di-.

c. The final name is taken by arranging the locants in increasing manner and attaching the parent name glycerol at the end.

Fats and oils

are mixture of triacylglycerol present in biological systems. have greasy feeling and non- volatile which causes the formation of translucent spot in paper. They are colorless, tasteless, and odorless.

Fats

* rich in saturated fatty acids
* solid at room temperature
* animals

Oil

* rich in unsaturated fatty acids
* liquid at room temperature
* plants, except for fish oils

Saturated fats

(rich in saturated FA) are bad fats. These types of fats can increase the risk of heart diseases.

Monounsaturated fats

(rich in MUFA) are good fats. These fats can decrease the risk of heart disease and breast cancer. MUFAs help reduce the stickiness of platelets, which helps prevent formation of blood clots and may dissolve clots once they form. Monounsaturated fats are present in olives, avocado, and canola oils. They are also present in tree nuts and peanuts.

Polyunsaturated fats

(rich in PUFA) can either be good or bad. The can reduce risk of heart disease, but promote the risk of certain types of cancers.

Fats Substitutes

These are substances that replicate the taste, texture, and cooking properties of fats but are themselves not lipids. There are two-types of artificial fats - calorie-reduced fat substitute and calorie-free fat substitute.

Acrolein Test

The glycerol backbone of a triacylglycerol can undergo dehydration reaction using potassium bisulfate (KHSO4) as dehydrating agent forming acrolein (2-propenal). Acrolein has black appearance and burnt fat odor. The same substance is produced when cooking oil is used repeatedly or when grilling pork fat for a long time.

Hydrogenation (Catalytic reduction)

Under high pressure and in the presence of a transition metal catalyst (e.g. Ni, Pt, Pd), the double bonds of unsaturated fatty acids present in triacylglycerols, undergoes addition reaction with hydrogen gas (H2). In the process, each carbon atom involved in a double bond is added with one hydrogen atom resulting to converting double bonds into single bonds.

Acidic Hydrolysis

In the presence of a strong acid (H+ ), triacylglycerol reacts with water breaking the ester bonds in between the glycerol backbone and the fatty acids. Consequently, 1 glycerol molecule and 3 fatty acid molecules are produced.

Enzymatic hydrolysis

Triacylglycerol are hydrolyzed in humans by the enzyme lipase Hydrolysis by lipase occurs in a stepwise manner following the order: first fatty acid —> third fatty acid —> second fatty acid. But lipase hydrolysis rarely goes to completion; most of the time, the products are 2 fatty acids and 1 monoacylglycerol (the 2nd fatty acid is not removed).

Saponification (Basic Hydrolysis)

In the presence of strong base, such as NaOH and KOH, the ester bonds in triacylglycerols are broken down without the action of water. In return, a glycerol molecule is produced together with 3 fatty acid salt whose cation is from the used base. The fatty acid salt products are commonly known as soap. Hard soap is produced when using NaOH, while liquid soap is produced when using KOH.

Rancidity

Fats and oils may develop unpleasant smell and taste over time

Hydrolytic rancidity

is due to partial hydrolysis of the triacylglycerol molecules due to traces of hydrolytic enzymes present in naturally occurring fats and oils.

Oxidative rancidity

is the result of partial oxidation of unsaturated fatty acids with resultant formation of epoxides of small molecular weight fatty acids by peroxides and free radicals. The Page 12 of 21 same process, if it occurs in vivo will affect the integrity of biomembranes, leading to cell death. Many fats and oils may contain antioxidants (e.g. vitamin E) to prevent oxidative rancidity. PUFA are more easily oxidized; so vegetable oils with a high content of PUFA are usually preserved with addition of antioxidants.