index
Announcements
Questions
Section 9.3: Triacylglycerols
Triacylglycerols are an Efficient Form of Energy Storage
Quantities of Fuel Reserves in Humans
Digestion of Triglyclerides
Conversion of Triglycerides to Fatty Acids
Lipid Transport in Chylomicrons
Structure of Chylomicrons
Section 9.4: Glycerophospholipids
Structure of Phosphatidates
Some Common Phosphatidates
Structures of Three Phosphatidates
Plasmalogens
Section 9.5: Sphingolipids
Glycosphingolipids
Section 9.6: Steroids
Section 9.7: Other Important Lipids
Eicosanoids
Aspirin Inhibits Prostaglandin Synthesis
Questions
References
Next Lecture: Sections 9.8 - 9.10

Fundamentals of Biochemistry

Biochem 380 - Fall 2006

Lecture 026

Outline

Announcements
Questions from previous lecture
Section 9.3: Triacylglycerols
Section 9.4: Glycerophospholipids
Section 9.5: Sphingolipids
Section 9.6: Steroids
Section 9.7: Other Important Lipids

Announcements

Lecture 25 notes are online

Questions

Any questions on the material from the previous lecture?

Section 9.3: Triacylglycerols

Fatty acids stored in the form of triacylglycerols. As mentioned previously, this is a concentrated form of energy, yielding about 37 kJ/g on oxidation, compared to around 16 kJ/g for proteins and carbohydrates
The structure of a triacylglycerol consists of three fatty acid chains joined to glycerol by ester linkages. Naturally-occurring fats and oils can may contain as many as 20 to 30 different types of triacylglcerols, with variations in chain length and degree of saturation
Examples include tripalmitin, which has three residues of palmitic acid, and triolein, with three oleic acid residues

Triacylglycerols are an Efficient Form of Energy Storage

Fats, stored as triacylglycerols, are the major fuel reserves in humans
The complete oxidation of 1 gram of fatty acids will produce 38 kJ of energy, compared to only 17 kJ per gram for carbohydrates and proteins
The greater energy density of fats is the result of two factors:
- The hydrocarbons in fats are more reduced than those in carbohydrates and proteins
- Carbohydrates and proteins are hydrated (interacting with water), reducing the amount of energy available per unit weight
The high concentration of fatty acids in adipocytes (fat cells) result in a purely lipid environment (no hydration)

Quantities of Fuel Reserves in Humans

The higher energy density of fat enables it to serve as the most efficient type of fuel reserve
In humans, the vast majority of the total energy in fuel reserves comes from stored fat
For comparison, a 70 kg man has the following amounts of fuel reserves:
- 170 kJ in glucose
- 2500 kJ in glycogen
- 100,000 kJ in protein (mostly from muscle)
- 420,000 kJ in fat

Digestion of Triglyclerides

When lipids are ingested in the form of triacylglycerols, they must be cleaved into individual fatty acids in order to be absorbed by intestinal epithelial cells
Because this digestion is done by pancreatic lipases in aqueous solution, the triglycerides must first be solubilized
Bile salts secreted from the gall bladder promote the solvation of triglycerides in the intestine
The amphipathic character of bile salts promotes the formation of micelles, where the ester bonds of the triglycerides are oriented toward the surface, where they can be accessible to the lipases
An example of a bile salt is glycocholate:

Conversion of Triglycerides to Fatty Acids

The fatty acid chains in a triglyceride are cleaved by distinct lipases
The process is sequential, beginning with cleavage of the first chain by a triacylglycerol lipase followed by cleavage by diacylglycerol lipase and monoacylglycerol lipase:

The end result consists of three fatty acid chains and a glycerol, which can subsequently converted to glyceraldehyde 3-phosphate (G3P)
G3P is then available for energy production through the glycolytic pathway or for conversion into glucose by gluconeogenesis

Lipid Transport in Chylomicrons

After digestion of triglycerides in the intestine and conversion into fatty acid chains, the fatty acids are absorbed by endothelial cells in the lining of the intestine, where they are subsequently converted back into triglycerides and packaged for transport to other cells in the body:

The packaging takes the form of lipoprotein particles called chylomicrons, which are released into the lymph system and then the blood

Structure of Chylomicrons

Chylomicrons belong to a family of lipid transport particles that also include HDL, LDL, and VLDL. Chylomicrons are the largest in size of these lipoproteins, and also have the highest lipid content
A chylomicron is composed mostly of triglycerides in the interior, surrounded by phospholipids, cholesterol and protein chains (apolipoproteins) on the outside:

Section 9.4: Glycerophospholipids

So far, we have seen how lipids such as triacylglycerols are used for energy storage. Another major function of lipids are as components in cell membranes. The most common types of lipids in membranes are glycerophospholipids
There are two types of glycerophospholipids: phosphatidates and plasmalogens. Both types have a phosphate group on the C3 carbon of a glycerol backbone.
Where they differ is in the linkage of the fatty acid groups. Phosphatidates have two fatty acids connected by ester linkages, and plasmalogens have a vinyl ether linkage

Structure of Phosphatidates

Like triacylglycerols, phosphatidates consist of fatty acids esterified to glycerol, but with just two chains, connecting to the C1 and C2 carbons
The third carbon is connected to a phosphate group, which in turn can be linked to an additional compound via a -OH group. This results in an amphipathic molecule with a long, nonpolar fatty acid tail and a polar head group
In general, phosphatidates have a saturated fatty acid esterified to C1 and unsaturated fatty acids esterified to C2

Some Common Phosphatidates

Structures of Three Phosphatidates

Plasmalogens

Plasmalogens are the other major type of glycerophospholipid. They are distinguished by a fatty acid chain connected to C1 by a vinyl ether linkage. The polar head group is usually ethnolamine or choline
Plasmalogens account for about 23% of the glycerophospholipids in the human central nervous system and are also present in peripheral nerve tissue and muscle tissue

Section 9.5: Sphingolipids

Sphingolipids are the next most abundant type of lipid found in cell membranes, and are especially abundant in the nervous systems of mammals
Their common structural component is sphingosine, an 18 carbon alcohol with a trans double bond at C4, amino group at C2 and hydroxyl groups at C1 and C3
The addition of a fatty acyl group at C2 produces ceramide
This is the precursor to two different families of sphingolipids: sphingomyelins and glycosphingolipids
Sphingomylein is a phospholipid with choline as the polar head group

Glycosphingolipids

Glycosphingolipids are lipids with carbohydrate groups attached to the C1 carbon of ceramide. They are classified into two different types: cerebrosides and gangliosides
Cerebrosides have one or more sugars attached by a β-glycosidic linkage. Common sugars include glucose and galactose
Gangliosides are more complex, with multiple sugars in the saccharide group, including a characteristic unit of N-acetylneuraminic acid (NeuNAc), a derivative of sialic acid

Section 9.6: Steroids

Steroids are a type of lipid built from the 5-carbon unit isoprene:
Steroids contain 4 rings: A, B, C and D, that result from ring closure reactions in the building block molecule squalene
Steroids are used as components in cell membranes (cholesterol), as hormones (testosterone) and as bile salts (sodium cholate, glycocholate)
With cholesterol, the highly planar ring structure results a rigid molecule that is used to modulate the fluid properties in cell membranes

Section 9.7: Other Important Lipids

Other important lipids include waxes, eicosanoids and the isoprene-derived vitamins such as Vitamin A, E and K that we have already seen
Waxes are nonpolar esters of long-chain fatty acids and alcohols. An example is myricyl palmitate, which is an ester of palmitate (16:0) and the 30-carbon alcohol myricyl alcohol:

The long, saturated chains produce highly hydrophobic solids at typical outdoor temperatures that provide protective waterproof coatings, used in plants and animals

Eicosanoids

Eicosanoids are oxygenated derivatives of C₂₀ polyunsaturated fatty acids such as arachidonic acid. They are used to regulate physiological processes including imflammation (prostaglandins), blood clotting (thromboxane) and smooth muscle contraction (leukotrienes)

Aspirin Inhibits Prostaglandin Synthesis

Aspirin (acetylsalicylic acid) is the most widely used drug in the world. It is a derivative of salicylic acid, an extract from willow bark long used for medicinal purposes
Aspirin reduces pain, fever and inflammation by inhibiting the synthesis of prostaglandins. It does this by selectively aceytlating the hydroxyl group of a serine residue in the cyclooxygenase (COX) portion of the prostaglandin H₂ synthase enzyme

Questions

Questions about the material covered today?

References

Prostaglandins

The mechanism of action of aspirin,
J.R. Vane, R.M. Botting,
Thrombosis Research 110 (2003) 255�258
(A brief history of aspirin and its mechanism in inhibiting PG2 synthesis)

Next Lecture: Sections 9.8 - 9.10

Read Sections 9.8 - 9.10

Table of Contents

Fundamentals of Biochemistry

Biochem 380 - Fall 2006

Lecture 026

Outline

Announcements

Questions

Section 9.3: Triacylglycerols

Triacylglycerols are an Efficient Form of Energy Storage

Quantities of Fuel Reserves in Humans

Digestion of Triglyclerides

Conversion of Triglycerides to Fatty Acids

Lipid Transport in Chylomicrons

Structure of Chylomicrons

Section 9.4: Glycerophospholipids

Structure of Phosphatidates

Some Common Phosphatidates

Structures of Three Phosphatidates

Plasmalogens

Section 9.5: Sphingolipids

Glycosphingolipids

Section 9.6: Steroids

Section 9.7: Other Important Lipids

Eicosanoids

Aspirin Inhibits Prostaglandin Synthesis

Questions

References

Prostaglandins

Next Lecture: Sections 9.8 - 9.10