Lipid Biochemistry (Part II)

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Simple lipids: Fats and oils 

(Storage lipids)

These are triglycerides or triacylglycerols.

These are esters of 3 fatty acid molecules with trihydroxy alcohol, glycerol.

A fat is solid at room temperature while oil is liquid.

Triglycerides are most abundant among all lipids, constituting about 98% of total dietary lipids.

They are major storage components or depot fats in plant and animal cells but are not normally found in membranes.

They are non-polar, hydrophobic molecules since they contain no electrically charged or highly polar functional groups.

Triglycerides which contain the same kind of fatty acids in all three positions are called simple triacylglycerols and are named after the fatty acid they contain. 

Simple triacylglycerols of 16:0, 18:0, and 18:1, for example, are tristearin, tripalmitin, and triolein, respectively.

Triglycerides which contain two or more different fatty acids are called mixed triacylglycerols.

To name these compounds unambiguously, the name and position of each fatty acid must be specified.

Functions of fats and oils

In eukaryotic cells, triacylglycerols form a separate phase of microscopic, oily droplets in the aqueous cytosol, serving as depots of metabolic fuel.

Adipocytes or fat cells store large amounts of triacylglycerols as fat droplets.

Triacylglycerols are also stored as oils in the seeds of many types of plants, providing energy and biosynthetic precursors during seed germination.

Adipocytes and germinating seeds contain lipases that catalyze the hydrolysis of stored triacylglycerols, releasing fatty acids for export to sites where they are required as fuel.

Humans have adipocytes under the skin, in the abdominal cavity, etc.

Seals, walruses, penguins, and other warm-blooded polar animals are amply padded with triacylglycerols which provide insulation.

In hibernating animals, the huge fat reserves accumulated before hibernation serve the dual purposes of insulation and energy storage.

In sperm whales, a store of triacylglycerols and waxes (low density) allows the animals to match the buoyancy of their bodies to that of their surroundings during deep dives in cold water.

Simple lipids: waxes 

(Energy Stores and Water Repellents)

Waxes are esters of fatty acids with high molecular weight monohydroxy alcohols.

Fatty acids range between C14 and C36 while alcohols range from C16 to C36.

Melting varies from 60° - 100ºC.

The term ‘wax’ originated from old English ‘weax’ meaning ‘material of the honeycomb’.

Carnauba wax is the hardest known wax. It consists of fatty acids esterified with tetracosanol [CH₃(CH₂)₂₂CH₂OH] and tetratriacontanol [CH₃(CH₂)₃₂CH₂OH].

Waxes are unusually inert due to their saturated nature of the hydrocarbon chain. However, they can be slowly split with hot alcoholic KOH.

Functions of waxes

In planktons, the chief storage form of metabolic fuel.

Certain skin glands of vertebrates secrete waxes to protect hair and skin and keep it pliable, lubricated, and waterproof.

Some aquatic birds, secrete waxes from their preen glands to keep their feathers water-repellent.

Leaves of many tropical plants are coated with a thick layer of waxes, which prevents excessive evaporation of water and protects against parasites.

Biological waxes find a variety of applications in the pharmaceutical, cosmetic, and other industries. 

Lanolin (from lamb’s wool), beeswax, carnauba wax (from a Brazilian palm tree), and wax extracted from spermaceti oil (from whales) are widely used in the manufacture of lotions, ointments, and polishes.

Compound lipids: Phospholipids 

(Structural lipids)

Phospholipids are the most abundant membrane lipids.

They serve primarily as structural components of membranes and are never stored in large quantities.

Phospholipids contain phosphorus in the form of phosphoric acid groups. They differ from triglycerides in possessing usually one hydrophilic polar ‘head’ group and usually two hydrophobic nonpolar ‘tail’ groups. They are often called polar lipids. Thus, phospholipids are amphipathic.

In phospholipids, two of the –OH groups in glycerol are linked to fatty acids while the third –OH group is linked to phosphoric acid. The phosphate is further linked to one of a variety of small polar head groups (alcohols).

Phospholipids can be classified into:

Phosphoglycerides

Phosphoinositides

Phosphosphingosides

Phosphoglycerides

Major phospholipids found in membranes.

It consists of two fatty acid molecules or ‘tails’ esterified with the first and second –OH groups of glycerol.

The third –OH group of glycerol forms an ester bond with phosphoric acid. An additional substituent group is esterified with the phosphoric acid. This is referred to as ‘head group’ as it is present at one end of the long phosphoglyceride molecule.

Of the two fatty acid molecules, the one on C1 is saturated (C16-C18) while the one on C2 is unsaturated (C18-C20).

C2 of glycerol is asymmetric in nature.

All phosphoglycerides contain a negative charge on the phosphoric acid at pH 7. In addition, the head group may also have one or more electrical charges at pH 7.

Phosphoglycerides are of 3 types:

Lecithins (Phosphatidyl cholines)

Cephalins (Phosphatidyl ethanolamine and phosphatidyl serine)

Plasmalogens (Phosphoglyceracetals)

Lecithins (Phosphatidyl cholines)

Found in various oil seeds like soybeans and yeasts. In animals, glandular and nervous tissues are rich in lecithins.

Required for normal transport and utilization of other lipids in the liver. In its absence, accumulation of lipids occurs in the liver to as much as 30% (against 3-4% in normal) giving rise to a condition called ‘fatty liver’. This may lead to fibrotic changes.

Lecithins contain two fatty acids esterified with any two –OH groups of glycerol while the third –OH group is esterified with a phosphoric acid group. The phosphoric acid group is again linked to a nitrogen base, choline.

Hydrolysis of lecithins

On complete hydrolysis, lecithins yield a mixture of choline, phosphoric acid, glycerol, and two moles of fatty acids.

But partial hydrolysis of lecithins by lecithinase (active components of snake venom) causes removal of one fatty acid to yield lysolecithins.

When subjected to the bloodstream (as a result of snakebite), lysolecithins cause rapid rupture of RBC (hemolysis).

Cephalins

These are closely associated with lecithins in animal tissues. They are also structurally similar to lecithins except that the choline is replaced by either ethanolamine or serine.

Accordingly, two types of cephalins are recognized:

Phosphatidyl ethanolamine

Phosphatidyl serine

Since the primary amino group is a weaker base than the quaternary ammonium group of choline, the cephalins are more acidic than lecithins. Cephalins are also comparatively less soluble in alcohol than lecithins.

Snake venoms containing lecithinase can also split off fatty acids from cephalins leaving hemolytic lysocephalins.  

Plasmalogens (phosphoglyceracetals)

Plasmalogens constitute about 10% of phospholipids in the brain and muscles. About half of heart phospholipids are plasmalogens.

These ether phospholipids are common in membranes of halophilic bacteria because they are resistant to phospholipases.

Structurally they are similar to lecithins and cephalins but have one of the fatty acid chains replaced by an unsaturated ether.

Since nitrogen base can be choline, ethanolamine, or serine, plasmalogens can be of three types:

Phosphatidal choline

Phosphatidal ethanolamine

Phosphatidal serine

Phosphoinositides (phosphatidyl inositols)

Phosphoinositides have been found to occur in phospholipids of brain tissues and soybeans. They play an important role in the transport process as well as a signaling intermediate in cells.

They have a cyclic hexahydroxy alcohol called inositol which replaces the base. The inositol is present as the stereoisomer, myo-inositol.

Number of phosphate groups may be one, two, or three. Accordingly, mono-, di- and triphosphoinositides are found.

Phosphosphingosides (sphingomyelins)

Commonly found in nerve tissues, especially in the myelin sheaths. Absent in plants and microorganisms.

In a syndrome called Niemann-Pick disease, the sphingomyelins are stored in the brain in large quantities.

They lack glycerol and instead contain sphingosine or a closely related dihydrosphingosine.

They are electrically charged molecules and contain phosphocholine as a polar head group.

Functions of phospholipids

In association with proteins, phospholipids form the structural components of membranes and regulate membrane permeability.

Phospholipids in the mitochondria maintain the conformation of electron transport chain components and thus cellular respiration.

They participate in the absorption of fats from the intestine.

They are essential for the synthesis of different lipoproteins and thus participate in the transport of lipids.

They prevent the accumulation of fats in the liver (lipotropic factors).

They participate in the transport of cholesterol and thus help in the removal of cholesterol from the body.

They act as surfactants (respiratory distress syndrome).

Cephalin participates in blood clotting.

Phosphatidyl inositol is the source of the second messenger that is involved in the action of some hormones.