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.

Lipid Biochemistry (Part I)

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Lipids

Lipids are water-insoluble biomolecules that are highly soluble in organic solvents like chloroform.

Lipids are a heterogeneous group of compounds related to fatty acids and include fats, oils, waxes, and other related substances.

Lipids are hydrophobic in nature.

The term ‘lipid’ was first coined by a German biochemist, Bioor, in 1943.

Chemically fats are defined as the esters of glycerol and fatty acids or triglycerides of fatty acids.

Functions of lipids

A diverse range of functions in biological systems:

Important constituents of diet because of their high energy value

Serve as a source of fat-soluble vitamins and essential fatty acids in natural foodstuffs.

Serve as a stored form of energy in adipose tissues.

Serve as an insulating material in subcutaneous tissues and around certain organs.

Lipoproteins are important constituents of biological membranes.

Plays crucial roles as:

enzyme cofactors, 

electron carriers, 

light absorbing pigments, 

emulsifying agent in digestive tracts, 

hormones and intracellular messengers

May act as chaperones to help membrane proteins fold.

Advantage of energy storage as fatty acids

The carbons in the fatty acids (mostly –CH₂ group) is almost completely reduced compared to the carbon in other simple biomolecules (sugars, amino acids, etc.). Therefore, oxidation of fatty acids will yield more energy (in form of ATP) than any other form of carbon.

Fatty acids are not hydrated as mono- and polysaccharides and thus can be packed more closely in storage tissues.

Fatty acids

Fatty acids are hydrocarbon chains of various lengths and degrees of unsaturation that terminate with the carboxyl group.

Fatty acids that occur in natural fats are usually monocarboxylic.

They contain an even number of carbon atoms as they are synthesized from condensation of 2 carbon units. 

These range from 4 to 36 C-atoms.

The chain may be saturated (no double bonds) or unsaturated (contain one or more double bonds).

Some fatty acids may have hydroxyl groups in the chain (hydroxy-fatty acid) and still, others may possess ring structures (cyclic-fatty acids).

Alcohols in lipid molecules

Alcohols present in lipid molecules commonly include glycerol, cholesterol, and higher alcohols like cetyl alcohol and mericyl alcohol.

In the structural formula of glycerol, the C-atoms are numbered 1, 2, 3 from any end. Since C1 and C3 are identical, they are also denoted as α, β, and α’.

Nomenclature of fatty acids

The systematic nomenclature of fatty acids is based on the Genevan System under the instruction of IUPAC.

The fatty acid is named after the hydrocarbon with the same number of carbon atoms, the suffix –oic is written in place of the final letter ‘-e’ in the name of the hydrocarbon.

The names of the saturated fatty acids end with the suffix –anoic and those of unsaturated fatty acids with the suffix –enoic.

The positions of the carbon atoms are denoted by numbering (carboxyl carbon atom is C1, the adjacent carbon atom is C2 and so on) or by using Greek letters (C2 is denoted as α-carbon, C3 is denoted as β-carbon and so on).

A widely used convention is to indicate the carbon atom number followed by the number and position of double bonds in the case of unsaturated fatty acids (position of a double bond is indicated by a lower number of the two carbon atoms involved in double bonding).

In nearly all naturally occurring unsaturated fatty acids, the double bonds are in the cis configuration.

Trans fatty acids are produced by fermentation in the rumen of dairy animals and are obtained from dairy products and meat.

They are also produced during hydrogenation of fish or vegetable oils. 

Diets high in trans fatty acids correlate with increased blood levels of LDL (bad cholesterol) and decreased HDL (good cholesterol).

French fries, doughnuts, fast foods, and cookies tend to be high in trans fatty acids.

Lipid classification

Bioor (1943) classified lipids on the basis of their chemical composition:

Simple lipids or homolipids

Compound lipids or heterolipids

Derived lipids

Conn and Stumpf (1976) traditionally classified lipids into the following categories:

Acyl glycerols

Waxes

Phospholipids

Sphingolipids

Glycolipids

Terpenoid lipids including carotenoids and steroids

Lipid types (on the basis of function)

Physical properties of lipids

State: Saturated fatty acids are solid at room temperature while unsaturated fatty acids are liquid, in general.

Colour, odour and taste: Pure fats are colourless, odourless and bland taste.

Solubility: Soluble in organic solvent. Longer the fatty acyl chain and fewer the double bonds, the lower is its solubility in water.

Specific gravity: Less than 1

Geometric isomerism: Present

Emulsification: Found

Melting point: Depends on the chain length of the constituent fatty acyl chain and the degree of unsaturation.

Chemical properties of lipids

Hydrolysis: Fats are hydrolyzed by enzyme lipase to yield fatty acids and glycerol.

Saponification: Hydrolysis of fats by alkali is called saponification which results in the formation of glycerol and salts of fatty acids (called soaps).

Hydrogenation: Unsaturated fatty acids react with gaseous hydrogen to yield saturated fatty acids.

Halogenation: Unsaturated fatty acids and their esters can take up halogens (Br2, I2, etc.) at their double bonds at room temperature in acetic acid or methanol solution. This reaction is the basis of ‘iodine number determination’.

Oxidation: Unsaturated fatty acids are susceptible to oxidation at their double bonds.

Acid number: It is the number of milligrams of KOH required to neutralize the free fatty acids present in 1 gram of fat. The acid number indicates the quantity of free fatty acid present in fat.

Saponification number: It is the number of milligrams of KOH required to saponify 1 gram of fat. The saponification number provides information on the average chain length of the fatty acids in fats.

Iodine number: It is the number of grams of iodine absorbed by 100 grams of fat. The iodine number indicates the degree of unsaturation of the fatty acids present in fat.

Acetyl number: It is the number of milligrams of KOH required to neutralize the acetic acid obtained by saponification of 1 gram of fat after it has been acetylated. The acetyl number indicates the number of –OH groups present in fat.

Rancidity: When lipid-rich foods are exposed for too long in the air (oxygen), they may spoil and become rancid. The unpleasant taste and smell associated with rancidity results from oxidative cleavage of the double bonds in unsaturated fatty acids, producing aldehydes and carboxylic acids of shorter chain length which are volatile.

Essential fatty acids

Our body can synthesize most of the fatty acids except a few. These fatty acids must be supplied through diet and are termed essential fatty acids.

Example-α-Linolenic acid (ω-3 fatty acid), Linoleic acid (ω-6 fatty acid)

It is recommended that essential fatty acids make up 3% to 6% of your daily caloric intake. Of this percentage, you should consume 2 to 4 times more omega-6 fatty acids than omega-3 fatty acids. 

Omega-3 sources include:

Nuts

Soybeans

Walnut oil

Canola oil

Flaxseed oil

Cold water fatty fish such as salmon, herring, cod, flounder, tuna, bluefish, and shrimp

Omega-6 sources include:

Leafy vegetables

Seeds

Nuts

Grains

Vegetable oils (corn, safflower, soybean, cottonseed, sesame, sunflower)

Roles of Essential fatty acids

• Help with cellular development and the formation of healthy cell membranes.
• Blocks tumor formation and growth of cancer cells
• Assist in the development and function of the brain and nervous system.
• Regulates proper thyroid and adrenal activity. 
• Plays a role in thinning your blood, which prevents blood clots, heart attacks and stroke.
• Regulates blood pressure, immune responses and liver function.
• Deficiency causes skin problems, including eczema, dandruff, split nails and brittle hair.
• Forms Lipid rafts which affects cellular signalling.
• Acts on DNA, activates or inhibits transcription factors.

Carbohydrate Biochemistry (Part II)

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Disaccharides

Disaccharides consist of two monosaccharides joined covalently by an O-glycosidic bond, which is formed when a hydroxyl group of one sugar reacts with the anomeric carbon of the other.

Example: maltose, lactose, and sucrose

Glycosidic bonds are readily hydrolyzed by acid but resist cleavage by base. Thus disaccharides can be hydrolyzed to yield their free monosaccharide components by boiling with dilute acid.

N-glycosyl bonds join the anomeric carbon of a sugar to a nitrogen atom in glycoproteins and nucleotides.

General formula: Cₙ(H2O)ₙ₋₁

The oxidation of a sugar’s anomeric carbon by cupric or ferric ion (the reaction that defines a reducing sugar) occurs only with the linear form, which exists in equilibrium with the cyclic form(s).

When the anomeric carbon is involved in a glycosidic bond, that sugar residue cannot take the linear form and therefore becomes a non-reducing sugar.

The end of a chain with a free anomeric carbon (one not involved in a glycosidic bond) is commonly called the reducing end.

The disaccharide maltose contains two D-glucose residues joined by a glycosidic linkage between C-1 (the anomeric carbon) of one glucose residue and C-4 of the other.

Because the disaccharide retains a free anomeric carbon (C-1 of the glucose residue on the right), maltose is a reducing sugar.

Nomenclature of disaccharides (or oligosaccharides)

By convention, the name describes the compound with its nonreducing end to the left.

Give the configuration (α or β) at the anomeric carbon joining the first monosaccharide unit (on the left) to the second.

Name the nonreducing residue; to distinguish five- and six-membered ring structures, insert “furano” or “pyrano” into the name.

Add suffix '-syl' to the name of the first residue. For example, 'glucopyranosyl'.

Indicate in parentheses the two carbon atoms joined by the glycosidic bond, with an arrow connecting the two numbers; for example, (1🠊4) shows that C-1 of the first-named sugar residue is joined to C-4 of the second. 

If the anomeric carbons from both residues are involved in bond formation, a double headed arrow is given.

Name the second residue similarly. Add suffix '-ose' to the name of the second residue in case of reducing sugars and '-oside' in case of non-reducing sugars.

If there is a third residue, describe the second glycosidic bond by the same conventions.

Short name:Glc(α1🠊4)Glc

Sucrose

Sucrose (cane sugar) is made up of α-D-glucose and β-D-fructose linked by a glycosidic bond (α1 ⟺ β2). 

The reducing  groups (anomeric carbon) of both glucose and fructose are involved in glycosidic bond formation. Hence, sucrose is non-reducing sugar and it cannot form osazones.

The systematic name of sucrose is α-D-glucopyranosyl-(1 ⟺ 2)- β-D-fructofuranoside. 

This indicates:

It is composed of two monosaccharides: glucose and fructose

Ring type: Glucose is pyranose and fructose is furanose

Linkage: oxygen on C1 of α-D-glucose  is linked to C2 of β-D-fructose 

Suffix '–oside' and '⟺' indicates that the anomeric carbons of both the monosaccharides participate in glycosidic bond formation

Sucrose is a major carbohydrate produced in photosynthesis. It has the advantage as storage and transport as its functional groups are held together and are protected from oxidative attacks.

Intestinal enzyme, sucrase hydrolyze sucrose to glucose and fructose.

Inversion of sucrose:

Sucrose is dextrorotatory (+66.5°). But when hydrolyzed, it becomes levorotatory (-28.2°). The process of change in optical rotation from dextrorotatory(+) to levorotatory (-) is referred to as inversion. The hydrolyzed mixture of sucrose, containing glucose and fructose, is known as invert sugar.

Sucrose first splits into α-D-glucopyranose (+) and β-D-fructofuranose (+). But β-D-fructofuranose is less stable and gets converted into β-D-fructopyranose (-). The overall effect in the mixture becomes levorotatory (-).

Lactose

Lactose (milk sugar) is composed of β-D-galactose and β-D-glucose held together by   β(1🠊4) glycosidic bond.

The anomeric carbon of C1 of glucose is free. Hence lactose exhibits reducing properties and forms osazones (powder-puff or hedgehog shape).

The systematic name is β-D-galactopyranosyl-(1🠊4)-β-D-glucopyranose.

It is hydrolyzed by intestinal enzyme lactase into glucose and galactose.

Maltose

Maltose (malt sugar) is produced during digestion of starch by enzyme amylase.

Maltose is composed of two α-D-glucose  units held together by α(1🠊4) glycosidic bond. 

A free aldehyde group is present on C1 of the second glucose unit and hence maltose exhibits reducing properties and forms osazones (sunflower shaped).

It can be hydrolyzed by dilute acid or enzyme maltase.

 

In isomaltose, the glucose units are held together by α(1🠊6) glycosidic bond.

Cellobiose

It is identical to maltose, except that in it the linkage is  β(1🠊4) glycosidic bond.

It is formed during hydrolysis of cellulose.

Trehalose

It is identical to maltose, except that in it the linkage is  (α1⇔α1) glycosidic bond.

It is formed during hydrolysis of cellulose.

It is a non-reducing sugar.

Examples of other oligosaccharides

Raffinose (trisaccharides): Fructose+Galactose+Glucose

Stachyose (Tetrasaccharide):  Galactose+Galactose+Glucose+Fructose

Verbascose (Pentasaccharide): Galactose+Galactose+Galactose+Glucose+Fructose

Polysaccharides

Carbohydrates containing repeating units (more than 10 units) of the monosaccharides or their derivatives linked by glycosidic linkages are called polysaccharides.

They are primarily concerned with 2 important functions:

Structural role

Storage of energy

Polysaccharides can be linear or branched. The occurrence of branched polysaccharides is due to the fact that glycosidic linkages can be formed at any one of the –OH groups of a monosaccharide.

Polysaccharides are of high molecular weight. They are usually tasteless (non-sugars) and form colloids with water.

Polysaccharides are of two types:

Homopolysaccharides (Homoglycans): They, on hydrolysis, yield only one type of monosaccharide. They are named based on the nature of the monosaccharide unit. 

Example: Glucan (polymer of glucose), Fructosan (polymer of fructose)

Heteropolysaccharides (heteroglycans): They, on hydrolysis, yield a mixture of a few types of monosaccharide units or their derivatives. 

Example: Peptidoglycan (polymer of N-acetylglucosamine and N-acetylmuramic acid residues)

Starch

Starch is the carbohydrate reserve of plants which is the most important dietary source for higher animals.

Starch is a homopolysaccharide composed of D-glucose units held by glycosidic bonds. 

It is known as glucosan or glucan.

Starch consists of two polysaccharide components:

Water soluble amylose (15-20%)

Water insoluble amylopectin (80-85%)

Chemically amylose is a long unbranched chain with 200-1000 D-glucose units held by (α1🠊4) glycosidic linkage.

Amylopectin is a branched chain with (α1🠊6) glycosidic bonds at the branching points and (α1🠊4) glycosidic bonds everywhere else.

Starches are hydrolyzed by amylases (pancreatic or salivary) to liberate dextrins and finally maltose and glucose units. Amylase acts specifically on the (α1🠊4) glycosidic bonds.

(b) amylopectin

Dextrins

These are the breakdown products of starch by the enzyme amylase or dilute acids.

Starch is hydrolyzed through different dextrins and finally to maltose and glucose. 

The various intermediates (identified by iodine coloration) are soluble starch (blue), amylodextrin (violet), erythrodextrin (red) and achrodextrin (no colour).

Inulin

Inulin is a polymer of fructose.

It occurs in dahlia bulbs, garlic, onion, etc.

It is a low molecular weight (~ 5000) polysaccharide easily soluble in water.

Inulin is not utilized by the body. 

It is used for assessing kidney function through measurement of glomerular filtration rate (CFR).

Cellulose

Cellulose occurs extensively in plants and is totally absent in animals.

Cellulose is composed of β-D-glucose units linked by β(1🠊4) glycosidic bonds.

Cellulose can not be digested by mammals due to lack of the enzyme that cleaves β-glycosidic bonds. Hydrolysis of cellulose yields a disaccharide, cellobiose, which is further broken down to β-D-glucose units.

It is a major constituent of fibers, the non-digestible carbohydrate.

Glycogen

Glycogen is the main storage polysaccharide of animal cells.

Like amylopectin, glycogen is a polymer of (α1🠊4)-linked subunits of glucose, with (α1🠊6)-linked branches, but glycogen is more extensively branched (on average, every 8 to 12 residues) and more compact than starch.

Glycogen is especially abundant in the liver, it is also present in skeletal muscle.

Chitin

Chitin is a linear homopolysaccharide composed of N-acetylglucosamine residues in β linkage.

The only chemical difference from cellulose is the replacement of the hydroxyl group at C-2 with an acetylated amino group. 

Peptidoglycan

The rigid component of bacterial cell walls is a  heteropolymer  of  alternating (β1🠊4)-linked N-acetylglucosamine and N-acetylmuramic acid residues.

The enzyme lysozyme kills bacteria by hydrolyzing the   (β1🠊4) glycosidic bond between N-acetylglucosamine and Nacetylmuramic acid.