18 Glycolipids and Waxes

Prof. M. N. Gupta

Objectives

• To learn about phosphatidyl inositol
• To understand the role of inositol derivatives obtained from these in signal transduction
• To learn about cerebroside and gangliosides
• To learn about waxes: their structure and biological roles

Concept Map

3. Description

The words glycolipids is unlikely to produce any excitement of the kind, say associated with rDNA!

Yet, we will see that none of the biological molecules are redundant. We will discuss how simple glycolipids can give rise to inositol phosphates which are so critical in signal transduction.

Glycolipids again are diverse in structure and function. Both glycol- and lipid components contribute to this diversity.

Phosphatidyl moiety, simple glycerol moiety and sphingosine are different lipid components. The carbohydrate part can be a single sugar, disaccharide, trisaccharide or oligosaccharide.

Apart from the usual monosaccharides, glycolipids also contain glucosamine and galactosamine units. Sialic acids are also found in glycosphingolipids called gangliosides.

Finally, we will also look at the waxes and their diverse functions.

The simplest glycolipids are those that contain inositol. The two isomeric inositols are meso-inositol and scyllitol. Meso-inositol is more commonly called myo-inositol.

This compound is widely distributed in both plants and animals. It was found in boar semen at the concentration of 1 g/ 100 mL. Scyllitol has been detected in dog fish liver and cartilage as well as in many plants. These compounds have been known for a long time since the days of Emil Fischer (1940s).

Myo-inositol is also found in microorganisms. Conversion of glucose to myo-inositol in yeast was shown way back in 1957 and its catabolism in Acetobacter was also investigated around the same time.

Phosphatidyl inositols also have been known for a long time. These were found in animal tissues, plant sources such as wheatgerm and soybean and in bacteria in the period 1949-1960.

These were earlier classified in terms of inositol derivatives which were obtained upon their hydrolysis. Inositol monophosphate or diphosphoinositols were obtained from these lipids. Some also contained galactose and arabinose as well.

Phosphatidyl inositol 4,5-bisphosphate is a phospholipid found in biomembranes. It is formed from phosphatidyl inositol which in turn arises from CDP-diacylglycerol and inositol.

This bisphosphate PIP2 is involved in a major signal transduction mechanism. This pathway is initiated by activation of an enzyme which is a phospholipase C and specifically called polyphosphoinositide diesterase or simply phosphoinositidase.

This enzyme is activated in response to a hormone like serotonin binding to cellular receptor. The enzyme action liberates inositol 1,4,5-triphosphate (IP3) and diacyl glycerol.

IP3 has a short life of only few seconds due to phosphatases stripping off all the phosphate and producing inositol. Alternatively, IP3 can also be further phosphorylated to 1,3,4,5-tetrakisphosphate (IP4) which then generates another isomer of original IP3. This isomer generated by a phosphatase is inositol 1,3,4-triphosphate.

Inositol 1,3,4-triphosphate is also converted to inositol by a phosphatase. This phosphatase is strongly inhibited by Li+ at a mM concentration. It is believed that therapeutic benefit of Li+ in the treatment of manic depression disorders is due to its inhibition of this phosphatase.

PIP2 generally has Arachidonate at C-2 position of glycerol. We have already learnt about the importance of this C20 PUFA as a precursor of eicosanoids prostaglandins and thromboxanes.

Phosphoinositide cascade of signal transduction in fact mediates diverse metabolic processes. This is as varied as glycogenolysis in liver cells to visual transduction in invertebrate photoreceptors.

It was Michael Berridge who discovered that IP3 is responsible for a quick release of Ca2+ from its intracellular stores. These stores are in endoplasmic reticulum, smooth muscle cells and sarcoplasmic reticulum.

It is this Ca2+ which is now present in the cytosol which triggers various processes like glycogenolysis and smooth muscle contraction.

IP3 at submicromolar concentration opens Ca-channels in the endoplasmic and sarcoplasmic reticulums. This is how the quick rise in cytoplasmic Ca2+ concentration happens.

Ca2+ in turn is an important intracellular messanger for many biological processes. Its transport systems ensure that most of the times Ca2+ concentration in cytosol is ~0.1 μM. The extracellular Ca2+ concentration is much higher.

It is necessary that cells keep Ca2+ concentrations low. From our studies of metabolism, we do know that many phosphate esters play an important role there. Pi is an important specie. Given the low solubility product of calcium phosphate, it will be undesirable to have high Ca2+ concentration in the cytosol.

The considerable difference in normal intracellular and extracellular concentrations makes it possible to quickly increase the cytosolic [Ca2+] once the Ca channel is opened in response to IP3 presence.

We see here how biology exploits a simple chemistry concept of solubility product to design finely controlled metabolic processes.

We also see that an inorganic specie Ca2+ is extremely important and dictates functions of more complicated molecules and processes!

Ca2+ can co-ordinate with multiple ligands. Asp and Glu side chains and even peptide bond carbonyls are good ligands via their oxygen. So, it binds protein very well. In fact, eukaryotic cells keep a control on [Ca2+] by sensing it via Calmodulin: a [Ca2+] detector.

Plant Glycosyl Glycerides

Plants contain glycosyl glycerides. These are generally mono- or di- galactosyl derivatives of diglycerides.

Α-D-galactosyl (1→6) β-D-galactosyl diglyceride is a glycolipid found in chloroplasts. These glycosyl diglycerides of leaves are rich in linolenic acid. Hence, these green leaves are a good source of this PUFA.

A sulpholipid found widely distributed in plant chloroplasts is a sulphonic acid derivative of 6-deoxyglucosyl diglyceride. Apart from plant chloroplasts, this glycolipid is also present in chromatophores of photosynthetic bacteria. This indicates its importance in photosynthesis.

Some simple compounds formed from sugars and glycerols are also known. As these are soluble in organic solvents, these can be classified as glycolipids. Alternatively, these may be viewed as carbohydrate derivatives.

Red marine algae Irideae laminarioides contain α-D-galactopyranosyl-2-glycerol. Wheat flour contains β-D-galactopyranosyl-1-glycerol. Please note that fatty acids are absent in these simple glycolipids.

Let us now switch over to the discussion of a more well known class of glycolipids. Just like glycerol backbone produces diverse kinds of lipids, many lipids are produced which use sphingosine as a basic scaffold.

Sphingosine itself is produced from palmitoyl CoA and serve as precursors. These molecules condense to produce dihydrosphingosine. A dehydrogenase action introduces the double bond to produce sphingosine.

Some important classes of glycolipids are formed from N-acyl sphingosine which is more commonly called ceramide. These glycolipids are sphingomyelins, cerebrosides and gangliosides.

In cerebrosides, glucose or galactose is bonded to terminal -OH of ceramide. This synthesis occurs by UDP-sugars acting as the sugar donor. Please note that wherever sugars are added to a preexisting compound in biochemistry UDP-sugars often act as sugar donors.

The cerebrosides of the neural tissues (in plasma membranes of their cells) generally have galactose. The corresponding cerebrosides in non neural tissues have glucose. The terms galactolipds/glucolipids or galactocerebrosides/glucocerebrosides have been sometimes used.

With the amino group of sphingosine already acylated in ceramide, the cerebrosides have no charge at pH 7 and are called neutral glycolipids. The other glycosphingolipids which have two or more sugars, generally, D-glucose, D-galactose or N-acetylgalactosamine are called globosides.

Cytolipin H is a ceramide lactoside. Cytolipin K is a globoside identified in kidney and it turned out to be abundantly present in the human erythrocyte stroma. The sugars present are 2 galactose molecules, glucose and N-acetylglucosamine linked to ceramide.

The nature of fatty acids in the ceramide portion of the cerebroside is also the source of their structural diversity. The cerebroside kerasin contains C24 saturated fatty acid lignoceric acid. Another cerebroside phrenosin contains cerebronic acid which is the 2-hydroxy derivative of lignoceric acid.

Brain white matter contains galactocerebrosides which are rich in sulphate ester analog of phrenosin with C-3 of the galactose esterified. Such sulfate esters have been sometime referred to as sulfatides.

The glycosphingolipids found in the nerve tissues and spleen are gangliosides. Structurally, gangliosides have oligosaccharide chain (rather than couple of sugar/sugar derivatives as in cerebrosides) attached to the ceramide.

Other characteristic features of the sugar composition of gangliosides is that the oligosaccharide chain contains at least one N-acetyl glucosamine or N-acetyl galactosamine. Also, at least one molecule of N-acetyl neuraminic acid (NAN) is present.

Gangliosides are formed by the stepwise addition of sugars to the cerebroside. The N-acetyl neuraminic acid is added, though with the CMP derivative acting as its donor.

In gangliosides of erythrocytes and spleen of horses, instead of NAN, N-glycolylneuraminic acid is present. Both acidic sugars are called sialic acid.

Sialic acid is also present in glycoproteins. Many isoforms of some glycoproteins simply differ in number of sialic acid units attached at the end of oligosaccharide chain. In both glycoproteins and gangliosides, sialic acid contributes negative charge to the molecules at physiological pH.

Johann Thudichum (1829-1901) who discovered sphingolipids was puzzled about their biological role and apparently he called these substances after the enigmatic sphinx. The major function of the glycosphingolipids in many membranes is structural. With the carbohydrate portion imparting them a specific orientation, these are present asymmetrically in membranes just like glycoproteins.

In the immunology paper, the Karl Landsteiner ABO blood group system is discussed. We do know, however, that for blood transfusion, the RBCs have to be of the compatible group. The „A‟ and „B‟ are surface antigens on erythrocytes.

These surface antigens are in fact carbohydrate component of glycosphingolipids. Thus, the person with „A‟ type blood group has a different glycosphingolipid on its erythrocytes as compared to the person with „B‟ type blood group.

These molecules are also part of cellular receptors. The ganglioside GM, is a receptor for cholera toxin. It is also believed that glycosphingolipids are involved in intercellular communication during growth and development.

Many metabolic disorder relates to catabolism of gangliosides and other glycosphingolipids. An example is Tay-Sach‟s disease in which the concentration of ganglioside GM2 becomes very high as a specific β-N-acetyl hexosaminidase which removes N-acetylgalactosamine is deficient. Tay-Sach‟s disease is usually fatal by age 3 of the infant.

Waxes

Waxes are sometimes called biological waxes to distinguish them from paraffin waxes.

These are esters where both alcohols and fatty acids are long chain compounds.

These long chains result in waxes being solid with melting points in the range of 60-100 °C. These, in general, are higher than fats.

These long chains of both acid and alcohol components also make waxes highly hydrophobic to the extent that these are described as a water repellant. This property is exploited in nature for various purposes.

Honeycomb provides bees complete shelter from the rain. The skin glands of vertebrates produce waxes which helps the skin and hair to remain soft, lubricated and protected from water.

Birds similarly produce waxes from their preen glands to make their feathers water repellant. The high priced down jackets filled with feathers of the birds like goose become protective wear against cold, snow and rain!

Waxes find large number of applications in the pharma, cosmetic and similar industries. Next time when you see/buy a high quality lip balm, look at the ingredients. It is likely to contain bees wax!

Sperm whale is not the only one which exploits waxes to adjust buoyancy. Dinoflagellates, krill and other crustaceans and other fishes have low density waxes in their swim bladder or other tissues to obtain desired buoyancy.

A large number of terrestrial arthropods have waxes on their cuticle surfaces to decrease loss of water from their body surfaces.

In general, waxes in marine animals contain higher amounts of unsaturated fatty acids and alcohols. However, waxes are quite diverse in their structure depending upon their function in the organism/plants.

The insect waxes generally contain saturated alcohols and fatty acids with their chains consisting of carbons in 12 to >20 range. The giant whitefly Aleurodicus dugesti has C-chains of upto 30 carbons in both components.

In general, waxes have primary alcohols. The lipids of cuticules of melanopline grasshoppers contain waxes with secondary alcohols.

In plants, cutins and suberin are the lipid polymers present in the hydrophobic layers of cell walls. Cuticle is the cutin based layer on the epidermis of aerial organs of the plants. This controls the water loss and movement of gases and solutes. In addition, cuticles contain waxes of C24-C34 saturated fatty acids and alcohols.

Lanolin present in lamb‟s wool, Carnauba wax obtained from a Brazilian palm tree and wax from Spermaceti oil of whales are industrial products used in ointments, polishes and lotions.

Sperm whale is a massive marine. About one-third of its weight is due to its head. About 90% of the weight of its head in turn is due to a blubbery part called spermaceti organ.

Typically spermaceti contains upto 18000 kg of lipids consisting of triglycerides and waxes. Presence of a large number of UFA ensures that this is liquid at 37 °C which is the normal body temperature of resting whale.

These whales dive down in deep sea in search of squids on which they feed. At that level, water is both colder and denser. Whales are able to wait quietly (without much swimming) for schools of squids to pass by.

Whale‟s physiology results in rapid cooling of the oil to become solid (it actually starts to crystallize even at 31 °C) during the dive. The buoyant density of whale now matches with the denser water around it.

At one time, before sperm whales became endangered, the spermaceti “oil” with waxes in it was a valuable lubricant. Relentless hunting for the waxy oil made them the endangered specie.

The dew drop nestling on the plant leaf is always considered a visual delight. The cutin is the waxy material which makes it happen!

The role of glycolipids in biomembranes, signal transduction and photosynthesis is well established. Glycolipids and waxes are interesting molecules and form part of the diversity of lipids as a class of biological molecules.

Summary

•  Glycolipids with inositol as sugar
•  Glycosyl glycerides
•  Cerebrosides
•  Gangliosides
•  Waxes and their applications