Discussion
3. New potential p38α transcriptional effectors in tumor suppression
Classifi cation
The GAGs are classifi ed in to six major types according to their monomeric compositions, type of glycosidic linkages, and degree and location of their sulphate units (Table 2.4). However, all types share some important features:
(a) Composition: They contain repeating disaccharide units of an amino sugar (glycosamine) and uronic Attachment of glycosaminoglycans to a protein
mole-cule forms proteoglycans. Proteoglycans are enormous molecules consisting of central strand of hyaluronic acid with attached core proteins that bear numerous glycos-aminoglycans and oligosaccharides. They bind water and minerals forming a hydrated gel which acts as a cushion against mechanical shock.
Proteoglycans versus glycoproteins: It is important to dis-tinguish proteoglycans from glycoprotein. Glycoproteins (i) have short oligosaccharide chains (1–20 sugars in length), which are (ii) highly branched, (iii) generally do not have a repeating sequence and (iv) constitute 1–30%
of the hybrid. On the other hand, sugar chains of proteo-glycans are longer (100 or more sugars), linear and unbranched, with disaccharide repeats.
Relationship Between Structure and Function
The peculiar structural features of GAGs that explain their suitability to perform a number of specialized func-tions in the body are described below.
Gel-forming component of extracellular matrix (ECM):
GAGs have polyanionic character, meaning numerous negative charges are present on a single molecule. Because of these charges, GAGs have two important properties:
The heteropolysaccharide chains repel one another and, therefore, tend to exist in extended conforma-tion in soluconforma-tions.
The anionic groups being strongly hydrophilic tend to associate with water.
The special ability of the chains to bind large amounts of water produces the gel-like matrix that forms the body’s ground substance. Due to charge repulsion the chains tend to “slip” past each other, in a way two magnets of the
Table 2.4. Structure and distribution of the proteoglycans
Proteoglycan Characteristic disaccharide Sulphation Tissue locations
1. Hyaluronic acid D-Glucuronic acid, N-Acetylglucosamine None Joint and ocular fl uids 2. Chondroitin sulphates D-Glucuronic acid,
N-Acetylgalactosamine 4-sulphate
GalNAc Cartilage, tendons, bone
3. Keratan sulphates D-Galactose, N-Acetylglucosamine 6-sulphate
GlcNAc Cartilage, cornea
4. Dermatan sulphate L-Iduronic acid, N-Acetylgalactosamine 4-sulphate
IdUA, GalNAc Skin, valves, blood vessels
5. Heparin Iduronic acid, N-Sulphoglucosamine GlcNH2, IdUA Mast cells, liver
6. Heparan sulphate Glucuronic acid, N-Sulphoglucosamine GlcNc Cell surfaces
GalNAc N-acetylgalactosamine, GlcNH2 glucosamine, GlcUA D-glucuronic acid, IdUA L-iduronic acid.
humor of the eye, umbilical cord and loose connective tissue. It primarily serves as a lubricant and shock absorber.
Some pathogenic bacteria secrete an enzyme hyaluroni-dase, which cleaves the glycosidic bonds of the hyaluronic acid. This renders the tissues more susceptible to invasion by the bacteria. Hyaluronidase also hydrolyzes the outer polysaccharide coat of the ovum and thereby makes the penetration by spermatozoa possible.
Chondroitin 4- and 6-sulphates (CS): These are the most abundant GAGs in the body that comprise D-glucuronic acid and N-acetylgalactosamine units: the latter are phated on either C-4 or C-6 (Fig. 2.15). Chondroitin sul-phate is mostly present in the cartilage where it binds collagen and holds fi bres in a tight, strong network.
Tendons, ligaments, bones and aorta contain relatively smaller amount of this mucopolysaccharide.
Keratan sulphate (KS): It is the most unusual of all GAGs: the only one that does not contain an acid sugar. The repeating disaccharide unit consists of N-acetylglucosamine and galactose. It is linked to protein by rather unusual link-ages (N-glycosidic), that are usually found in glycopro-teins. It helps to keep cornea transparent.
Dermatan sulphate (DS): It was originally isolated from skin, but is also found in blood vessels and heart valves.
acid. Keratan sulphate is an exception for it contains galactose in place of uronic acid.
(b) Linkage: With exception of heparin and heparin sul-phate, where the linkage between the amino sugar and uronic acid is uniformly 14, in all other GAGs it is alternating 14/13.
(c) Core protein: GAGs are usually attached to a core protein by a core trisaccharide, Gal-Gal-Xyl. The latter is attached to a serine or threonine residue of the core protein by O-glycosidic linkage. Exceptions are hyaluronic acid and keratan sulphate. In case of kera-tan sulphate, the linkage to core protein is N-glycosidic (discussed in Chapter 5). Hyaluronic acid is a rather unusual GAG for it is not attached to a core protein.
Hyaluronic acid: The repeating disaccharide unit of hyaluronic acid comprises D-glucuronic acid and N-acetylglucosamine, joined by b(13) linkage (Fig. 2.15).
Hyaluronic acid is different from other GAGs in lacking sulphate groups, not being covalently attached to protein and for not being limited to animal tissue (it is found in bacteria also). The polysaccharide chain is longest of all the GAGs, with molecular weight of 1 105 to 1 107 (250–
25,000 repeating disaccharide units). It is a viscous jelly-like substance that fi lls the intercellular spaces of the animal tissues. It is found in synovial fl uid of joints, vitreous
H
Fig. 2.15. Repeating disaccharide units of some glycosaminoglycans.
are hydrolytic in action and catalyze degradation of the GAGs at an optimum pH of 5.0. Such low optimum pH has a protective value since it prevents destruction of the cellular constituents in case the enzyme leaks out of the lysosomes. Half-life of the GAGs is relatively short (3–10 days), with exception of keratan sulphate (120 days). Therefore, with impaired degradation, the cellular concentration of the GAGs rapidly builds up.
Features of mucopolysaccharidoses: In mucopolysacchari-doses, the tissues that produce GAGs are most affected. In these tissues, degradation of GAGs is impaired. Lysosomal vesicles become swollen with incompletely degraded mucopolysaccharides. A characteristic fi nding of diagnos-tic signifi cance is excessive elimination of the GAGs in urine. Diagnosis can be confi rmed by measuring the concentration of lysosomal hydrolases.
The mucopolysaccharidoses are autosomal and reces-sively inherited, with an exception of Hunter’s syndrome which is X-linked. Prenatal diagnosis of these disorders is possible. Children who are homozygous for these disorders are apparently normal at birth. The condition deteriorates gradually, but in severe cases the progression is rapid and death occurs in childhood. Unfortunately, no effective treatment exists at present.
Clinical types: Several types of mucopolysaccharidoses are recognized depending on the lysosomal hydrolase that is defi cient (Table 2.5).
Exercises
Essay type questions
1. Defi ne carbohydrates and give one example from each class/subclass. Explain why starch can be uti-lized in humans but not cellulose?
It helps maintain shape of these tissues. The predomi-nant acid sugar is iduronic acid, though a variable amount of glucuronic acid is also present. The repeating disaccharide unit is made up of iduronic acid and N-acetylgalactosamine 4-sulphate. The L-iduronic acid, a C-5 epimer of D-glucuronic acid, is formed in an unusual reaction by epimerization of the latter, after it has been incorporated into the polymer.
Heparin: The repeating disaccharide unit comprising glu-cosamine and glucuronic acid (or iduronic acid), is rich in sulphate groups. Almost all glucosamine residues are bound to sulphate group. An average of 2.5 sulphate groups per disaccharide unit is seen. The linkages between the amino sugar and the uronic acid is uniformly 14 in heparin (and heparin sulphate). (Alternating 14/13 linkages seen in other GAGs.) Unlike other GAGs (which are extracellular compounds), heparin is an intracellular component of mast cells that line arteries, especially in liver, lungs, and skin. Heparin is a potent anticoagulant that helps to prevent clotting of the circulating blood. It is therapeutically used to prevent clotting during intrave-nous therapy, and to inhibit clotting in various patho-logical conditions, such as following a heart attack.
Heparan sulphate (HS): It contains the same disaccharide units as heparin except that some of the glucosamine units are acetylated and there are fewer sulphate groups.
It is an extracellular GAG, found in basement membrane and is an essential component of cell surfaces.
The structures of GAGs are quite variable. Thus, hyalo-ronic acid is a huge polysaccharide, chondroitin 4-sulphate has sulphur on C-4 rather than C-6 of the amino sugar;
dermatan sulphate contains some glucuronic acid besides iduronic acid, and the sulphate of the amino sugar may be either on C-4 or on C-6; heparan sulphate contains some iduronic acid besides glucuronic acid; and hepar-ian contains both glucuronic acid and iduronic acid.
Mucopolysaccharidoses
Mucopolysaccharidoses are a group of hereditary disor-ders of proteoglycan metabolism that are clinically pro-gressive and characterized by excessive intralysosomal accumulation of GAGs in various tissues. Such accumu-lation accounts for clinical manifestations of mucopoly-saccharidoses such as coarse facial features, thick skin, and corneal opacity. Defective cell function leads to mental retardation, growth defi ciency, and skeletal dysplasia.
Biochemical defect: The underlying defect in mucopoly-saccharidoses is impaired degradation of mucopolysac-charides by lysosomal enzymes. The lysosomal enzymes
Table 2.5. Mucopolysaccharidoses
Type Syndrome Enzymatic defect Accumulated metabolite
I Hurler’s a-L-Iduronidase DS; HS
II Hunter’s Iduronate sulphatase DS; HS III Sanfi lippo’s Heparin sulphatase HS IV Morquio’s Galactosamine sulphatase KS; CS
V Scheie’s L-Iduronidase DS
VI Maroteaux-Lamy’s
N-Acetylgalactosamine 4-Sulphatase
DS
VII SLY’s b-Glucuronidase DS; HS
CS chondroitin sulphate, DS dermatan sulplate, HS heparan sulphate, KS keratan sulphate.
2. What are mucopolysaccharides? Name some and explain their biological signifi cance.
3. How do physical properties of glycosaminoglycans relate to their biological roles?
4. Name some homopolysaccharides and explain their biological signifi cance.
5. Compare and contrast structures and functions of cellulose, chitin, starch and glycogen.
Write short notes on
1. Mutarotation 2. Enantiomers
3. Chondroitin sulphate 4. Invert sugar
5. Cardiac glycoside 6. Anomers
7. Howarth and Fisher projections 8. Derived sugars
Lipids, carbohydrates and proteins form bulk of the organic matter in living systems. However, unlike the other two, lipids are not well defi ned chemically. In general, lipids are a heterogeneous group of water-insoluble, oily or greasy substances that can be extracted with non-polar solvents (e.g. benzene or ether), but not with the polar solvents. Because of their insolubil-ity in aqueous solutions, body lipids are mostly found in isolated compartments. For example, droplets of triacylglycerols are present in adipocytes and some lipids are membrane bound. When present in blood, they form complexes with pro-teins so that they can be transported in the aqueous plasma. In contrast to other major biomolecules, lipids do not form polymers.
In this chapter, various types of lipids have been described, with a special emphasis on structure-function relationships.
After going through this chapter, the student should be able to understand:
Basic chemistry, nomenclature, properties and signifi cance of fatty acids in the body.
Chemistry, classifi cation, properties and functions of various types of lipids and the related medical implications.
C H E M I S T RY OF L I P I D S
Lipids serve as fuel molecules, highly concentrated energy stores, signal molecules, components of cell membranes, and carriers of fat soluble vitamins, e.g.
A, D, E and K. A layer of subcutaneous fat acts as thermal insulator and acts as a cushion, which protects several delicate organs by absorbing mechanical shocks. Both lipids and lipid derivatives serve as vitamins and hor-mones (Table 3.1).
The building blocks of most lipids are fatty acids.
Some lipids such as cholesterol and terpenes lack fatty acids. However, these lipids are potentially related to fatty acids because they are synthesized from the cata-bolic end product of fatty acid degradation (i.e. acetyl CoA).
Lipids are a diverse group of molecules that are soluble in organic solvents, and in contrast to other major types of biomolecules, do not form polymers.
3
Table 3.1. Lipids and their major biological functions in the human body
Lipid Biological function
Triacylglycerols Energy storage, thermal insulation Waxes Keeps skin lubricated and waterproof Phospholipids Membrane components, detergents,
surfactant, second messengers
Sphingolipids Components of membranes, especially of nervous tissue and the myelin sheath
Sterols Precursors of biologically useful compounds, such as bile acids, steroids, sex hormones, vitamin D, component of membranes
I. Fatty Acids
A. General Characteristics
The fatty acid molecule consists of a long hydrocarbon chain with a polar carboxyl group at its end (Fig. 3.1). Since the
Functions: Fatty acids perform following functions in the body:
1. Membrane lipids: They are the components of the more complex membrane lipids.
2. Fuel molecules: They are components of stored fat in the form of triacylglycerols.
3. Hormones: Derivatives of fatty acids serve as hor-mones (such as prostaglandins) and intracellular second messengers (such as IP3 and DAG).
4. Numerous proteins are covalently modifi ed by fatty acids. Palmitic acid and myristic acid, for example, are directly attached to some proteins.
B. Nomenclature
The nomenclature of fatty acids is based on the following characteristics of the hydrocarbon chain:
Chain length
Presence of double bonds and their positions.
As shown in Table 3.2, the abbreviation 18;1 indicates an 18-carbon fatty acid having a single double bond.
Likewise, 18;2 indicates an 18-C fatty acid with two dou-ble bonds. ⌬n indicates position of these bonds; for example, ⌬9,12 indicates double bonds at C-9 and C-12, starting from the carboxyl end.
Thus, the 18-C saturated fatty acid (Fig. 3.2a) is called octadecanoic acid: octa (8) and deca (ten) imply chain length (18 carbons). Its common name is stearic acid and it is the most abundant fatty acid in nature. Introduction of a double bond at the C-9 position results in the for-mation of octadeca (mono) enoic acid (common name, oleic acid). Similarly, the 18-C fatty acid having two pK value of the carboxylate group is around 4.85
(between 4.7 and 5.0), it rapidly ionizes at the physio-logical pH to form the carboxylate ion (COO⫺). The car-boxylate ion has polar characteristics, with high affi nity for water (hydrophilic). However, the predominant por-tion of the fatty acid molecule is the long hydrocarbon chain in which carbon atoms are in their lowest oxida-tion state. Being non-polar in nature, hydrocarbon chain accounts for predominantly non-polar character of the fatty acid molecule. This in turn accounts for the oily or greasy nature of lipids.
Thus, a fatty acid molecule contains both polar (hydro-philic) and non-polar (hydrophobic) regions; such mol-ecules are called amphipathic molmol-ecules. Fatty acids are simplest of all amphipathic substances in the body.
Naturally occurring fatty acids contain even number of carbon atoms (most contain 14 to 24). Of these, most abundant are the fatty acids containing 16 or 18 carbon atoms. The fatty acids with hydrocarbon chains contain-ing one or more double bonds are called unsaturated fatty acids, whereas those lacking any double bonds are referred to as saturated fatty acids.
Fatty acids are carboxylic acids, whose chain length and degree of unsaturation varies. Most fatty acids have even number of carbons in an unbranched chain.
ω γ α
β Hydrocarbon chain CH3 (CH2)n COO–
COO–
Fig. 3.1. Structure of a fatty acid.
Table 3.2. Some naturally occurring fatty acids
Carbon atoms Systematic name Common name Melting point (°C) Occurence % of total Saturated
12 n-Dodecanoic acid Lauric acid 44.2 ⬍ 1
14 n-Tetradecanoic acid Myristic acid 53.9 3
16 n-Hexadecanoic acid Palmitic acid 63.1 23
18 n-Octadecanoic acid Stearic acid 69.6 6
20 n-Eicosanoic acid Arachidic acid 76.5 ⬍ 1
Unsaturated
18;1–⌬9 n-Octadeca-enoic acid Oleic acid 13.4 50
18;2–⌬9,12 n-Octadeca-dienoic acid Linoleic acid –5 10
18;3–⌬9,12,15 n-Octadeca-trienoic acid Linolenic acid –11 ⬍ 1
20;4–⌬5,8,11,14 n-Eicosa-tetraenoic acid Arachidonic acid –49.5 ⬍ 1
18;2–⌬9,12 implies 18-C fatty acid with 2 double bonds at C-9 and C-12, 20;4–⌬5,8,11,14 implies 20-C fatty acid with 4 double bonds at C-5, C-8, C-11, and C-14.
2. Chain length of fatty acid: Increase in chain length results in elevating the melting point. Saturated fatty acids of less than eight carbon atoms are liquid at physiological temperature, whereas those containing more than 10 are solids at this temperature.
In saturated fatty acids, hydrocarbons chains are in an extended conformation (Fig. 3.2a) and so can be packed together into a compact structure.
In unsaturated fatty acids, compact packing is pre-vented due to presence of rigid bends in their hydrocarbon chains; these bends are produced by the double bonds.
Consequently, the unsaturated chains are loosely packed and are, therefore, more easily disturbed by thermal energy. This accounts for the lower melting point of the latter. For example, melting point of stearic acid (69.6°C) is about fi vefolds higher than that of oleic acid (13.4°C).