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Term Paper on Carbohydrates
Term Paper Contents:
- Term Paper on the Introduction to Carbohydrates
- Term Paper on Aldoses and Ketoses
- Term Paper on the Stereoisomers
- Term Paper on the Polysaccharides
- Term Paper on the Glycoproteins
1. Term Paper on the Introduction to Carbohydrates:
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Carbohydrates are poly alcohols which also possess an aldehyde or a ketone group in a free or combined form. Most of the carbohydrates are compounds of carbon, hydrogen and oxygen in which there are two hydrogen atoms and one oxygen atom (as in water) for each carbon atom. (Hence the name carbohydrate meaning ‘hydrates of carbon’).
However, some carbohydrates may contain nitrogen, sulfur or phosphorus in addition to carbon, hydrogen and oxygen. Further, some carbohydrates may not be strictly hydrates of carbon. Rhamnose, for example, has a formula C6H12O5. Moreover, all compounds having the formula (CH2O)n (hydrates of carbon) may not be carbohydrates. Formic, acetic and lactic acids are some examples of such compounds.
The carbohydrates can be classified into three main groups as – (a) monosacharides, (b) oligosaccharides and (c) polysaccharides, based on number of monomeric sugar units present.
Monosaccharides are the simplest sugars consisting of single polyhydioxy aldehyde or ketone group that cannot be hydrolyzed into smaller units under reasonable mild conditions. They serve as the building-blocks for the more complex sugars. Oligosaccharides (Greek Oligo ‘few’) contain from two to ten mono-saccharide units joined through glycosidic linkage or bond. They are hydrolysable into constituent monosaccharide units.
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Polysaccharides are polymers of monosaccharide units joined in long linear or branched chains through glycosidic bonds. Hydrolysis of polysaccharides yields many units of constituent monosaccharides. Polysaccharides have two major biological function – (a) As a storage form of fuels and (b) As structural elements in living organisms.
2. Term Paper on Aldoses and Ketoses:
A carbohydrate is composed of carbon (carbo-), and hydrogen and oxygen (-hydrate). The simplest carbohydrates are the monosaccharide’s that have the general formula (CH2O)n where n is 3 or more.
A monosaccharide or simple sugar, consists of a carbon chain with a number of hydroxyl (OH) groups and either one aldehyde group (often written as – CHO) or one ketone group. A sugar that bears an aldehyde group is called an aldose whereas a sugar with a ketone group is a ketose.
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The smallest carbohydrates, for which n = 3, are called trioses. The terms can be combined. Thus glyceraldehyde is a triose that has an aldehyde group and so is an aldose. Thus it can also be called an aldotriose. Similarly, dihydroxyacetone is a ketotriose.
Sugars that contain a free aldehyde or ketone group in the open-chain configuration can reduce cupric ions (Cu2+) to cuprous ions (Cu+) and hence are called reducing sugars. This is the basis of the Fehling’s and Benedict’s tests for reducing sugars. The reducing end of such a sugar chain is thus the end that bears the aldehyde or ketone group.
Note that glyceraldehyde and dihydroxyacetone have the same chemical composition, C3H6O3, but differ in structure (i.e. they are structural isomers).
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3. Term Paper on the Stereoisomers:
Glyceraldehyde has a single asymmetric carbon atom (the central one) and so two stereoisomers (also called optical isomers) are possible, that is two forms of glyceraldehyde, denoted as D- and L-glyceraldehyde, which are mirror images of each other. Stereoisomers also exist for amino acids.
Sugars with four, five, six or seven carbons are called tetroses, pentoses, hexoses and heptoses respectively. In these cases the sugars may have more than one asymmetric carbon atom.
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The convention for numbering carbon atoms and naming configurations is as follows:
i. The carbon atoms are numbered from the end of the carbon chain starting with the aldehyde or ketone group, which is carbon 1 (C-1);
ii. The symbols D and L refer to the configuration of the asymmetric carbon atom furthest from the aldehyde or ketone group.
Thus, for example, glucose, an aldohexose, exists as D and L forms. The furthest asymmetric carbon from the aldehyde group is C-5. D-Glucose is called D because the configuration of the atoms bonded to C-5 is the same as for D-glyceraldehyde. Similarly D-fructose (a ketohexose) is designated D because the configuration at C-5 matches that for D-glyceraldehyde. D sugars that differ in configuration at only a single asymmetric carbon atom are called epimers. Thus D-glucose and D-galactose are epimers, differing only in their configuration at C-4.
Ring Structures:
The aldehyde or ketone group can react with a hydroxyl group to form a covalent bond. Formally, the reaction between an aldehyde and the hydroxyl group of a sugar (an alcohol) creates a hemiacetal whereas a ketone reacts with a hydroxyl group (alcohol) to form a hemiketal.
For tetroses and larger sugars, the reaction can take place within the same molecule so that the straight-chain form of the sugar cyclizes. For example, the cyclization of D-glucose to form a six-carbon ring. The ring structures are called Haworth projections in which the plane of the ring can be imagined as approximately perpendicular to the plane of the paper with the thick lines of the ring in the diagram pointing towards the reader.
Note that a new asymmetric center is formed during cyclization, at C-1. Thus two isomers of D-glucose exist, α-D-glucose (in which the OH group at C-1 lies below the plane of the ring) and (β-D- glucose (in which the OH group at C-1 lies above the plane of the ring). The C-1 carbon is called the anomeric carbon atom and so the α and β forms are called anomers. In aqueous solution, the α and β forms rapidly interconvert via the open chain structure, to give an equilibrium mixture. This process is called mutarotation.
Because of its structural similarity to the ring compound called pyran, the six-membered ring structures of hexoses such as glucose are called pyranoses. Thus β-D-glucose may also be written as β- D-glucopyranose.
Five-carbon sugars, such as D-ribose and D-deoxyribose, and six-carbon ketose sugars (ketohexoses), such as D-fructose, form rings called furanoses by comparison with the compound furan. Again furanoses can exists in both α and β forms except here the nomenclature refers to the hydroxyl group attached to C-2 which is the anomeric carbon atom.
The pyranose ring of a six-carbon aldose sugar can exist in either a boat or a chair configuration. The substituents attached to the ring carbons that extend parallel to the symmetry axis are said to be axial (a) whilst those that extend outward from this axis are said to be equatorial (e).
In the boat form, there is considerable steric hindrance between the various groups attached to the carbon atoms of the ring and therefore this form is less favourable energetically. Hence the chair form predominates, as shown for (3-D- glucose, where all the axial positions are occupied by hydrogen atoms.
4. Term Paper on the Polysaccharides:
There is no sharp dividing line between disaccharides and polysaccharides, but few oligosaccharides (i.e. compounds containing 5-20 sugar residues) occur free except during the enzymic hydrolysis of starch. The so-called blood-group oligosaccharides, for example, are attached covalently to amino acid residues of proteins, and are considered below under glycoproteins.
The polysaccharides can be conveniently divided into those which contain only one kind of repeating unit, and those, called the heteropolysaccharides, which contain two or more repeating monomers.
(a) Heteropolysaccharides:
Plant Polysaccharides:
Roughage. The structural homopolysaccharide of plants is cellulose. This is a straight-chain polysaccharide consisting of β – 1, 4 linked glucose units. The β-configuration at C-1, which is apparently a trifling difference, is very important, as no vertebrate has a digestive enzyme able to attack it. Ruminants digest cellulose with the aid of symbiotic micro-organisms. Cellulose is very insoluble.
Chemical modification gives carboxymethyl-cellulose, which is soluble but bulky, and is sometimes used in slimming products. There are some other homopolysaccharides, and a few have a storage, rather than a structural, function, e.g. inulin, a polymer of fructose found in some tubers. None of these compounds is of great importance in mammalian biochemistry.
It is rare for cellulose to occur pure in plant structures. It is usually accompanied by pentose polymers such as xylans (D-xylose) and arabinans (L-arabinose), and by hetero-polymers containing acidic groups. All these polymers are known collectively as hemicelluloses. None of them is significantly attacked by pancreatic amylase, and they are excreted from the gut unchanged, except insofar as they have been digested by gut bacteria.
This bacterial metabolism is often associated with the production of uncomfortable volumes of gas, chiefly CH4 and H2. Nevertheless it has been recognized that Western diets containing much white flour and meat products are likely to lead to a decrease in faecal bulk, with the consequent possibility of serious constipation. There has been a trend towards the deliberate re-introduction of non-digested polysaccharides (‘roughage’) into the diet.
Cereal husks (bran) provide an abundant source of hemicelluloses, but other sources exist. Gum arabic contains arabinans; the seaweed product agar, containing agarose, a galactose polymer, has been used therapeutically for many years. Many heteropolymers, especially agarose and those containing sulphate ester or amino sugar residues, are associated in solution with large volumes of water, which aids in the maintenance of optimal faecal consistency. Cellulose does not possess this water-retaining property.
(b) Glycosaminoglycans:
This group of compounds used to be known as the mucopolysaccharides. They are characterized by the presence, in regular alternation with other residues, of amino sugars (glucosamine or galactosamine), which are not found in plant heteropolysaccharides.
It is now known that most of the glycosaminoglycans are attached by covalent bonds to a protein ‘core’, which typically accounts for some 10% of the dry weight. The size of the core, and the number of attached carbohydrate chains, are both very variable. This group of compounds is called the proteoglycans; each carbohydrate chain is usually unbranched, whereas in the glycoproteins branching is common Hyaluronic acid and heparin are the most important of the glycosaminoglycans that are not attached covalently to protein.
Taken as a group, the glycosaminoglycan are fairly acidic, both because ‘uronic’ acids (glucuronic and galacturonic) occur regularly in the chains, and because esterification by sulphate is common.
(c) Hyaluronic Acid:
This is a large linear polymer made up of the disaccharide repeating unit. Note that there is no sulphate ester in hyaluronic acid. Typically there are 400-4000 repeating units, giving a molecular weight range of 1.5 x 105 – L5 x 106. The largest molecules would be about 0.4 mm long if they were uncoiled, but in fact the molecules coil and entwine to make a very firm gel at very low concentration (∼0.1%).
The gel excludes other large molecules and also microorganisms, so that the rate of spread of bacterial infection is hindered. Many micro-organisms secrete a hyaluronidase which by shortening the average chain length of the polymer, reduces dramatically the viscosity of the gel.
Gels made from hyaluronic acid have good resistance to compression, and appear as lubricants and shock absorbing components in synovial fluid, in subcutaneous connective tissue (where it is progressively replaced in adult life by the arguably less elastic dermatan sulphate) and in many other tissues. In cartilage hyaluronic acid is found in very small quantities, but plays a rather special role.
(d) Heparin:
This is a small molecular weight polymer (mol. wt 15-20000) derived from the mast cells lining the walls of blood vessels, especially in liver and lung; it does not occur in connective tissue. Unlike hyaluronic acid, heparin is very highly sulphated, and hence very acidic. The sulphate groups on the —NH2 groups of the glucosamine residues are very readily hydrolysed by acid. When this happens, heparin loses its biological activity.
Heparin is a powerful inhibitor of blood clotting. There is still some doubt about its structure.
(e) Proteoglycans:
Our knowledge of connective tissue polysaccharides has been very dependent on advances in the technique of extracting them. For many years they were always extracted with alkali, which happens to hydrolyse the bond between xylose, the terminal sugar, and the hydroxy-amino acid in the peptide chain. Thus until recently the major polymers chondroitin sulphate (esterified either in the 4- or the 6-position), keratan sulphate, dermatan sulphate and heparan sulphate-were not recognized as proteoglycans.
The substances are fairly, widely distributed in the extracellular matrix of tissues, although dermatan sulphate is more especially found in skin, and chondroitin-4-sulphate in cartilage, where it may form up to 40% of the dry weight.
With the exception of heparin, which is primarily a less highly sulphated version of heparin, the other polymers have a repeating disaccharide unit that is similar to chondroitin, but with different monomers, as indicated below:
i. Chondroitin α-glucuronic acid-β-N-acetylgalactosamine
ii. Dermatan α-iduronic acid-, β-N-acetylgalactosamine
iii. Keratan β-galactose-β-N-acetylglucosamine
In diaphysial bone and tendon there is very little proteoglycan, but in cartilage, which is resilient, there is a great deal, and it is rather highly organized. The bulk of the structure is provided by proteoglycan subunits (PGS), which consist of a core protein rich in serine and if threonine, to which are attached chains of keratan sulphate (5-6 disaccharide units long) and chondroitin sulphate (40-50 units long), by α-Gal-Xyl- link.
The protein content of a PGS is about 11%. One end it of the protein core is free from carbohydrate and this associates non-covalently with 1 hyaluronic acid. A small link protein helps to stabilize the arrangement, which extends over several disaccharide units. There will be a PGS unit about every 25 repeating units of hyaluronic acid, which gives from 16 to 160 units per assembly.
The total molecular weight of even the smallest assemblies is many millions of daltons. The assemblies are to some extent flexible, and are of various sizes, so that the ground substance does not have a pseudo-crystalline structure, but it is probable that the PGS carbohydrate associates with the short carbohydrate chains attached to collagen. This association may well be strengthened by divalent metal ions.
(f) Glycogen:
This polysaccharide is found only in animals. It has a structure very like that of amylopectin, except that it is even more highly branched. The average chain-length of the exterior chains is only 8 glucose units (13-18 in amylopectin), and in the main chains there is a branch point every 3 units on the average (every 5-6 units in amylopectin).
The molecular weight is very high, about 5000000 (=25000 units). It gives a red colour with iodine (amylopectin gives a red-violet). Glycogen forms a colloidal solution, but in cells is often found as glycogen particles, mol. wt a up to 2 x 107, and containing associated enzymes.
5. Term Paper on the Glycoproteins:
In this article, emphasis must be on the carbohydrate component of complex polymers, but it is worth noting that there is no absolute dividing line between proteoglycans and glycoproteins; the mucoproteins described below contain only an average of 15 protein, compared with the 11% of the proteoglycans. Most glycoproteins do indeed contain much less carbohydrate than this, often as branched, bushy structures.
(a) Mucins:
These are the chief components of the viscous slimy secretions of the intestinal tract, and also of the bronchi (sputum); large quantities of the characteristic carbohydrates have been obtained from ovarian cysts. The overall structure resembles that of PGS, with a core protein rich in serine and threonine, to which the carbohydrate chains are attached, but the two classes of glycoprotein are quite distinct.
In particular, the carbohydrate of mucins contains no uronic acids. Instead, galactose and glucosamine predominate. In addition, in about 80%, of the population (secretors), some of the mucin polysaccharides have blood-group antigenicity. Connective tissue polysaccharides do not possess this property.
It is presumed that mucins have lubricating and protective properties, although direct evidence for this is sparse. In cystic fibrosis, over-production of mucin occurs both in the digestive system and in the lungs, where it forms insoluble plugs obstructing the airways. It is not yet known whether the glycoprotein is also abnormal in structure.
(b) Blood Group Substances:
At least nine antigen systems can be detected in human erythrocytes, of which only ABO and rhesus are clinically important. The antigen in all nine systems is an oligosaccharide. This is not inevitably the case; the histocompatibility antigens are glycoproteins, but the protein moiety is the antigen. The blood group antigens can be surprisingly small, and a change in a single monosaccharide residue can alter the antigenicity.
Note how the 1-3 glycoside bonds promote coiling of the chain. Red blood cell agglutination depends on a rather larger oligosaccharide covalently bonded to a membrane protein (not glycophorin for the ABO system). However, the same minimal configuration occurs in this highly branched structure. A very similar oligosaccharide is found in the mucoprotein of secretors.
(c) Starch:
This is a mixture of two main components, one soluble in boiling water and making up to 10-20% of the total, called amylose; the other 80-90% is insoluble in boiling water and is
called amylopectin. Both are made up of D-glucose units. Amylose is un-branched, containing 200-2000 glucose units linked α-1, 4 in a straight line. Amylopectin, on the other hand, is highly branched.
It has one end-group to 24-30 glucose units, which means that the outer chains are about 13-18 residues long. The molecule is very large, containing 250-5000 units. The main linkage is α- 1, 4, but at the branch points a third molecule of glucose is joined in the 6-position. The mixture of amylose and amylopectin known as starch forms a gel when concentrated solutions cool. It gives a blue colour with iodine.
(d) Dextran:
This is a polysaccharide, consisting of relatively un-branched 1, 6 linked glucose molecules, produced by the bacterium, Leuconostoc mesenteroides, acting on sucrose. It is soluble, but colloidal, and can be used as a plasma substitute in the treatment of shock. A similar polymer, produced by bacteria from sucrose, forms the plaque that coats teeth, and precedes decay.