Main Menu

Advertisement

Subjective Questions

Q.1- What are storage polysaccharides? Give a brief description of each of them.

Answer- Glycogen, starch and Inulin are storage polysaccharides.

1) Glycogen- Glycogen is a readily mobilized storage form of glucose. It is a very large, branched polymer of glucose residues (Figure-1) that can be broken down to yield glucose molecules when energy is needed. Most of the glucose residues in glycogen are linked by α-1,4-glycosidic bonds. Branches at about every tenth residue are created by α-1,6-glycosidic bonds. It is the storage polysaccharide in animals and is sometimes called animal starch, but it is more branched than amylopectin present in starch.

Figure- 1-showing the structure of glycogen, Note the α-1,6-glycosidic linkage at the branch point

It is hydrolyzed by both α and β-amylases and by glycogen phosphorylase. The complete hydrolysis yields glucose. Glycogen on reaction with iodine gives a reddish brown color.

Glycogen is stored in muscle and liver. The concentration of glycogen is higher in the liver than in muscle (10%versus 2% by weight), but more glycogen is stored in skeletal muscle overall because of its much greater mass. Glycogen is present in the cytosol in the form of granules ranging in diameter from 10 to 40 nm. In the liver, glycogen synthesis and degradation are regulated to maintain blood-glucose levels as required to meet the needs of the organism as a whole. In contrast, in muscle,these processes are regulated to meet the energy needs of the muscle itself.

Glycogen is not as reduced as fatty acids are and consequently not as energy rich. But still animals store energy as glycogen? All excess fuel is not converted to fatty acids. Glycogen is an important fuel reserve for several reasons.

The controlled breakdown of glycogen and release of glucose increase the amount of glucose that is available between meals. Hence, glycogen serves as a buffer to maintain blood-glucose levels. Glycogen’s role in maintaining blood glucose levels is especially important because glucose is virtually the only fuel used by the brain, except during prolonged starvation. Moreover, the glucose from glycogenis readily mobilized and is therefore a good source of energy for sudden,strenuous activity. Unlike fatty acids, the released glucose can provide energy in the absence of oxygen and can thus supply energy for anaerobic activity.

2) Starch- It is a polymer of glucose, found in roots, rhizomes, seeds, stems, tubers and corms of plants, as microscopic granules having characteristic shapes and sizes. Most animals,including humans, depend on these plant starches for nourishment. The intact granules are insoluble in cold water, but grinding or swelling them in warm water causes them to burst. The released starch consists of two fractions.

About 20% is a water soluble material called Amylose. The majority of the starch is a much higher molecular weight substance, consisting of nearly a million glucose units, and called amylopectin.

(a) Amylose is a linear polymer of α-D-glucose, linked together by α 1→4 glycosidic linkages. It is soluble in water, reacts with iodine to give a blue color and the molecularweight of Amylose ranges between 50, 000 – 200, 000.

 (b) Amylopectin is a highly branched polymer,insoluble in water, reacts with iodine to give a reddish violet color. The molecular weight ranges between 70, 000 – 1 000, 000. Branches are composed of 25-30 glucose units linked by α 1→4 glycosidic linkage in the chain and by α 1→6 glycosidic linkage at the branch point.

Figure-2 Showing the structure of Amylopectin

Hydrolysis: Hydrolysis of starch with hot dilute acids or by enzymes gives dextrins of varying complexities, maltose and finally D-glucose

3) Inulin- Inulin is a polysaccharide of fructose (and hence a fructosan) found in tubers and roots of dahlias,artichokes,

and dandelions. It is readily soluble in water and is not hydrolysed by intestinal enzymes. It has a lower molecular weight than starch and colors yellow with iodine.

It is used to determine the glomerular filtration rate, Inulin is of particular use as it is not secreted or reabsorbed in any appreciable amount at the nephron allowing GFR to be calculated, rather than total renal filtration. However, due to clinical limitations, Inulin is rarely used for this purpose and creatinine values are the standard for determining an approximate GFR.

 

Q.2- What are structural polysaccharides? Give a brief account of each of them.

Answer- Cellulose and chitin are structural polysaccharides.

1) Cellulose-Cellulose is the chief constituent of plant cell walls. It is the most abundant of all carbohydrates .It is insoluble in water, gives no colour with iodine and consists of β -D-glucopyranose units linked byβ 1 →4 bonds to form long, straight chains strengthened by cross-linking hydrogen bonds. Mammals lack an enzyme that hydrolyzes the β 1→ 4 bonds, and so cannot digest cellulose. It is an important source of “bulk” in the diet, and the major component of dietary fibre. Microorganisms in the gut of ruminants and other herbivores can hydrolyze the linkage and ferment the products to short-chain fatty acids as a major energy source. There is some bacterial metabolism of cellulose in the human colon.

Figure-3- showing the structure of cellulose 

 

Figure-4- Showing the intrachain and interchain hydrogen bonding in cellulose molecule

Cellulose yields Glucose upon complete hydrolysis. Partial hydrolysis yields cellobiose.

Products obtained from Cellulose-

•         Microcrystalline cellulose : used as binder-disintegrant in tablets

•         Methylcellulose:suspending agent and bulk laxative

•         Oxidized cellulose:hemostat

•         Sodiumcarboxymethyl cellulose: laxative

•         Cellulose acetate:rayon; photographic film; plastics

•         Cellulose acetatephthalate: enteric coating

•         Nitrocellulose:explosives; collodion (pyroxylin)

2) Chitin is a structural polysaccharide in the exoskeleton of crustaceans and insects, and also in mushrooms. It consists of N-acetyl-D-Glucosamine units joined by β 1→ 4 glycosidic bonds. Chitin is the second most abundant carbohydrate polymer and is used commercially incoatings (extends the shelf life of fruits and meats).

 

Figure-5- showing the structureof chitin

Q.3 – What is the difference between dextrins and dextrans?

Answer-

Dextrins- are

•produced along with maltose andglucose by the partial hydrolysis of starch

•dextrins are often referred to aseither Amylo dextrins, erythrodextrins or achrodextrins

•used as mucilages (glues)

•also used in infant formulas(prevent the curdling of milk in baby’s stomach)

Dextrans- are

•products of the reaction of glucose and the enzyme Transglucosidase from Leuconostoc mesenteroides

•contains α (1,4), α (1,6) and α (1,3) linkages

•MW: 40,000; 70,000; 75,000

•used as plasma expanders(treatment of shock)

•also used as molecular sieves to separate proteins and other large molecules (gel filtration chromatography)

•Components of dental plaques.

 

Q.4- What are Glycosaminoglycans? Discuss the structure and functions of various Glycosaminoglycans.

Answer- The most abundant heteropolysaccharides in the body are the glycosaminoglycans (GAGs). GAGs arehighly negatively charged molecules, with extended conformation that imparts high viscosity to the solution. GAGs are located primarily on the surface ofcells or in the extracellular matrix (ECM). Along with the high viscosity ofGAGs comes low compressibility, which makes these molecules ideal for a lubricating fluid in the joints. At the same time, their rigidity provides structural integrity to cells and provides passageways between cells, allowing for cell migration.

The specific GAGs of physiological significance are hyaluronic acid, dermatan sulfate, Chondroitinsulfate, heparin, heparan sulfate, and keratan sulfate. These molecules are long unbranched polysaccharides containing a repeating disaccharide unit. [acidic sugar-amino sugar]n

Although each of these GAGs has a predominant disaccharide component, heterogeneity does exist in the sugarspresent in the make-up of any given class of GAG. The disaccharide units contain either of two modified sugars, N-acetyl galactosamine (GalNAc) orN-acetylglucosamine (GlcNAc), as amino sugars and uronic acid such as glucuronate or Iduronate as acidic sugars.

The amino sugar may also be sulfated on carbon 4 or 6 or on non acetylated nitrogen. The acidic sugarscontain carboxyl groups that are negatively charged at physiological pH, and together with the sulfate groups, give glycosaminoglycans their strongly negative nature.

Because of their large number of negative charges,these heteropolysaccharide chains tend to be extended in solution. They repel each other and are surrounded by a shell of water molecules. When brought together they “slip” past each other. This produces the slippery consistency of mucous secretions and synovial fluid. When a solution of GAG is compressed, the water is squeezed out and GAGs are forced to occupy a smaller volume. When the compression is released the GAGs  get back to their original, hydrated volume because of the repulsion of the negative charges. This property contributes to resilience of synovial fluid and vitreous humor of eye.

THE SPECIFIC GAGs OF PHYSIOLOGICAL SIGNIFICANCE ARE:

1) Hyaluronic acid – The repeating disaccharide unit is N-Acetylglucosamineand Glucuronic acid.

(D-glucuronate + GlcNAc) n

Figure-6-showing the structure of Hyaluronic acid

Occurrence:  Hyaluronic acid is found in synovial fluid, ECM of loose connective tissue, umbilical cord and vitreous humor of the eye. It serves as a lubricant and shock absorber. It is the only GAG that is not limited to animal tissue but is also found in bacteria.

Hyaluronic acid is unique among the GAGs because it does not contain any sulfate and is not found covalently attached to proteins. It forms non-covalently linked complexes with Proteoglycans in the ECM.

Hyaluronic acid polymers are very large (100 – 10,000 kDa) and can displace alarge volume of water.

2) Dermatan sulfate- The repeating disaccharide unit is N-Acetyl Galactosamineand L-Iduronic acid, with variable amount of Glucuronic acids.

(L-Iduronate + GalNAc sulfate) n

 Figure-7-showing the structure of Dermatan Sulfate

Occurrence:  It is found in skin, blood vessels and heart valves

3) Chondroitin sulfate- The repeating disaccharide unit is N-Acetyl galactosamine with sulfate on eitherC-4 or C-6 and Glucuronic acid. Based on presence of sulfate group, it may belabeled as Chondroitin-4-Sulfate or Chondroitin-6-Sulfate.

(D-glucuronate + GalNAc sulfate)n

 

Figure-8-showing the structure of Chondroitin Sulfate

Occurrence:  It is found in cartilages,tendons, ligaments, heart valves and aorta.

It is the most abundant GAG. In cartilages they bind collagen and hold fibers in a tight, strong network.

4) Heparin sulfate – The repeating disaccharide unitis

D- Glucosamine and L-Iduronic acid with variable amounts of Glucuronic acid. Most glucosamine residues are bound in Sulfamide linkages. Sulfate is also found on C-3 or C-6of Glucosamine and C-2 of uronic acid (An average of 2.5 Sulfate per disaccharide unit)

(D-glucuronate sulfate +N-sulfo-D-glucosamine) n

 

Figure-9-Showing the structure of Heparin Sulfate

Occurrence: Heparin is a component of intracellular granules of mast cells lining the arteries of the lungs, liver and skin (Contrary to other GAGs that are extra cellular compounds, it is intracellular). It servesas an anticoagulant.

5) Heparan sulfate: Heparans haveless sulfate groups than heparins. The repeating disaccharide unit is same as Heparin. Some glucosamines are acetylated

Occurrence-  It is an extracellular GAG found in basement membrane and as a ubiquitous component of cell surfaces

6) Keratan sulfate –The repeating disaccharide unitis N-Acetyl glucosamine and galactose (No uronic acid). The sulfate content isvariable and may be present on C-6 of either sugar.

(Gal + GlcNAc sulfate) n

Most heterogeneous GAGs because they contain additional monosaccharides such as L-Fucose, N-Acetyl Neuraminic acid and Mannose.

 Figure-10- showing the structure of Keratan sulfate

Occurrence:  cornea,bone, cartilage;

Keratan sulfates are often aggregated with Chondroitin sulfates.

Proteoglycans (mucoproteins) are formed of glycosaminoglycans (GAGs) covalently attached to the core proteins. They are found in all connective tissues, extracellular matrix (ECM) and on the surfaces of many cell types. Proteoglycans are remarkable for their diversity (different cores, different numbers of GAGs with various lengths and compositions).

Structure of Proteoglycans

All of the GAGs, except Hyaluronic acid are found covalently attached to protein forming proteoglycan monomers.

Structure of Proteoglycan monomer

A Proteoglycan monomer foundin cartilage consists of a core protein to which the linear GAG chains are covalently linked. These chains  which each may be composed of more than 100 monosaccharides, extend out from the core protein and remain separated from each other because of charge repulsion. The resulting structure resembles a ‘Bottle brush’(see figure). In cartilage proteoglycans,the species of glycosaminoglycans include Chondroitin sulfate and Keratan sulfate.

 

 

Figure- 11-showing the structure of Proteoglycan  monomer(Bottle Brush)

 Linkage between the carbohydrate chain and the protein

The linkage of GAGs such as (heparan sulfates andChondroitin sulfates) to the protein core involves a specific 

trisaccharide linker(Galactose-galactose-Xylose). The protein cores of Proteoglycans are rich in Ser and Threonine residues which allow multiple GAG attachments.

An O-Glycosidic bond is formed between the Xylose andthe hydroxyl group of Serine. Some forms of keratan sulfates are linked to the protein core through an

N-asparaginyl bond (N-Glycosidic linkage )

Proteoglycan Aggregates- The proteoglycan monomers associate with a molecule of Hyaluronic acid to form Proteoglycan aggregates. The association is not covalent, but occurs primarily through ionic interactions between the core protein and Hyaluronic acid. The association is stabilized by additional small proteins called Link proteins.

 

Figure-12- showing the structure of proteoglycan aggregate

Functions of Proteoglycans

They perform numerous vital functions within the body.

GAG dependent functions can be divided into two classes:the biophysical and the biochemical.

1) The biophysical functions depend  on the unique properties of GAGs: the ability to fill the space, bind and organize water molecules and repel negatively charged molecules. Because of high viscosity and low compressibility they are ideal for a lubricating fluid in the joints. On the other hand their rigidity provides structural integrityto the cells and allows the cell migration due to providing the passageways between cells.

2) The other, more biochemical functions of GAGs are mediated by specific binding of GAGs to other macromolecules,mostly proteins. Proteoglycansparticipate in cell and tissue development and physiology.

3) Heparin acts as an anticoagulant and is used in the clinical practice.

Q.5-What do you understand by the term mucopolysaccharidoses? Give a brief accountin a tabular manner.

Answer-Glycosamino glycans are degraded by Lysosomal Hydrolases. A deficiency of one of the Hydrolase results in a mucopolysaccharidoses.These are hereditary disorders in which the Glycosamino glycans accumulate in tissues, causing symptoms such as skeletal and extra cellular matrix deformities, and mental retardation.

Type

Main diseases

Deficient enzyme

Accumulated products

Symptoms

MPS I

Hurler’s syndrome

α-L-Iduronidase Heparan sulfateDermatan sulfate
  • Mental retardation
  • Micrognathia
  • Coarse facies
  • Macroglossia
  • Retinal degeneration
  • Corneal clouding
  • Cardiomyopathy

MPS II

Hunter syndrome

Iduronate sulfatase
  • Heparan sulfate
  • Dermatan sulfate
  • Mental retardation

(similar, but milder, symptoms to Hurler syndrome)

MPS III

Sanfilippo syndrome A

Heparan sulfamidase
  • Heparan sulfate
  • Developmental delay
  • Severe hyperactivity
  • Spasticity
  • Motor dysfunction
  • Death by the second decade

Sanfilippo syndrome B

N-acetylglucosaminidase

Sanfilippo syndrome C

Acetyl-CoA:alpha-glucosaminide acetyltransferase

Sanfilippo syndrome D

N-acetyl glucosamine 6-sulfatase

MPS IV

Morquio syndrome A

Galactose-6-sulfate sulfatase
  • Keratan sulfate
  • Chondroitin 6-sulfate
  • Severe skeletal dysplasia
  • Short stature
  • Motor dysfunction

Morquio syndrome B

Beta-galactosidase
  • Keratan sulfate

MPS VI

Maroteaux-Lamy syndrome

N-acetylgalactosamine-4-sulfatase
  • Dermatan sulfate
  • Severe skeletal dysplasia
  • Short stature
  • Motor dysfunction
  • Kyphosis
  • Heart defects

MPS VII

Sly syndrome

β-glucuronidase
  • Heparan sulfate
  • Dermatan sulfate
  • Chondroitin 4,6-sulfate
  • Hepatomegaly
  • Skeletal dysplasia
  • Short stature
  • Corneal clouding
  • Developmental delay

MPS IX

Natowicz syndrome

Hyaluronidase
  • Hyaluronic acid
  • Nodular soft-tissue masses around joints
  • Episodes of painful swelling of the masses
  • Short-term pain
  • Mild facial changes
  • Short stature
  • Normal joint movement
  • Normal intelligence

Laboratory Investigations

1)   Urine test,which shows the excessive excretion of undegraded mucopolysaccharides, which isspecific for a specific type.

2)   Cetyl Trimethyl ammonium bromide test is undertaken to confirm the presence of gylcosaminoglycans in urine.

3)    Absence of Lysosomal enzyme in cultured fibroblasts.

4)    Culture of cells from amniotic fluid obtained by amniocentesis for enzyme testing (prenatal testing)

5)    X-ray of spine and chest.

Prenatal diagnosis using amniocentesis and chorionic villous sampling can verify if a fetus either carries a copy of the defective gene or is affected with the disorder. Genetic counselling can help parents who have a family history of the Mucopolysaccharidoses determine if they are carrying the mutated gene that causes the disorders.

Treatment

This disease can be treated by bone marrow transplantation (BMT) and umbilical cord blood transplantation (UCBT) preferably before the age of 18 months. Abnormal physical characteristics, except for those affecting the skeleton and eyes, can be improved, and neurologic degeneration can often be halted. BMT and UCBT are high-risk procedures with high rates of morbidity and mortality. There is no cure for Mucopolysaccharidoses.

Gene therapy is under  trial as a permanent cure. Enzyme replacement therapies are currently in use, they have proven useful in reducing non-neurological symptoms and pain.

Please help "Biochemistry for Medics" by CLICKING ON THE ADVERTISEMENTS above!

Q.1- Give a brief account of the disaccharides of biological importance.

Answer- The disaccharides are sugars composed of two monosaccharide residues linked by a glycoside bond. The glycosidic bonds are readily hydrolyzed by acids but resist cleavage by alkaline hydrolysis.The glycosidic linkage is formed by loss of one molecule of water as shown in the reaction below

The physiologically important disaccharides are as follows

1) Maltose- It is also called ‘Malt sugar’ and is composed of 2 glucose monomers in an  α-(1,4) glycosidic bond. 

Figure-1- showing the structure of Maltose

It is produced by the partial hydrolysis of starch (either salivary amylase or pancreatic amylase) and is used as a nutrient (malt extract; Hordeum vulgare); as a sweetener and as a fermentative reagent. It is hydrolyzed to glucose by maltase.

Since it has a free active group hence- it is reducing in nature, can exist in α and

βanomeric forms and can exhibit mutarotation. 

2) Lactose- is also called ‘Milk Sugar’ and is composed of galactose joined to glucose by a β-1,4-glycosidic linkage.

Figure-2- showing the structure of Lactose

Lactose is the only carbohydrate of milk (7gm% and 4gm% in human& bovine milk respectively). It is synthesized by mammary glands during lactation and is the best food for infants [Least sweet-laxative-non fermentable). It has a free active group, shows reducing properties and exhibits mutarotation.

Milk contains the α and β-anomers in a 2:3 ratio. β-lactose is sweeter and more soluble than ordinary α – lactose. It is used in infant formulations, medium for penicillin production and as a diluent in pharmaceuticals. Lactose is hydrolyzed to its component monosaccharides by lactase in human beings and by β -galactosidase in bacteria. Lack of lactase (alactasia) leads to lactose intolerance—diarrhea and flatulence; Lactose may be excreted in the urine in pregnancy

 3) Sucrose- Sucrose (common table sugar) is obtained commercially from cane or beet. The anomeric carbon atoms of a glucose unit and a fructose unit are joined in this disaccharide; the configuration of this glycosidic linkage is α-for glucose and β-for fructose. Sucrose can be cleaved into its component monosaccharides by the enzyme sucrase. Sucrase, lactase, and maltase are located on the outer surfaces of epithelial cells lining the small intestine. Rare genetic lack of sucrase leads to sucrose intolerance— diarrhoea and flatulence.

 Figure-3- showing the structure of Sucrose.

 Sucrose has no free reactive group because the anomeric carbons of both monosaccharides units are involved in the glycosidic bond. So, sucrose neither shows reducing nor mutarotation characters. Sucrose is called invert sugar because the optical activity of sucrose ( dextrorotatory) is inverted after hydrolysis [by an acid or an enzyme (invertase or sucrase)] into equimolar mixture of its two components glucose (+52.5) and fructose (-92.5) and the optical activity of the mixture becomes levorotatory.  

 4) Lactulose- is composed of β-galactose and β fructose in a β -(1,4) glycosidic bond. It is a semi-synthetic disaccharide (not naturally occurring) and is not absorbed in the GI tract. It is used either as a laxative or in the management of portal systemic encephalopathyIt is metabolized in distal ileum and colon by bacteria to lactic acid, formic acid and acetic acid. In chronic liver diseases the level of ammonia in the blood is increased, so, the oral Lactulose by microfloral conversion in the colon to organic acid will relieve the high ammonia. On the other hand, the osmotic activity of the disaccharide will cause diarrhea which will remove toxic products.

 5) Isomaltose- is composed of two glucose unite linked by α 1,6 glycosidic linkage. It is produced by enzymatic hydrolysis of starch (at the branch point in Amylopectin).

Figure-4- showing the structure of Isomaltose, 2 glucose units are linked together by α 1,6 glycosidic linkage
Q.2– Enlist the Oligosaccharides of biological importance

 Answer- Oligosaccharides are condensation products of three to ten monosaccharides. Most are not digested by human enzymes. Some of the Oligosaccharides of biological importance are as follows-

Trisaccharide: Raffinose (glucose, galactose and fructose)

Tetrasaccharide: Stachyose (2 galactose, glucose and fructose)

Pentasaccharide: Verbascose (3 galactose, glucose and fructose)

Hexasaccharide: Ajugose (4 galactose, glucose and fructose)

 

Figure-5- showing the structure of Stachyose (2 galactose, glucose and fructose).

Stachyose is a constituent of many plants and is used to prevent constipation. Melezitose- a constituent of honey also contains Glucose, fructose and some volatile oils. Cycloheptamylose- a breakdown product of starch is used in chromatographic procedures

.                        

 

 Figure –6-showing the structure of Cycloheptamylose

Oligosaccharides occur widely as components of antibiotics derived from various sources- e.g.- Bleomycin A(An antitumor agent) and Streptomycin used as broad spectrum Antibiotic are oligosaccharides.

Please help "Biochemistry for Medics" by CLICKING ON THE ADVERTISEMENTS above!

Q.1- What are sugar alcohols? How are they produced in the body? What is their biological or clinical significance?

 Answer- Sugar alcohols are produced by the reduction of the carbonyl group (Aldehyde/ ketone group) of monosaccharides. The reaction can be represented as follows-

a) Reduction of aldoses takes place at C-1 to form sugar alcohol

Figure-1 -showing the formation of Sugar alcohol from Aldose sugar

b) Reduction of Ketose sugars takes place at C-2 to form Sugar Alcohol

Figure-2 showing the formation of Sugar alcohol from ketose sugar

Details of Reaction-

Under specific conditions of temperature and pressure, sugars can be reduced in the presence of hydrogen. The resultant product is a polyol or sugar alcohol (alditol)

but reduction of ketose sugar produces a new asymmetric carbon atom (See figure ), thus two types of  sugar alcohols can be produced, which are epimer of each other.

Examples

1)   Glucose form Sorbitol (glucitol)

2)   Mannose forms mannitol

3)   Fructose forms a mixture of mannitol and sorbitol

4)   Galactose forms Dulcitol

5)   Glyceraldehyde gives glycerol

6)   Ribose forms Ribitol

Structure and Significance of some Sugar alcohols-

1) Sorbitol- In Diabetes Mellitus excess of Glucose is converted to Sorbitol. The osmotic effect of Sorbitol is responsible for many of the complications of diabetes mellitus e.g. Cataract formation in lens. Clinically sorbitol is dehydrated and nitrated to form Isosorbide mono and dinitrate, both of which are used for the treatment in Angina.

2) Mannitol- Mannitol is also osmotically active and is used as an infusion to lower the intracranial tension by producing forced diuresis.

3) Dulcitol- excess of galactose in galactosemia is converted to Dulcitol. The osmotic effect of Dulcitol is similar to Sorbitol and is responsible for premature cataract formation in affected patients of galactosemia.

4) Xylitol- is produced in Uronic acid pathway of Glucose utilization; it is subsequently oxidized to produce D- Xylulose.

5) Glycerol- is produced from Glyceraldehyde. Glycerol is used for the formation of Triglycerides and phospholipids. Clinically glycerol is nitrated to form Nitroglycerine , that is used for the treatment of angina.

6) Myo- Inositol- It is hexahydroxy alcohol, also considered a vitamin It is present in the plasma membrane and acts as a second messenger for the action of hormones.

7) Ribitol- is used in the formation of vitamin B2- (Riboflavin )

Figure-3- Showing the structures of commonly found sugar alcohols

Q.2- What are Glycosides? Discuss the clinical significance of Glycosides.

Answer- Acetal or ketal derivatives formed when a monosaccharide reacts with an alcohol are called glycosides. They are formed by the reaction of the hydroxyl group of anomeric carbon (hemiacetal or hemiketal)of monosaccharide with hydroxy group of second molecule with the loss of an equivalent of water.

Figure-4-showing the formation of Glycosides

The second molecule may be-

1)   Another sugar (Glycon)-  e.g. formation of disaccharides and polysaccharides.

2)   Non Carbohydrate (Aglycon)- such as Methanol, Glycerol, Sterol or Steroids etc.

In naming of glycosides, the” ose” suffix of the sugar name is replaced by “oside”, and the alcohol group name is placed first. For example, D-glucose reacts with methanol in an acid-catalyzed process: the anomeric carbon atom reacts with the hydroxyl group of methanol to form two products, methyl α -D-glucopyranoside and methyl β -D-glucopyranoside. These two gluco pyranosides differ in the configuration at the anomeric carbon atom. The new bond formed between the anomeric carbon atom of glucose and the hydroxyl oxygen atom of methanol is called a glycosidic bond specifically, an O-glycosidic bond. See figure below.

Figure- 5-Showing Methyl Glucopyranoside

The anomeric carbon atom of a sugar can be linked to the nitrogen atom of an amine to form an N-glycosidic bond. Nucleosides are adducts between sugars such as ribose and amines such as adenine (the linkage between them is N-Glycosidic linkage).

Examples of Glycosides-Glycosides are present in many drugs, spices and in the constituents of animal tissues. Glycosides comprise several important classes of compounds such as hormones, sweeteners, alkaloids, flavonoids, antibiotics, etc. The glycosidic residue can be crucial for their activity or can only improve pharmacokinetic parameters.

1) Cardiac Glycosides

Cardiac glycosides all contain steroids or genin component as aglycone in combination with sugar molecules. These include derivatives of digitalis and strophanthus such as oubain.

 2) Other glycosides such as streptomycin are used as antibioticsPhloridzin is another glycoside which is obtained from the root and bark of apple tree. It blocks the transport of sugar across the mucosal cells of small intestine and also renal tubular epithelium. It displaces Na+ from the binding site of “carrier protein” and prevents the binding of sugar molecule and produces Glycosuria.

 3) Glycosides of vitamins, both hydrophilic and lipophilic often occur in nature. Glycosylated vitamins have an advantage over the respective aglycone in their better solubility in water (especially the lipophilic ones), stability against UV-light, heat and oxidation, reduction of the bitter taste and odor(e.g., thiamine), and resistance to an enzymatic action. Some of the vitamin glyco conjugates have altered or improved Pharmacokinetic properties.

 Q.3- What are Amino Sugars and amino sugar acids? Discuss in brief about their biological importance?

 Answer- Amino groups may be substituted for hydroxyl group of sugars to give rise to amino sugars. Generally, the amino group is added to the second carbon of the hexoses. The most common aminosugars are Glucosamine and Galactosamine.

Figure- 6-Showing amino sugars.The OH group present at the second position is replaced by NH2 group

 The amino group in the sugar maybe further acetylated to produce N-Acetylated sugars such as N-AcetylGlucosamine (GluNac ) and N-Acetyl-Galactosamine(GalNAc), etc. These are important constituents of glycoproteins, mucopolysaccharides and cell membrane antigens. Glucosamine is the chief constituent of cell wall of fungi and a constituent of shells of crustaceae (Crabs, Lobsters etc), where it is found as a polymer of N-Acetyl Glucosamine called Chitin. Hence this amino sugar is also called Chitosamine.

Galactosamine occurs as N-Acetyl Galactosamine in Chondroitin sulphates which are present in cartilages, bones, tendons and heart valves. Hence Galactosamine is also called Chondrosamine.

Certain antibiotics, such as Erythromycin, Carbomycin contain amino sugars.

In some amino sugar the anomeric OH group is replaced by amino group. e.g. Ribosylamine, which is used for the de novo synthesis of Purine nucleotides.

Amino sugar acids are produced by condensation of amino sugar with Pyruvic or lactic acid. E.g.Muramic acid is produced by the condensation of lactic acid with D- Glucosamine. Certain bacterial cell walls contain Muramic acid.

N-Acetyl Neuraminic acid is formed from the condensation of Pyruvic acid with N-Acetyl Mannosamine.(Figure)

 

Figure-7- showing the structure of amino sugar acids

N-Acetyl Neuraminic acid (NANA), also called Sialic acid, is a nine carbon derivative and is an important component of glycoproteins and gangliosides (lipids). Neuraminidase is the enzyme which removes NANA from its binding with other compounds.

Q.4- What are deoxy sugars? How are they produced in the body? What is their biological significance?

Answer- Deoxy Sugars are monosaccharides which lack one or more hydroxyl groups on the molecule. They are formed by the removal of oxygen, generally from OH group present at C-2  or other locations of monosaccharides 

Examples of deoxy Sugars-
1) One quite ubiquitous deoxy sugar is2’-deoxy ribose which is the sugar found in DNA.
2) 6-deoxy-L-mannose (L-rhamnose) is used as a fermentative reagent in bacteriology.
3) L-Fucose (6-deoxy.L-galactose) is a component of glycoproteins and gangliosides of cell membranes.

Figure-8 -showing the structures of commonly found deoxy sugars.

Q.5- What is the effect of strong acids on monosaccharides?

Answer- Monosaccharides are normally stable to dilute acids, but are dehydrated by strong acids.

•  D-ribose (Pentoses) when heated with concentrated HCl yields furfural (cyclic anhydride)

• D-glucose(Hexoses) under the same conditions yields 5-hydroxymethyl furfural

Practical Applications– The furfural derivatives can condense with phenolic compounds to give colored products. This forms the basis for Molisch test. This test is a sensitive test but it is nonspecifically given by all carbohydrates. Alpha nephthol is used in this test. A purple colored ring develops if carbohydrate is present.

Similar to this Seliwanoff Test is undertaken with Resorcinol, a cherry red color is produced if fructose is present.
The other tests are Anthrone test and Bial’s test etc.

Q.6- Enlist the important reactions of monosaccharides.

Or

Describe the chemical properties of the anomeric carbon in monosaccharides.

Answer- The reactions/ properties of monosaccharides are as follows-
All the reactions are taking place at C-1  (CHO) in aldoses and C-2(C=O) in ketoses, that is why these are called functional groups.

Reactions of monosaccharides-

a) Osazone formation

b) Cyanohydrin reaction

c) Reduction

d) Oxidation

e) Action of base

f) Action of acid

g) Glycoside formation

h) Ester formation

a) Osazone formation-This test is used for the identification of sugars. It involves the reaction of monosaccharide with phenyl hydrazine, a crystalline compound. All reducing sugars form osazones with excess of phenyl hydrazine when kept at boiling temperature. Each sugar has a characteristic crystal form of osazones.  The reaction involved can be represented as follows-

Reactions involved in the formation of Osazone crystals

Three molecules of phenyl hydrazine are required, the reaction takes place at first two carbon atoms. The upper equation shows the general form of the osazone reaction, which affects an alpha-carbon oxidation with formation of a bis- phenylhydrazone, known as an osazone.

D-fructose and D-mannose give the same osazone as D-glucose. The difference in these sugars present on the first and second carbon atoms are masked when osazone crystals are formed. Hence these three sugars form similar needle shaped crystals arranged like sheaves of corn or a broom.  It is seldom used for identification these days . HPLC or mass spectrometry is used for the identification of sugars present in the biological fluids.

 

Figure-9-a) showing formation of osazone crystals,     

b) Needle shaped crystals of Glucose, Mannose and Fructose.

b) Cyanohydrin reaction- It involves the reaction of an aldose with HCN. It is used to increase the chain length of monosaccharides. It results in a Cyanohydrin formation   which is then hydrolyzed to an acid and reduced to the aldehyde. It is also known as the Kiliani -Fischer synthesis. It can prepare all monosaccharides from D-glyceraldehyde.

Figure-10- showing the chain lengthening procedure by Kiliani -Fischer synthesis

c) Reduction-Sugar alcohols are produced by the reduction of the carbonyl group (Aldehyde or ketone) of monosaccharides (Check the details in sugar alcohols).

d) Oxidation- Sugar acids are produced by the oxidation of the AldehydeC-1 (Aldonic acid) or terminal hydroxyl group at C-6 of Aldo sugar (Uronic acid) or both C-1 and C-6 (Saccharic acid). Check the details in sugar acids.

e) Action of Base- Sugars are weak acids and can form salts at high pH. A1,2-enediol salt is formed as the result. This allows the interconversion of D-mannose, D-fructose and D-glucose. The reaction is known as the Lobry de Bruyn-Alberta von Eckenstein reaction.  

Enediols obtained by the action of base are quite susceptible to oxidation when heated in the presence of an oxidizing agent. Copper sulfate is frequently used as the oxidizing agent and a red precipitate of Cu2O is obtained. Sugars which give this reaction are known as reducing sugars. Some of the frequently used solutions for detecting the presence of reducing sugars in biological fluids are as follows-
1) Fehling’s solution: KOH or NaOH andCuSO4
2) Benedict’s solution: Na2CO3 and CuSO4
3) Clinitest tablets are used to detect urinary glucose in diabetics.

f) Action of Acids and

g) Glycoside formation- Check the details above.

 h) Ester formation-The –OH groups of monosaccharides can behave as alcohols and react with acids (especially phosphoric acid) to form esters.

Figure 11- Showing sugar ester

These esters serve as intermediates in several metabolic pathways

 Figure-12- showing the summary of reactions of monosaccharides

Please help "Biochemistry for Medics" by CLICKING ON THE ADVERTISEMENTS above!

Q.1- Discuss in brief about the isomerism in monosaccharides.

 Answer- The sugar molecules having asymmetric carbon atoms exhibit isomerism.

Asymmetric carbon atom- It is the carbon atom that is attached to four different groups. All monosaccharides except- Dihydroxy acetone, have asymmetric carbon atoms. Based on the presence of asymmetric carbon atoms the following types of isomerism of monosaccharides are observed in the human system-

1) D and L isomerism- The designation of a sugar isomer as the D form or of its mirror image as the L form is determined by its spatial relationship to the parent compound of the carbohydrates, the three-carbon sugar glycerose (glyceraldehyde), and also called reference sugar. The L and D forms of this sugar are shown in Figure-1. The orientation of the —H and —OH groups around the carbon atom adjacent to the terminal primary alcohol carbon (carbon 5 in glucose) determines whether the sugar belongs to the D or L series. When the —OH group on this carbon is on the right (as seen in Figure-1), the sugar is the D isomer; when it is on the left, it is the L isomer.

 Figure-1 showing D and L isomers of Glyceraldehyde

Glyceraldehyde has a single asymmetric carbon and, thus, there are two stereoisomers of this sugar. D-Glyceraldehyde and L-glyceraldehyde are enantiomers, or mirror images of each other. Dihydroxyacetone lacks asymmetric carbon atom thus it has no D and L isomers.

Most of the monosaccharides occurring in mammals are D sugars, and the enzymes responsible for their metabolism are specific for this configuration. Simple monosaccharides with four, five, six, and seven carbon atoms have multiple asymmetric carbons, they exist as diastereoisomers, isomers that are not mirror images of each other. In regard to these monosaccharides, the symbols D and L designate the absolute configuration of the asymmetric carbon farthest from the aldehyde or keto group.

D-Ribose, the carbohydrate component of RNA, is a five-carbon aldose. D-Glucose, D-mannose, and D -galactose are abundant six-carbon aldoses.

Some sugars naturally occur in the L form e.g. L-Arabinose and L-Fucose are found in glycoproteins, while L- Xylulose is produced during the metabolism of Glucose in Uronic acid pathway. It is subsequently converted to its D form.

 2) Optical Isomerism- The presence of asymmetric carbon atoms also confers optical activity on the compound. When a beam of plane-polarized light is passed through a solution of an optical isomer, it rotates either to the right, dextrorotatory (+), or to the left, levorotatory (–). The direction of rotation of polarized light is independent of the stereochemistry of the sugar, so it may be designated D(–), D(+), L(–), or L(+). For example, the naturally occurring form of fructose is the D(–) isomer. In solution, glucose is dextrorotatory, and glucose solutions are sometimes known as dextrose.

Measurement of optical activity in chiral or asymmetric molecules using plane polarized light is called Polarimetry. The measurement of optical activity is done by an instrument called Polarimeter.

 3) Epimers- Isomers differing as a result of variations in configuration of the —OH and —H on carbon atoms 2, 3, and 4 of glucose are known as epimers. Biologically, the most important epimers of glucose are mannose and galactose, formed by epimerization at carbons 2 and 4, respectively.  Mannose and Galactose are not epimers of each other as they differ in configuration around 2 carbon atoms.

D- Xylulose is the C-3 epimer of D-Ribulose. See the figure-2 for the number of possible isomers of aldoses and ketoses.

 4) Pyranose and furanose ring structures: The ring structures of monosaccharides are similar to the ring structures of either pyran (a six-membered ring) or furan (a five-membered ring) . For glucose in solution, more than 99% is in the pyranose form.

 5) Anomers- The ring structure of an aldose is a hemiacetal, since it is formed by combination of an aldehyde and an alcohol group. Similarly, the ring structure of a ketose is a hemiketal.  The ring can open and reclose allowing the rotation to occur around the carbon bearing the reactive carbonyl group yielding two possible configurations- α and β  of the hemiacetal and hemiketal. The carbon about which this rotation occurs is called Anomeric carbon and the two stereoisomers are called Anomers. Crystalline glucose is α-D-glucopyranose. The cyclic structure is retained in solution, but isomerism occurs about position 1, the carbonyl or anomeric carbon atom, to give a mixture of α-D-glucopyranose (38%) and β-D glucopyranose (62%). Less than 0.3% is represented by α and β- anomers of Glucofuranose.

 6) Aldose-ketose isomerism: Fructose has the same molecular formula as glucose but differs in its structural formula, since there is a potential keto group in position 2, the anomeric carbon of fructose (Figures3), whereas there is a potential aldehyde group in position 1, the anomeric carbon of glucose.

 

 

Figure-2- showing the possible isomers of  Aldose D sugars containing 3,4,5 and 6 carbon atoms. Each of them will have the L isomer also. Thus Glucose has 8+8 =16 isomers i.e. 8 D isomers and 8 L isomers. The number of possible isomers of a sugar is derived from the formula 2n , where n represents the number of asymmetric carbon atoms. Taking in to account the α  and β  anomers , there are 32 possible isomers of Glucose.

 

Figure- 3- Showing the possible isomers of ketose sugars (D) with 3, 4 ,5 and 6  carbon atoms. The number of asymmetric carbon atoms are less in ketose sugars, thus there are less isomers as compared to aldose with the same number of carbon atoms. Dihydroxy acetone has no isomer while fructose has 3 asymmetric carbon atoms, so it has in total 8 isomers, 4 D and 4 L isomers, taking in to account the α and β anomers, there are 16 possible isomers of fructose.

 Q.2- What is Mutarotation? Describe in context to Glucose.

 Answer- Carbohydrates can change spontaneously between α and β configurations through intermediate open chain formation, this leads to a process known as Mutarotation. There is gradual change in optical rotation of the solution. This can be explained as follows-

 

 

 Figure-4- Showing mutarotation of Glucose

When D Glucose is crystallized at room temperature and a fresh solution is prepared, its specific rotation of polarized light is +112ο , but after 12-18 hours it changes to +52.5 ο

If the initial crystallization takes place at 98 ο and then solubilized, the specific rotation is found to be +19 ο, which also changes to +52.5 ο within a few hours. This change in rotation with time is called Mutarotation. At room temperature the alpha form predominates and the specific rotation is+112ο, there is transient ring opening and change in configuration. In the second condition when the crystallization takes place at 98 ο , the Beta form predominates and the specific rotation is+19 ο. Both undergo Mutarotation and at equilibrium one third molecules are α type and two third are β variety to get the specific rotation of +52.5 ο.

 

 Figure -5-showing graphical representation of Mutarotation.

 Q.3- What are sugar acids? Give examples of such acids and state their biological importance.

 Answer- Sugar acids are formed by the oxidation of –

1)  Aldehyde group(C1) to form Aldonic acid, or

2)  Primary Alcoholic group (C5) in an aldohexose to form uronic acid or

3)  Both groups to form Saccharic acid.

Details of Reactions- (Figure-6)

1)   Oxidation of Aldehyde group- Under mild conditions, in the presence of Hypobromous acid, the aldehyde group is oxidized to form Aldonic acid. Thus, Glucose is oxidized to Gluconic acid, Mannose to form Mannonic acid and Galactose to form Galactonic acid. Formation of Gluconic acid by the activity of Glucose oxidase is the basis for the Quantitative estimation of urinary and blood Glucose.(See details below )

2)   Oxidation of Primary Alcoholic acid- Under special conditions when the aldehyde group is protected, and the molecule is oxidized at the primary alcoholic group the product is a Uronic acid. Thus Glucose is oxidized to form Glucuronic acid, Galactose to form Galacturonic acid and Mannose is oxidized to Mannuronic acid. Glucuronic acid is used in the body for conjugation reactions to convert the toxic water insoluble compounds in to nontoxic water-soluble form, which can be easily excreted in urine. Glucuronic acid and its epimer Iduronic acid are used for the synthesis of heteropolysaccharides.

3)   Oxidation of both Aldehyde and Primary Alcoholic group-Under strong acidic conditions (Nitric acid and heat)  the first and the last carbons are simultaneously oxidized to form dicarboxylic acids, known as Saccharic acids. Glucose is thus oxidized to form Gluco Saccharic acid, Mannose to Mannaric acid and Galactose to Mucic Acid .The mucic acid forms insoluble crystals and is the basis for a test for identification of Galactose.

 

Figure-6- showing the formation of sugar acids.

Please help "Biochemistry for Medics" by CLICKING ON THE ADVERTISEMENTS above!

Advertisement