Carbohydrates Note M.N Chatterjea For Nurses Part IV

Carbohydrates Note For Nurses Part IV  

Disaccharides, Biomedical Importance of Disaccharides, Oligosaccharides, Biomedical Importance of Oligosaccharides.

Disaccharides

Three most common disaccharides of biological impor[1]tance are: Maltose, Lactose and Sucrose. Their general molecular formula is C12H22O11 and they are hydrolysed by hot acids or corresponding enzymes as follows

1:Maltose =D Gllucose +D Glucose

2: Lactose =D Gllucose +D Galactose

3:Sucrose = D Glucose +D Fructose

The disaccharides are formed by the union of two constituent monosaccharides with the elimination of one molecule of water. 

The points of linkage, the glycosidic linkage varies, as does the manner of linking and the properties of the disaccharides depend to a great extent on the type of the linkage. 

If both of the two potential aldehyde/or ketone groups are involved in the linkage the sugar will not exhibit reducing properties and will not be able to form osazones, e.g. sucrose. 

But if one of them is not bound in this way, it will permit reduction and osazone formation by the sugars, e.g. Lactose and Maltose

Invert Sugars and ‘Inversion’ Sucrose is dextrorotatory (+62.5°) but its hydrolytic products are laevorotatory because fructose has a greater specific laevorotation than the dextrorotation of glucose.

Biomedical Importance of Disaccharides

• Various food preparations, such as baby and invalid foods available, are produced by hydrolysis of grains and contain large amounts of maltose. From nutritional point of view they are thus easily digestible.

• In lactating mammary gland, the lactose is synthesised from glucose by the duct epithelium and lactose present in breast milk is a good source of energy for the newborn baby.

• Lactose is fermented by ‘Coliform’ bacilli (E. coli) which is usually non-pathogenic (lactose fermenter) and not by Typhoid bacillus which is pathogenic (lactose non[1]fermenter). This test is used to distinguish these two microorganisms.

• ‘Souring’ of milk: Many organisms that are found in milk, e.g. E. coli, A. aerogenes, and Str. lactis convert lactose of milk to lactic acid (LA) thus causing souring of milk.

 • Sucrose if introduced parenterally cannot be utilised, but it can change the osmotic condition of the blood and causes a flow of water from the tissues into the blood. 

Thus clinicians use it in oedema like cerebral oedema. If sucrose or some other disaccharides are not hydrolysed in the gut, due to deficiency of the appropriate enzyme, diarrhoea is likely to occur

Sr.No	Lactose	Sucrose
01	Known also as ‘milk sugar’	Common table sugar (cane sugar)
02	Structurally one molecule of D-Glucose and one molecule of D-Galactose are joined together by glycosidic linkage (ẞ 1 →4)	Structurally one molecule of D-Glucose and one molecule of D-Fructose joined together (a 1→→2)
03	Hydrolysed to give one molecule of glucose and one molecule of galactose	Hydrolysed to give one molecule of glucose and one molecule of fructose
04	Specific enzyme which hydrolyses is called lactase, which is present in intestinal juice	Specific enzyme which hydrolyses is called Sucrase (Invertase) which is present in intestinal juice
05	Dextrorotatory disaccharide	Also dextrorotatory (+66.5°), but hydrolytic products are laevorotatory (-19.5") Hydrolytic products are called invert sugars and process is called Inversion.
06	As anomenic carbon is free, can form a and B forms and exhibits mutarotation	As both anomenic carbons are involved in linkage, cannot form a and B-forms
07	Specific rotation of the solution is +55.2°	Does not exhibit mutarotation
08	Can reduce alkaline copper sulphate solution like Benedict's qualitative reagent, Fehling's solution	Does not reduce alkaline copper sulphate solution
09	Does not reduce Berfoed's solution	Does not reduce Berfoed's solution
10	Forms Osazone. Lactosazone crystals have typical hedge-hog shape or Powder puff	Cannot form osazones
11	Hydrolytic products on treatment with conc. HNO3 can form "mucic acid"	Cannot form mucic acid
12	Fearon's test is positive	Fearon's test is negative
13	Can be synthesised in lactating mammary gland from glucose	Not so
14	In lactating mother lactose may appear in urine, producing Lactosuria	Not so

Oligosaccharides

Biomedical Importance:

Integral membrane proteins contain covalently attached carbohydrate units, oligosaccharides, on their extracellular face. Many secreted proteins, such as antibodies and coagulation factors also contain oligosaccharide units. 

These carbohydrates are attached to either the side-chain O2 atom of serine or threonine residues by O-glycosidic linkages or to the side chain nitrogen of Asparagine residues by N-glycosidic linkages. 

N-linked oligosaccharides contain a common pentasaccharide core consisting of three mannose and two N-acetyl glycosamine residues. Additional sugars are attached to this common core in many different ways to form the great variety of oligosaccharide patterns found in glycoproteins. 

The diversity and complexity of the carbohydrate oligosaccharide units of glycoprotein suggest that they are rich in information and are functionally important. Carbohydrates participate in molecular targeting and cell-cell recognition. 

The removal of glycoproteins from the blood is accomplished by Surface Protein Receptors on Liver cells, e.g. Asialo-glycoprotein receptor. Many newly synthesised glycoproteins such as, immuno[1]globulins (antibodies) and peptide hormones, contain oligosaccharide carbohydrate units with terminal sialic acid residues. 

When the function of particular protein is over, in hours or days, terminal sialic acid residues are removed by Sialyses on the surface of blood vessels. 

The exposed galactose residues of these trimmed proteins are detected by the asialoglycoprotein receptors on liver cell membrane. The complex of the asialoglycoprotein and its receptor is then internalised by the liver, by “endocytosis”, to remove the trimmed glycoprotein from the circulating blood. 

The oligosaccharide units actually mark the passage of time and determines when the proteins carrying them should be taken out of circulation. The rate of removal of sialic acid from glycoproteins is controlled by the structure of the protein itself. 

Thus, proteins can be designed to have life times ranging from a few hours to many weeks depending on the physiological and biological needs.

Reference:

Notes Made By The Help of "The Text Book of Medical Biochemistry By MN. Chatterjea 8th Edition"