Carbohydrates Note For Nurses Part III
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Monosaccharides
Biological Importance
(a) Trioses: Both
D-glyceraldehyde and dihydroxyacetone occur in the form of phosphate esters, a intermediates
in glycolysis. They are also the precursors of glycerol, which the organism synthesizes
and incorporates into various types of lipids.
• D-ribose is a constituent of nucleic acid RNA; also, as a
constituent of certain coenzymes, e.g., FAD, NAD, coenzyme A.
• D-2-deoxyribose is a constituent of DNA.
• Phosphate esters of ketopentoses—D-ribulose and D-xylulose occur
as intermediates in HMP shunt.
• L-xylulose is a metabolite of D-glucuronic acid and is excreted
in urine of humans afflicted with a hereditary abnormality in metabolism called
pentosuria.
• L-fucose (methyl pentose): occurs in glycoproteins.
• D-Lyxose: It forms a constituent of lyxoflavin isolated from
human heart muscle whose function is not clear.
(d) Hexoses
1. D-Glucose: (Synonyms:
Dextrose, Grape Sugar)
• It is the chief physiological sugar present in normal blood
continually and at fairly constant level, i.e. about 0.1 per cent.
• All tissues utilize glucose for energy. Erythrocytes and Brain
cells utilize glucose solely for energy purposes.
• Occurs as a constituent of disaccharide and polysaccharides.
• Stored as glycogen in liver and muscles mainly.
• Shows mutarotation
2. D-galactose: Seldom found
free in nature. In combination it occurs both in plants and animal
• Occurs as a constituent of milk sugar lactose and also in tissues
as a constituent of galactolipid and glycoproteins.
• It is an epimer of glucose and differs in orientation of H and OH
on carbon-4.
• It is less sweet than glucose and less soluble in water.
• It is dextrorotatory and shows mutarotation.
• On oxidation with hot HNO3, it yields dicarboxylic acid, mucic
acid; which helps in its identification, since the crystals of mucic acid are
not difficult to produce and have characteristic shape.
3. D-fructose: It is a
ketohexose and commonly called as fruit sugar, as it occurs free in fruits.
• It is very sweet sugar, much sweeter than sucrose and more
reactive than glucose. It occurs as a constituent of sucrose and also of the
polysaccharide inulin. It is levorotatory and hence is also called laevulose.
• Exhibits mutarotation.
Biomedical Importance
Seminal fluid is rich in fructose and sperms utilize fructose for
energy. Fructose is formed in the seminiferous tubular epithelial cells from
glucose.
4. D-mannose:
It does not occur free in nature but is widely distributed in
combination as the polysaccharide mannan, e.g. in ivory nut. In the body, it is
found as a constituent of glycoproteins.
5. Sedoheptulose: It is a
ketoheptose found in plants of the sedum family. Its phosphate is important as
an intermediate in the HMP-shunt and has been identified as a product of
photosynthesis.
Important Properties Of Monosaccharides
1. Iodo compounds: An aldose
when heated with conc. HI loses all of its oxygen and is converted into an iodo
compound.
Since the resulting derivative is a straight chain compound related
to normal hexane, thereby suggesting the lack of any branched chains in
structure of glucose.
2. Acetylation or ester formation: The ability to form sugar esters, e.g. acetylation with acetyl
chloride (CH3 – COCl) indicates the presence of alcohol groups. Due to
alcoholic –OH groups, it can react with anhydrides and chlorides of many
organic and inorganic acids, like acetic acid, phosphoric acid, sulphuric and
benzoic acids to form esters of corresponding acids.
3. Osazone formation: It is a
useful means of preparing crystalline derivatives of sugars. Osazones have
characteristic
• Melting points
• Crystal structures
• Precipitation time and thus are valuable in identification of sugars. Preparation:
Types of Crystals
• Glucosazone crystals: These are fine, yellow needles in
fan-shaped aggregates or sheaves or crosses, typically described as Bundle of
Hay.
Melting point = 204 to 205°C. Note: Glucose, mannose and fructose
due to similarities of structures form the same osazones. But since the
structure of galactose differs on C-4, that part of the molecule unaffected in
osazone formation, it would form a different osazone.
• Lactosazone crystals: These are irregular clusters of fine
needles and look like a Powder puff.
• Maltosazone: These are star-shaped and compared to Sunflower
petals.
4. Interconversion of sugars:
Glucose, fructose and mannose are interconvertible in solutions of
weak
5. Oxidation to produce sugar acids:
When oxidised under different conditions, the aldoses may form:
• Monobasic Aldonic acids
• Dibasic Saccharic acids
• Monobasic uronic acids containing aldehyde groups thus possessing
reducing properties.
Biomedical importance of D-Glucuronic acid
In the body D-Glucuronic acid is formed from Glucose in liver by uronic acid pathway, an alternative pathway for glucose oxidation. It occurs as a constituent of certain mucopoly[1]saccharides.
In addition, it is of importance in that it conjugates toxic substances, drugs,
hormones and even bilirubin (a break down product of Hb) and converts them to a
soluble nontoxic substance, a glucuronide, which is excreted in urine.
6. Reduction of sugars to form sugar alcohols:
The monosaccharides may be reduced to their corresponding alcohols
by reducing agents such as Na-Amalgam. Similarly, ketoses may also be reduced
to form keto alcohol.
Examples
• D-Glucose yields D-Sorbitol.
• D-Galactose yields D-Dulcitol.
• D-Mannose yields D-Mannitol.
• Ketosugar D-Fructose yields D-Mannitol and D-Sorbitol
Practical Application
In microbiology sugar alcohols have been used to identify type of
bacteria. Different bacteria gives different pattern.
7. Action of acids on carbohydrates:
Polysaccharides and the compound carbohydrates in general are
hydrolyzed into their constituent monosaccharides by boiling with dilute
mineral acids (0.5 to 1.0 N) such as HCl or H2SO4.
• With conc. mineral acids the monosaccharides are decomposed.
• Pentoses yield the cyclic aldehyde “furfural”. Twelve percent
(12%) HCl has been found most satisfactory for decomposition
Examples
• Molisch’s test:
With α-naphthol (in alcoholic solution) gives red-violet ring. A
sensitive reaction but non-specific, given by all sugars.
8. Action with alkalies:
With alkalies, monosaccharides react in various ways:
(a) In dilute alkali: The sugar will
change to the cyclic α and β forms with an equilibrium between the two isomeric
form (See mutarotation).
• On standing: A rearrangement will occur which produce an
equilibrated mixture of glucose, fructose and mannose through the common
“enediol” form (see interconversion).
• If it is heated to 37°C, the acidity increases, and a series of
Enols are formed in which double bond shifts from the oxygen-carbon atoms
(b) In conc. alkali:
The sugar caramelises and produces a series of decomposition products, yellow and brown pigments develop, salts may form, many double bonds between C-atoms are formed, and C bond C bonds may rupture.
9. Reducing action of sugars in alkaline solution:
All the sugars that contain free sugar group undergo enolization and various other changes when placed in alkaline solution. The enediol forms of the sugars are highly reactive and are easily oxidised by O2 and other oxidising agents and forms sugar acids.
As a consequence they readily reduce oxidising ions such as Ag+. Hg+, Bi+++, Cu++ (cupric) and Fe(CN)6 – – –.
Practical Application
This reducing action of sugars in alkaline solution is utilised for
both qualitative and quantitative determinations of sugars. Reagents containing
Cu++ (ic) ions are most commonly used. These are generally alkaline solution of
cupric sulphate containing
• Sodium potassium tartrate (Rochelle salt) and strong alkali
NaOH/KOH as in Fehling’s solution (not used now).
• Sodium citrate and weak alkali sodium carbonate as in Benedict’s
Qualitative reagent
Other Sugar Derivatives of Biomedical Importance
1. Deoxy sugars: Deoxy sugars
represent sugars in which the oxygen of a –OH gr. has been removed, leaving the
hydrogen. Thus, –CHOH or –CH2OH becomes –CH2 or –CH3. Several of the deoxy
sugars have been synthesised and others are natural products.
2. Amino sugars (hexosamines):
Sugars containing an –NH2 group in their structure are called amino sugars.
Types: Two types of amino sugars of physiological importance are:
• Glycosylamine: The anomeric –OH group is replaced by an –NH2
group
3. Amino Sugar Acids
• Neuraminic acid: It is an amino sugar acid and structurally an aldol condensation product of pyruvic acid and D-Mannosamine.
Neuraminic acid
is unstable and found in nature in the form of acylated derivatives known as
Sialic acids (N-acetyl Neuraminic acid —NANA)
4. Glycosides Definition:
Glycosides are compounds containing a carbohydrate and a noncarbohydrate residue in the same molecule.
In these compounds the carbohydrate residue is attached by an acetal linkage of carbon-I to the non-carbohydrate residue. The non-carbohydrate residue present in the glycoside is called as Aglycone.
The aglycones present in glycosides vary in complexity from simple substances as methyl alcohol, glycerol, phenol or a base such as adenine to complex substances like sterols, hydro-quinones and anthraquinones.
The glycosides are named according to the carbohydrate they contain. If it contains glucose, forms glucoside. If galactose, it forms galactoside and so on.
Reference:
Notes Made By The Help of "The Text Book of Medical Biochemistry By MN. Chatterjea 8th Edition"
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