Lyxose is a constituent of lyxoflavin isolated from human heart muscle. It is a monosaccharide containing 5 carbon atoms. D- Lyxose is present in nucleic acids of yeast, human heart muscle and Kanamycin antibiotic.
The compounds with the same molecular formula but differing in configuration around one carbon atom are called Epimers. Epimeric carbon is the asymmetric carbon atom other than carbon of aldehyde or Ketone group e.g . carbon number 2, 3 and 4 of glucose .
Isomers differing as a result of variations in configuration of the —OH and —H on carbon atoms 2, 3, and 4 of glucose are referred as epimers of Glucose. 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.
Figure-1 showing epimers of glucose
Similarly D- Xylulose is the C-3 epimer of D-Ribulose
Blood Group Antigens
The antigens which determine blood types belong to glycoproteins and glycolipids. There are three types of blood-group antigens: O, A, and B. They differ only slightly in the composition of carbohydrates.
L –Fucose is a methyl pentose and is an important component of blood group antigens. (See figure).
L-Fucose is rare L sugar found of the oligosaccharide chains of N- and O-linked glycoproteins. Glycoproteins (also known as mucoproteins) are proteins containing branched or unbranched oligosaccharide chains; they occur in cell membranes and many other situations.
Figure- 2-showing Blood-group antigens.
All humans contain enzymes which catalyze the synthesis of the O antigen. Humans with A-type blood also contain an additional enzyme (called A-type enzyme here) which adds N-Acetyl galactosamine to the O antigen. Humans with B-type blood contain another enzyme (called B-type enzyme here) which adds Galactose to the O antigen. Humans with AB-type blood contain both A-type and B-type enzymes while humans with O-type blood lack both types of enzymes. N-Acetyl glucosamine and N-Acetyl galactosamine are acetylated amino sugars.
Trehalose is a non reducing sugar. Sugars containing free aldehyde or ketone group can reduce other reagents e.g. . They can reduce cupric ions of Fehling and Benedict’s reagents into cuprous ions :
Cupric ( blue ) + sugar ________> Cuprous ( red ) + oxidized sugar
These tests are one of the earliest tests to detect the presence of reducing sugar in urine.
Trehalose is a disaccharide containing two glucose residues linked together by alpha1-1 glycosidic linkage. Since both the functional groups are involved in the linkage and there is no free functional aldehyde or ketone group to carry out reduction, thus Trehalose is a non reducing sugar. On the same ground Sucrose is a non reducing sugar. 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.
Figure- 3-showing structure of Trehalose- 2 glucose residues linked together by Alpha 1-1 glycosidic linkage
Tests to differentiate Glucose, Fructose and Mannose
Glucose and Fructose both give –
1) Positive Molisch Test- since both are carbohydrates. Molisch test is a general test for all types of carbohydrates.(See the details below)
2) Positive Benedict’s test, since both are reducing sugars.
3) Osazone formation- Both sugars give similar shaped crystals. 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-
Figure-4- Reaction showing formation of osazones
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-5- showing formation of osazone crystals,
Figure-6- Needle shaped crystals of Glucose, Mannose and Fructose.
4) Seliwanoff test- 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. Sucrose also gives same reaction with Seliwanoff reagent.
The other tests are Anthrone test and Bial’s test etc.
Adenosine and Adenosine Mono Phosphate
The compound that consists of ribose linked by an N-glycosidic bond to N-9 of adenine is- Adenosine
Adenosine is a nucleoside. Nucleosides are derivatives of purines and pyrimidines that have a sugar linked to a ring nitrogen of a heterocycle called heterocyclic “base by N-Glycosidic linkage.
a) Base –Adenine and
b) sugar- Ribose, a pentose sugar.
Base and sugar are linked together by N- glycosidic linkage.
Figure- 7-Showing the structure of Adenosine
Mononucleotides are nucleosides with a phosphoryl group esterified to a hydroxyl group of the sugar. Additional phosphoryl groups linked by acid anhydride bonds to the phosphoryl group of a mononucleotide form nucleoside diphosphates and triphosphates
Figure-8- Showing the structure of Adenosine Mono PhosphatePlease help "Biochemistry for Medics" by CLICKING ON THE ADVERTISEMENTS above!