Carbohydrate Biochemistry (Part II)

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Disaccharides

Disaccharides consist of two monosaccharides joined covalently by an O-glycosidic bond, which is formed when a hydroxyl group of one sugar reacts with the anomeric carbon of the other.

Example: maltose, lactose, and sucrose

Glycosidic bonds are readily hydrolyzed by acid but resist cleavage by base. Thus disaccharides can be hydrolyzed to yield their free monosaccharide components by boiling with dilute acid.

N-glycosyl bonds join the anomeric carbon of a sugar to a nitrogen atom in glycoproteins and nucleotides.

General formula: Cₙ(H2O)ₙ₋₁

The oxidation of a sugar’s anomeric carbon by cupric or ferric ion (the reaction that defines a reducing sugar) occurs only with the linear form, which exists in equilibrium with the cyclic form(s).

When the anomeric carbon is involved in a glycosidic bond, that sugar residue cannot take the linear form and therefore becomes a non-reducing sugar.

The end of a chain with a free anomeric carbon (one not involved in a glycosidic bond) is commonly called the reducing end.

The disaccharide maltose contains two D-glucose residues joined by a glycosidic linkage between C-1 (the anomeric carbon) of one glucose residue and C-4 of the other.

Because the disaccharide retains a free anomeric carbon (C-1 of the glucose residue on the right), maltose is a reducing sugar.

Nomenclature of disaccharides (or oligosaccharides)

By convention, the name describes the compound with its nonreducing end to the left.

Give the configuration (α or β) at the anomeric carbon joining the first monosaccharide unit (on the left) to the second.

Name the nonreducing residue; to distinguish five- and six-membered ring structures, insert “furano” or “pyrano” into the name.

Add suffix '-syl' to the name of the first residue. For example, 'glucopyranosyl'.

Indicate in parentheses the two carbon atoms joined by the glycosidic bond, with an arrow connecting the two numbers; for example, (1🠊4) shows that C-1 of the first-named sugar residue is joined to C-4 of the second. 

If the anomeric carbons from both residues are involved in bond formation, a double headed arrow is given.

Name the second residue similarly. Add suffix '-ose' to the name of the second residue in case of reducing sugars and '-oside' in case of non-reducing sugars.

If there is a third residue, describe the second glycosidic bond by the same conventions.

Short name:Glc(α1🠊4)Glc

Sucrose

Sucrose (cane sugar) is made up of α-D-glucose and β-D-fructose linked by a glycosidic bond (α1 ⟺ β2). 

The reducing  groups (anomeric carbon) of both glucose and fructose are involved in glycosidic bond formation. Hence, sucrose is non-reducing sugar and it cannot form osazones.

The systematic name of sucrose is α-D-glucopyranosyl-(1 ⟺ 2)- β-D-fructofuranoside. 

This indicates:

It is composed of two monosaccharides: glucose and fructose

Ring type: Glucose is pyranose and fructose is furanose

Linkage: oxygen on C1 of α-D-glucose  is linked to C2 of β-D-fructose 

Suffix '–oside' and '⟺' indicates that the anomeric carbons of both the monosaccharides participate in glycosidic bond formation

Sucrose is a major carbohydrate produced in photosynthesis. It has the advantage as storage and transport as its functional groups are held together and are protected from oxidative attacks.

Intestinal enzyme, sucrase hydrolyze sucrose to glucose and fructose.

Inversion of sucrose:

Sucrose is dextrorotatory (+66.5°). But when hydrolyzed, it becomes levorotatory (-28.2°). The process of change in optical rotation from dextrorotatory(+) to levorotatory (-) is referred to as inversion. The hydrolyzed mixture of sucrose, containing glucose and fructose, is known as invert sugar.

Sucrose first splits into α-D-glucopyranose (+) and β-D-fructofuranose (+). But β-D-fructofuranose is less stable and gets converted into β-D-fructopyranose (-). The overall effect in the mixture becomes levorotatory (-).

Lactose

Lactose (milk sugar) is composed of β-D-galactose and β-D-glucose held together by   β(1🠊4) glycosidic bond.

The anomeric carbon of C1 of glucose is free. Hence lactose exhibits reducing properties and forms osazones (powder-puff or hedgehog shape).

The systematic name is β-D-galactopyranosyl-(1🠊4)-β-D-glucopyranose.

It is hydrolyzed by intestinal enzyme lactase into glucose and galactose.

Maltose

Maltose (malt sugar) is produced during digestion of starch by enzyme amylase.

Maltose is composed of two α-D-glucose  units held together by α(1🠊4) glycosidic bond. 

A free aldehyde group is present on C1 of the second glucose unit and hence maltose exhibits reducing properties and forms osazones (sunflower shaped).

It can be hydrolyzed by dilute acid or enzyme maltase.

 

In isomaltose, the glucose units are held together by α(1🠊6) glycosidic bond.

Cellobiose

It is identical to maltose, except that in it the linkage is  β(1🠊4) glycosidic bond.

It is formed during hydrolysis of cellulose.

Trehalose

It is identical to maltose, except that in it the linkage is  (α1⇔α1) glycosidic bond.

It is formed during hydrolysis of cellulose.

It is a non-reducing sugar.

Examples of other oligosaccharides

Raffinose (trisaccharides): Fructose+Galactose+Glucose

Stachyose (Tetrasaccharide):  Galactose+Galactose+Glucose+Fructose

Verbascose (Pentasaccharide): Galactose+Galactose+Galactose+Glucose+Fructose

Polysaccharides

Carbohydrates containing repeating units (more than 10 units) of the monosaccharides or their derivatives linked by glycosidic linkages are called polysaccharides.

They are primarily concerned with 2 important functions:

Structural role

Storage of energy

Polysaccharides can be linear or branched. The occurrence of branched polysaccharides is due to the fact that glycosidic linkages can be formed at any one of the –OH groups of a monosaccharide.

Polysaccharides are of high molecular weight. They are usually tasteless (non-sugars) and form colloids with water.

Polysaccharides are of two types:

Homopolysaccharides (Homoglycans): They, on hydrolysis, yield only one type of monosaccharide. They are named based on the nature of the monosaccharide unit. 

Example: Glucan (polymer of glucose), Fructosan (polymer of fructose)

Heteropolysaccharides (heteroglycans): They, on hydrolysis, yield a mixture of a few types of monosaccharide units or their derivatives. 

Example: Peptidoglycan (polymer of N-acetylglucosamine and N-acetylmuramic acid residues)

Starch

Starch is the carbohydrate reserve of plants which is the most important dietary source for higher animals.

Starch is a homopolysaccharide composed of D-glucose units held by glycosidic bonds. 

It is known as glucosan or glucan.

Starch consists of two polysaccharide components:

Water soluble amylose (15-20%)

Water insoluble amylopectin (80-85%)

Chemically amylose is a long unbranched chain with 200-1000 D-glucose units held by (α1🠊4) glycosidic linkage.

Amylopectin is a branched chain with (α1🠊6) glycosidic bonds at the branching points and (α1🠊4) glycosidic bonds everywhere else.

Starches are hydrolyzed by amylases (pancreatic or salivary) to liberate dextrins and finally maltose and glucose units. Amylase acts specifically on the (α1🠊4) glycosidic bonds.

(b) amylopectin

Dextrins

These are the breakdown products of starch by the enzyme amylase or dilute acids.

Starch is hydrolyzed through different dextrins and finally to maltose and glucose. 

The various intermediates (identified by iodine coloration) are soluble starch (blue), amylodextrin (violet), erythrodextrin (red) and achrodextrin (no colour).

Inulin

Inulin is a polymer of fructose.

It occurs in dahlia bulbs, garlic, onion, etc.

It is a low molecular weight (~ 5000) polysaccharide easily soluble in water.

Inulin is not utilized by the body. 

It is used for assessing kidney function through measurement of glomerular filtration rate (CFR).

Cellulose

Cellulose occurs extensively in plants and is totally absent in animals.

Cellulose is composed of β-D-glucose units linked by β(1🠊4) glycosidic bonds.

Cellulose can not be digested by mammals due to lack of the enzyme that cleaves β-glycosidic bonds. Hydrolysis of cellulose yields a disaccharide, cellobiose, which is further broken down to β-D-glucose units.

It is a major constituent of fibers, the non-digestible carbohydrate.

Glycogen

Glycogen is the main storage polysaccharide of animal cells.

Like amylopectin, glycogen is a polymer of (α1🠊4)-linked subunits of glucose, with (α1🠊6)-linked branches, but glycogen is more extensively branched (on average, every 8 to 12 residues) and more compact than starch.

Glycogen is especially abundant in the liver, it is also present in skeletal muscle.

Chitin

Chitin is a linear homopolysaccharide composed of N-acetylglucosamine residues in β linkage.

The only chemical difference from cellulose is the replacement of the hydroxyl group at C-2 with an acetylated amino group. 

Peptidoglycan

The rigid component of bacterial cell walls is a  heteropolymer  of  alternating (β1🠊4)-linked N-acetylglucosamine and N-acetylmuramic acid residues.

The enzyme lysozyme kills bacteria by hydrolyzing the   (β1🠊4) glycosidic bond between N-acetylglucosamine and Nacetylmuramic acid.