Carbohydrates: Structure & Classification

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Carbohydrates are one of the most important components of the biological world in addition to being one of the most abundant classes of biological molecules. The word ‘carbohydrate’ is derived from the Greek word ‘sakcharon’ meaning ‘sugar’. Carbohydrates are nothing but aldehyde or ketone compounds with multiple hydroxyl groups. The literal meaning of carbohydrates is ‘carbon hydrates’ which originates from their chemical composition. The chemical composition of carbohydrates or saccharides is (CH2O)n where n>3 or n=3.

Carbohydrates

Basic Functions Of The Carbohydrates

  1. Energy reserves: Carbohydrates make up energy stores, fuels and metabolic intermediates.
  2. Structural framework of genetic material: The sugars ribose and deoxyribose are a part of the structural framework of genetic material RNA and DNA.
  3. Structural element of cell wall: Polysaccharides are the structural elements of the cell wall of bacteria and plants.
  4. Cellulose, a polysaccharide and a principal component of the cell wall of pants, is one of the most abundant organic compounds in the biosphere.
  5. Conjugate with lipids and proteins: Carbohydrates are extensively linked protein and lipid molecules. These glycoprotein and glycolipids are critical in choreographing interactions between cells and other biological elements.

Classification Of Carbohydrates

Carbohydrates can be classified into 2 categories-

  1. mono-, oligo- and polysaccharides and
  2. reducing and non- reducing sugars

Depending upon whether they undergo hydrolysis or not and if yes, then the number of products formed, the carbohydrates are classified into the following:

1. Monosaccharides: Monosaccharides are the simplest They cannot be hydrolyzed further into hydroxyl aldehyde and ketone unit.

2. Oligosaccharides: Oligosaccharides are polymers with two to ten monosaccharide units. The individual monosaccharide units are joined together by glycosidic linkages. They are often present in association with proteins (glycoprotein) and lipids (glycolipids). These two conjugates of carbohydrates with proteins and lipids are collectively called glycoconjugates. Depending upon the monosaccharide unit present, the oligosaccharides are further grouped into:

  • Disaccharides- with two monosaccharide units.
  • Trisaccharides- with three monosaccharide units.
  • Tetrasaccharides- with four monosaccharide units.
  • Pentasaccharides- with five monosaccharide units.

3. Polysaccharides: Polysaccharides have hundreds and even thousands of monosaccharide units linked covalently. The molecular mass of these polymers ranges into millions of Dalton. They have a critical role in maintaining the structural integrity of the living organisms. Cellulose is a major structural polysaccharide in plants. Starch in plants and glycogen in case of animals are principal nutritional reserves.

Monosaccharides Or Simple Sugars

Monosaccharides are the simplest aldehyde or ketone derivatives which cannot be hydrolyzed further; for example D-glucose and D- ribulose cannot be hydrolyzed further.

Monosaccharides

Monosaccharides are further classified into two subgroups depending upon the

  • Number of carbon atoms present: the smallest monosaccharide is the one with three carbon atoms, known as trioses. Therefore, the monosaccharide with four, five, six or seven carbon atoms are called tetroses, pentoses, hexoses and heptoses
  • Chemical nature of their carbonyl group or presence of aldehyde or ketone unit: if the carbonyl group is aldehyde in nature, the monosaccharide is called aldose. If the carbonyl group is ketone, then the monosaccharide is called ketose.

The monosaccharide glucose can hence be referred to as ‘aldohexose’. This implies that it is a six-carbon monosaccharide with carbonyl group which is aldehyde in nature. Similarly, fructose is a ‘ketohexose’ containing a six-carbon monosaccharide and a ketone group.

The smallest monosaccharides or trioses (n=3) are dihydroxyacetone, D- and L- glyceraldehyde.

Types of Monosaccharides

Glyceraldehyde with C-2 atom is chiral or asymmetric in nature, henceforth, there are two stereoisomers of this sugar. Chiral compounds like glyceraldehyde usually exist in two forms which are non- superimposable mirror images of each other. These non- superimposable mirror images are known as enantiomers. They are often represented as Fischer projections. In Fischer projections, atoms which are linked to an asymmetric carbon atom by horizontal bonds are present in front of the plane of the page whereas those linked to asymmetric carbon by vertical bonds are present behind. In case of glyceraldehyde, when the hydroxyl group which is attached to the asymmetric carbon is present on the left of the Fischer projection, the configuration is referred to as ‘L’ and when the hydroxyl group is present on the right, the configuration is ‘D’.

L and D Configuration of Monosaccharides

Since other polymers have more than one chiral or asymmetric carbon, they generally exist as diastereoisomers. Diastereoisomers are not mirror images of each other. A compound with ‘n’ chiral carbon atoms will have a maximum of 2n stereoisomers. Taking into consideration glucose here, we observe that 4 out of its 6 carbons are chiral. Going by the general formula for calculating the number of stereoisomers, 2n, 16 possible stereoisomers can arise comprising all possible aldohexoses. The absolute configuration of monosaccharides which contain multiple chiral carbons is, however, determined by comparison of the configuration at the highest- numbered chiral carbon to the configuration of the single chiral carbon of glyceraldehyde. Except for dihydroxyacetone, all monosaccharides occur in optically active isomeric forms.

D Aldoses

D Ketoses

Epimers– sugars which differ at only a single asymmetric or chiral carbon are called epimers. For example, D- glucose, and D- mannose differ only at C-2. In addition to this, even D- glucose and D- galactose differ at C-4.

Cyclic Forms- Pentoses And Hexoses Cyclize To Form Pyranose And Furanose Ring Structure

Monosaccharide polymers like glucose, fructose, and others do not exist as open chains in solution. The open chains of these simple sugars cyclize to form rings. Aldehyde and ketone groups react readily with alcohols to form hemiacetals and hemiketals respectively. In aldohexoses like glucose, the aldehyde at C-1 in the open chain of glucose reacts with the hydroxyl group at C-5 in order to give rise to hemiacetal. The result of this process is a cyclic structure of six carbons known as pyranose.

Formation of Hemiacetal

In a similar way, a ketone reacts with alcohol in order to yield hemiketal. In the open chain form of ketohexose, say fructose, the keto group at C-2 reacts with either the hydroxyl group at C-6 to form a six-membered cyclic hemiketal or the hydroxyl group at C-5 to form a five-membered cyclic hemiketal. The five-membered cyclic ring thus formed is called furan.

Formation of Furan and Pyran

The whole process of pyranose and furanose formation is described as follows. The depictions of glucopyranose and fructofuranose are referred to as Howarth projections. During the process of formation of cyclic hemiacetal, an additional asymmetric center is created. The C-1 in case o fan open-chain glucose becomes the asymmetric center. The end product is the formation of two ring structures, α– D- glucopyranose and β– D- glucopyranose. In case of the D- sugars represented as Howarth projections, the symbol α represents that the C-1 hydroxyl group is below the plane of the ring; β represents that the same hydroxyl group is above the plane of the ring. These two diastereoisomers are called anomers.  A similar process occurs during the formation of the furanose ring of fructose. The only difference is that the hydroxyl group is attached to the C-2 carbon atom.

The α and β forms interconvert via the open-chain form to give an equilibrium mixture. This process of interconversion is commonly referred to as mutarotation. A mixture of glucose at equilibrium contains approximately one-third α anomer, two-thirds β anomer and less than 1% of the open-chain.

Open Chain and Cyclic Forms of Monosaccharadies

Conformations Of Pyranose And Furanose Forms

The pyranose form can easily adopt two conformations called chair and boat. The substituents in case of the chair form have two orientations, the axial and the equatorial orientation. The axial groups which are close- fitting usually extend parallel to the threefold rotational axis of the ring. If they manage to extend out from the same side of the ring, they hinder each other sterically. Equatorial orientation is usually less crowded compared to the axial substituents. In case of glucose, the chair form of β– D- glucopyranose predominates and is more stable only because all the axial positions are occupied by hydrogen atoms.

Chair and Boat form of Pyranose

Furanose rings are not planar. The four atoms are nearly coplanar and so the conformation can be puckered. Just because this particular conformation resembles an opened envelope, it is called envelope form. Say, for example, the ribose moiety has either C-2 or C-3 is out of the plane and on the exact same side as C-5.  These conformations are particularly called C2- endo and C3- endo, respectively.

C 2 Endo C3 Endo Forms of Furanose

Monosaccharides And Their Derivatives

Monosaccharides react readily with alcohols and amines to form modified products called adducts. Alcohols react with hemiacetals to yield acetals and when they react with hemiacetal of sugars to form the acetal, it is commonly called glycoside. When glucose is the hemiacetal, the result is the formation of glucoside, if galactose, then galactoside. Ouabain is the most common glycoside. It particularly inhibits the action of enzymes, which pump Na+ and K+ across the biological cell membranes. Antibiotics such as streptomycin are also glycosides.

Formation of Acetal and Hemiacetal

Say, for example, methanol reacts via an acid- catalyzed process with D- glucose. Two products are formed by the reaction between the anomeric carbon and the hydroxyl group of methanol, methyl α -D-glucopyranoside and methyl β -D-glucopyranoside.

Glycosidic Bond

Some other modified sugars are as follows:

Examples of Modified Sugars

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Complex Sugars Are Formed By Glycosidic Linkage Between Monosaccharides

Monosaccharides readily form glycosidic bonds because of the presence of multiple hydroxyl groups. Disaccharide sugars are a result of 2 monosaccharides linked by O- glycosidic bond and oligosaccharides are formed by joining of 2 or more monosaccharide by O- glycosidic bond.

Complex Sugar and Glycosidic Bond

In this example, two molecules are linked by an O- glycosidic bond to yield the disaccharide, maltose.

Disaccharides And Glycosidic Linkage

When a disaccharide is formed, two monosaccharides are joined to each other by glycoside or acetal formation. Loss of water molecule occurs when the hemiacetal -OH of one monosaccharide and the -OH of the second monosaccharide react in order to establish a glycosidic bond. Therefore, it can be said that the glycosidic bond results because of the reaction between the anomeric carbon and the alkoxy oxygen. Going by the convention, the glycosidic linkages are read from left to right.

The most abundant disaccharides are lactose, maltose, and sucrose (common table sugar).

Sucrose, which is commercially available, is obtained from cane or beet and is the result of a reaction between α- anomeric carbon of glucose residue (C1) and β- anomeric carbon of fructose residue (C2). Therefore, the glucose and the fructose residues are joined via α1-2β glycosidic linkage. The configuration is always α for glucose and β for fructose. Sucrose can in turn be cleaved into its constituents monosaccharides by the action of sucrase. The hydrolysis of sucrose is often accompanied by the change in optical rotation from dextro to levo. Therefore, sucrose is also known as invert sugar or invertose. This process is catalysed by an enzyme called invertase or β-D- fructofuranosidase.

Glycosidic Linkage

Maltose is a disaccharide of glucose. The glycosidic linkage is formed between α- anomeric C-1 of one glucose and C-4 hydroxyl atom of the adjacent glucose residue. Hence, such a linkage is known as α-1, 4- glycosidic bond.

Monosaccharides and Disaccharides

The dissacharide of milk, lactose, is a linkage of galactose with glucose through β- 1,4- glycosidic linkage. Lactose is cleaved by lactase in humans and β- galactosidase in bacteria.

D Lactose and D Glucose

Biochemical Roles of Disaccharides

Sucrose, Lactose, and Maltose

Polysaccharides

Multiple monosaccharides link to form large polymeric oligosaccharides called polysaccharides, which are also known as glycans. Polysaccharides are versatile in their functions. They are classified into two groups,  homopolysaccharides (which contain only one type of monomeric unit) and heteropolysaccharide (which contain more than one or different types of monomeric units).

Polysaccharides

Homopolysaccharide

A branch of D- glucose units give rise to starch. Starch is a major storage form of glucose in plants. It contains amylose and amylopectin. Amylopectin is a branched structure consisting of α- D- glucose with α1-4 glycosidic linkages and α1-6 branching points. These branching points occur at approximate intervals of 25 to 30 α-D- glucose residues. Amylose is an unbranched linear polymer of α-D- glucose units with a repeat sequence of α1-4 glycosidic linkages. Iodine test is widely used to detect the presence of starch. The deep blue colour, which is formed in the presence of iodine is because of the presence of amylose in starch.

A major storage form of carbohydrate in animals is glycogen. It is found in liver and muscle. This large, branched polymer of glucose has glucose residues linked by α-1,4- glycosidic bonds. The branches present about once in 10 units are formed by α-1,6- glycosidic bonds.

Branch Point in Glycogen

Another linear, unbranched homopolysaccharide of D-glucose is cellulose. The individual glucose residues in cellulose are joined by β-1,4- glycosidic linkages. It is important  for maintaining the structural integrity of plant cells. Human enzyme systems are unable to hydrolyse cellulose. Cellulose is famous for being one of the most abundant organic compounds in the biosphere.

Cellulose, Starch, and Glycogen

It is to be noted that straight chains are more favoured by β-1,4- linkages. They ae optimal for structural purposes whereas α-1, 4- linkages favor bent structure. The bent structures are much favourable for storage purposes.

Chitin is yet another homopolysaccharide which is composed of N- acetyl- D- glucosamine residues. These residues are joined together by β-1,4- glycosidic linkage. It is critical for maintaing structural integrity in the exoskeleton of insects and crustaceans.

Chitin

Heteropolysaccharides

Gylcosaminoglycans are polysaccharides, which are unbranched as well as negatively charged heteropolysaccharides. These heteropolysaccharides are composed of repeating dissacharide units, [Acidic sugar- Amino sugar]n.. Amino sugars in most of the cases is either N-acetylglucosamine or N- acetylgalactosamine and the acidic sugar is uronic acid derivative, mostly glucuronic acid.

One of the simplest heteropolysaccharide is hyaluron or hyaluronic acid. It contains alternating residues of D- glucuronic acid and N- acetylglucosamine. Other major glycosaminoglycans are chondroitin sulfate, keratin sulfate, heparin, heparan sulfate, dermatan sulfate and hyaluronate. These polysaccharides are unique in the sense that their presence is only limited to bacteria and animals.

Heteropolysaccharides

Glycosaminoglycans usually link to proteins in order to yield proteoglycans except for hyaluronic acid. The site for assembly of polysaccharides is the core protein in the golgi bodies. A specific link tetrasaccharide is initially assembled on a serine residue. It is only after the assembly on serine residue that the GAG chain is synthesised with a single sugar residue being added at one time. The O-glycosidic bond formation takes place between Ser residue of protein and xylose sugar residue of the link tetrasaccharide.

Tetrasaccharides

Characteristics of GAG

Peptidoglycan or murein is present widely in the bacterial cell wall. It is a heteropolymer consisting of alternating (β1-4) linked N- acetyl- glucosamine (NAG) and N- acetyl- muramic acid (NAM) units. Lysosyme hydrolyses this linkage and henceforth degrades cell wall.

Carbohydrates

Reducing And Non- Reducing Sugars

Reducing sugars are the ones which are capable of reducing ferric or cupric ions. Reducing sugars always have a free aldehyde group which enables them to act as the reducing agents. It is interesting to note that all the monosaccharides (aldoses or ketoses) in their hemiketal or hemiacetal form are reducing sugars. The free anomeric carbon of the dissacharide or polysaccharide chain, which is not involved in the glycosidic linkage, is usually referred to as reducing end of the chain.

Also, all dissachrides with the exceptions of sucrose and trehalose are reducing sugars. All the sugars which act as reducing agents undergo mutarotation in aqueous solution. Since sucrose and trehalose are not capable of reducing ferric or cupric ions, they are commonly referred to as non- reducing sugars. The two non- reducing sugars have anomeric carbon engaged in glycosidic bond and so, have no free reducing end.

Osazone Formation

Famous German chemist Emil Fischer in 1875 prepared phenylhydrazine (PhNHNH2) by the reduction of phenyldiazonium salt. This compound phenylhydrazine has been extensively popular for the study of the stereochemistry of glucose.

The aldohexoses namely, D-glucose 1 and D-mannose 3, and the D-ketohexose, D-fructose 2 in the presence of calcium hydroxide are inter-convertible into one another. This reaction, which involves free carbonyl group (reducing end), takes place in the presence of excess phenylhydrazine when kept at boiling temperature, does not alter the stereochemistry at C3, C4, and C5. Therefore, it can easily be said that osazones are nothing but carbohydrate derivates which are formed only when sugars react with phenylhydrazine (present in excess). Osazones are formed from all reducing sugars. Sucrose fails to form osazone crystals since it is a non-reducing sugar.

Accompanying the oxidation of hydroxymethyl group of alpha carbon (carbon atom next to the chiral carbon), a pair of phenylhydrazone group is also formed. Enolization occurs and leads to the formation of the intermediate in this process, enediol 4. The process is known as the Lobry de Bruyn-Alberda van Eckstein rearrangement.

Osazone Formation

Osazone formation is important because it helps in the identification of monosaccharides. This process occurs in two steps. Firstly, phenylhydrazine and glucose react with each other to yield glucosephenylhydrazone concomitant to the elimination of water molecule from the functional group. In the second step, one equivalent of glucosephenylhydrazone reacts with two equivalents of phenylhydrazine (present in excess). The first phenylhydrazine initially oxidises the alpha carbon to a carbonyl group and the second phenylhydrazine removes one water molecule with the newly- formed carbonyl group of previously oxidised carbon. This gives rise to a carbon- nitrogen bond. The alpha carbon involved in this reaction is much more reactive than the other carbon atoms.

Osazones can be easily detected because they are vibrant in colour and crystalline in nature. Each sugar forms a distinctive crystalline osazone form.

  • Maltose forms petal-shaped crystals.
  • Lactose forms powder puff-shaped crystals.
  • Galactose forms rhombic-plate shaped crystals.
  • Glucose, fructose and mannose form broomstick or needle-shaped crystals.

Formation of Glucose Osazones

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