Biomolecules of the Cell - Carbohydrates (Part 2)

In the previous post, we have completed the introduction of carbohydrates and the characteristics of monosaccharides. In this post, we will discuss about characteristics of disaccharides, oligosaccharides and some polysaccharides. 

Let us first understand the disaccharides.

A. Disaccharides:

Disaccharides are formed by the condensation reaction (or dehydration reaction) of two monosaccharide units and water is the by-product of this reaction (as can be seen in the adjacent figure). The covalent linkage that is formed between two monosaccharides is called O-glycosidic bond and it represents the formation of an hemiacetal from an aldehyde and an alcohol. Similarly, a hemiketal is formed from a ketone and an alcohol.

In the adjacent figure (right side), is the disaccharide, maltose, the linkage is α(1-4) linkage. We can see the hemiacetal and acetal ends in the figure.

Types of Disaccharides (Sugars):
a. Reducing and Non-reducing Sugars:

The disaccharides are classified as reducing disaccharide (or sugar) and non-reducing disaccharide (or sugar). The disaccharides that have hemiacetals, are grouped under reducing sugar. Hemiacetals contain a free aldehyde group which can be oxidized into carboxylic acids (or diverse products). Thus, these types of sugars are reducing in nature, hence, they are called reducing sugars.

Another type is non-reducing sugars (disaccharides). Here, the sugar or the disaccharide in an acetal or ketal which cannot be oxidized readily and neither monosaccharide has a free hemiacetal unit. This is so, because both of its anomeric carbon atoms are involved in glycosidic linkage.

In the figure, the glucose on the right is designated as the reducing end of the disaccharide molecule as it can participate in a reduction reaction. In contrast, the glucose on the left represents the non-reducing end as the C-1 carbon atom is the part of the  α(1-4) linkage and it cannot form the open chain. Thus, as maltose contains one reducing end, it is called, reducing sugar. 

b. Properties of Disaccharides: 

The glycosidic bond can be formed between the hydroxyl groups on its component monosaccharide. So, even if both monosaccharides (forming a disaccharide) are the same (e.g., glucose), different bond combinations which is regiochemistry and stereochemistry (like alpha- or beta-) result in disaccharides that are isomers of each other. These are diastereoisomers with different chemical and physical properties.
For example, both maltose and cellobiose are the disaccharides of glucose monomers. However, maltose is the disaccharide with α(1-4) linkage between C1 hydroxyl of one glucose and C4 hydroxyl of another glucose. The configuration is 'α' because the O at the anomeric carbon atom points down from the ring. On the other hand, cellobiose, is the disaccharide with β(1-4) linkage but here, the configuration is β as O points up from the ring. (This β glycosidic linkage is generally depicted by a zig-zag line; however, one glucose molecule is actually flipped over relative to the other).

c. Nomenclature:

As per the standard conventions, the disaccharide is named such that first listing is that of non-reducing monosaccharide on the left followed by glycosidic linkage between the two monosaccharides and then the monosaccharide on the right. For example, with this nomenclature, maltose can be described as Glc(α1-4)Glc (where Glc stands for glucose).
Here is the table of some common reducing and non-reducing disaccharides with their glycosidic linkages.

Reducing Disaccharides
Disaccharide	Unit 1	Unit 2	Bond/Linkage
Cellobiose	Glucose	Glucose	β(1-4)
Gentiobiose	Glucose	Glucose	β(1-6)
Isomaltose	Glucose	Glucose	α(1-6)
Lactose	Galactose	Glucose	β(1-4)
Maltose	Glucose	Glucose	α(1-4)
Mannobiose	Mannose	Mannose	Either α(1-2) α(1-3), α(1-4) or α(1-6)
Xylobiose	Xylopyranose	Xylopyranose	β(1-4)
Non-reducing Disaccharides
Sucrose	Glucose	Fructose	α(1-2)β
Trehalose	Glucose	Glucose	α(1-1)α

B. Oligosaccharides:
Most of the oligosaccharides are not found as isolated molecules. Instead, they may be attached to other biomolecules like proteins or lipids, generally referred to as glycoconjugates. For example, the blood group serotypes (A, B, AB and O) are the result of various oligosaccharides involved in cellular recognition. The lipids on the surface of the erythrocytes are conjugated with various oligosaccharides.

C. Polysaccharides:

Polysaccharides are long chains of monosaccharides joined together by glycosidic bonds (linkages). As mentioned in previous post, polysaccharides maybe branched or unbranched. When all the monosaccharides in a polysaccharide are of the same type, the polysaccharide is called a homopolysaccharide or homoglycan, but when more than one type of monosaccharide is present, then they are called heteropolysaccharides or heteroglycans.

Here, we are going to see about some polysaccharides:

a. Cellulose: It is the most abundant (structural) polysaccharide on the earth. It is a straight chain homopolymer consisting of thousands of glucose moieties attached together by β(1-4) glycosidic linkage present in plants. It is a polymer of cellobiose units (repeats of Glcβ(1-4)Glc) as can be seen in the figure. Humans and many other animals lack the enzyme cellulose which is required to hydrolyze β-glycosidic linkages.
Many hydroxyl groups on the glucose molecules from one chain form hydrogen bonds with the oxygen atoms on the same or neighboring chain thereby holding the chains firmly together side-by side forming microfibrils giving high tensile strength.

b. Chitin: Chitin is to animal kingdom what cellulose is to plant kingdom. It is another abundant linear polysaccharide that forms the structural components of many invertebrates exoskeletons of insects and crustaceans. It is a polymer of units of N-acetyl glucosamine (abbreviated as NAG or GlcNAc) which is linked by β(1-4) glycosidic bond. The only difference between the structure of glucose and N-acetyl-glucosamine is the replacement of the C-2 hydroxyl group with that of an acetylated amino group. This allows for increased hydrogen bonding between adjacent polymers giving chitin more strength than cellulose.

c. Starch: Starch is the homopolymer in which the glucose units are linked via alpha linkages. It is made up of amylose (15-20%) and amylopectin (80-85%). Amylose is a linear polysaccharide linked by α(1-4) linkages while amylopectin is a branched polysaccharide connected by α(1-4) and α(1-6) linkages. The linear linkage is α(1-4) while the branched linkage is α(1-6) between glucose residues which greatly increases the number of free ends in the homopolymeric molecule. The branch points occur in chain after every 20-30 residues. Being the alpha linkages, these can easily be hydrolyzed by alpha amylase which cleaves α(1-4) glycosidic bonds.

d. Glycogen: What starch is to plants, glycogen is to animals and human. The structure is similar to amylopectin meaning the linear, glucose molecules are linked together by α(1-4) glycosidic bond and the branches are linked to these linear chains branching off from α(1-6) glycosidic bond between first glucose of new branch and a glucose on the stem branch.

There are certain differences between starch (amylopectin, more specifically) and glycogen. One of them is that of branching. In glycogen, the branching occurs more frequently  i.e.; branch point is after every 6-10 residues.
The glucose units can be added or removed only from the non-reducing ends of amylopectin and glycogen. The more branch points, the more ends are available for glucose retrieval and storage. Another difference is between the macromolecular structures of both. Amylopectin contains one free glucose at the reducing end of the 'tree branch' whereas glycogen lacks a free reducing end. This is because the glucose residue at the center of the glycogen 'spiral' is covalently linked to a protein called glycogenin (see adjacent figure).

e. Heparin: Heparin is a polysaccharide that is heteropolysaccharide with anti-clotting properties. It has medicinal value for surgery and is used to treat thrombosis. Heparin is found in arterial walls where it facilitates interactions between antithrombin (an inhibitor of blood coagulation) and thrombin (a clot-forming protein).