Carbohydrates are essential biomolecules that serve as a major source of energy and play vital roles in various biological processes. Understanding their biochemistry provides insights into their diverse structures. This article explores the fundamental aspects of carbohydrate biochemistry, shedding light on their significance in biological systems.
Structure of Carbohydrates
Carbohydrates are the most abundant compounds in nature, consisting of poly hydroxylated (having multiple -OH ligands) compounds with at least three carbon atoms. They have a general formula of Cx(H2O)y. Understanding the structure and properties of carbohydrates is crucial for comprehending their biochemical activities. In this article, we will explore the biochemistry of carbohydrates, including their structure, isomerism, oxidation and reduction, glycosidic bonds, and various types of carbohydrates.
Carbohydrates are polyhydroxylated compounds that are widely found in nature. They consist of carbon, hydrogen, and oxygen atoms. The general formula for carbohydrates is Cx(H2O)y, where "x" and "y" represent the number of carbon and water units, respectively.
Monosaccharides
Monosaccharides are the simplest form of carbohydrates that cannot be hydrolyzed into simpler carbohydrates. They are classified based on the number of carbon atoms they contain. For example, triose has three carbon atoms, tetrose has four carbon atoms, and pentose has five carbon atoms. Monosaccharides can be further divided into aldoses and ketoses based on the presence of aldehyde or ketone groups (carbonyl groups) in their structures.
Isomerism is a crucial aspect of monosaccharides, wherein compounds with the same chemical formula have different arrangements of their atoms in space and exhibit distinct physical properties. For instance, fructose and glucose are isomers of each other.
Epimers
Epimers are isomers that differ in the configuration around one carbon atom. For example, glucose and galactose are C4 epimers, while glucose and mannose are C2 epimers.
Enantiomers/Optical Isomers
Enantiomers, also known as optical isomers, are mirror images of each other with the same chemical and physical properties. They can be classified as either "D" or "L" isomers based on their rotation of plane-polarized light. Dextrorotatory (d) isomers rotate light to the right, while levorotatory (l) isomers rotate light to the left.
The D and L isomers of monosaccharides are important, with naturally occurring isomers primarily belonging to the D configuration. Enzymes, which play a crucial role in carbohydrate metabolism, are typically specific to the D isomers.
Anomeric Carbon
In an aqueous medium, more than 99% of monosaccharides with more than five carbon atoms adopt a ring structure through a process called cyclization. When a monosaccharide forms a ring structure, the carbonyl carbon of the straight chain becomes the anomeric carbon. Cyclization results in the formation of two isomers of the carbon ring: α and β.
Oxidation and Reduction
Carbonyl compounds, including monosaccharides, can undergo oxidation to form carboxylic acids. This oxidation reaction forms the basis of tests like the Benedict's Test. If a sugar contains an anomeric carbon with a free hydroxyl (OH) group, it can be oxidized and is referred to as a reducing sugar. Additionally, sugars can be catalytically or enzymatically reduced to produce polyols or sugar alcohols.
Glycosidic Bond
A glycosidic bond is formed between the hydroxyl (OH) group of an anomeric carbon in a monosaccharide and the hydroxyl (OH) or amino (NH) group of another compound. Depending on the type of group involved, the bond can be classified as an O-glycosidic bond
or an N-glycosidic bond. The naming of glycosidic bonds is important and involves the following steps:
- Note whether the monosaccharide that used its anomeric carbon is in the α or β configuration.
- Identify the number assigned to the anomeric carbon.
- Identify the number of the carbon assigned to the other monosaccharide.
For example, an α(1→4) bond indicates an O-glycosidic bond formed between the anomeric carbon of one monosaccharide and the fourth carbon of another monosaccharide.
Polysaccharides
Polysaccharides are complex carbohydrates that can be categorized as either homopolysaccharides or heteropolysaccharides. Homopolysaccharides, such as glycogen and starch, are composed of a single type of monosaccharide, while heteropolysaccharides, like glycosaminoglycans (GAGs), consist of different types of monosaccharides.
Glycogen
Glycogen is a homopolysaccharide composed of glucose units joined by α(1→4) glycosidic bonds, except at branching points where α(1→6) glycosidic bonds are present. It serves as a storage form of glucose in animals and humans and contains multiple non-reducing ends.
Starch
Starch exists in two forms: amylose and amylopectin. Amylose is a linear structure consisting of α(1→4) links between α-glucose units. On the other hand, amylopectin has a branched structure with both α(1→4) and α(1→6) glycosidic bonds between α-glucose units.
Cellulose
Cellulose is a polysaccharide that forms the structural component of plant cell walls. It consists of β-glucose units joined by β(1→4) glycosidic bonds. Humans lack the enzymes required to digest cellulose fibers, although certain bacteria can break it down.
Glycosaminoglycans (GAGs)
Glycosaminoglycans are linear polymers of repeating disaccharide units. These units are modified with various functional groups, including -COOH, -NH2, sulfated groups, and amino groups. The two monosaccharides present in GAGs are an acidic sugar (glucuronic acid or iduronic acid) and an amino sugar (N-acetyl glucosamine or N-acetyl galactosamine). GAGs have a high negative charge due to the presence of sulfated groups and play important roles in various biological processes.
Some examples of GAGs include chondroitin sulfate, hyaluronic acid, heparin, keratan sulfate, heparan sulfate, and dermatan sulfate. These GAGs have distinct features, tissues of occurrence, and functions, as shown in the table below:
| Features | Tissues | Function |
|---|---|---|
| Chondroitin sulfate | Tendons, ligaments, cartilages, wall of aorta, cornea, skin | Bind collagen and form a tight network |
| Hyaluronic acid | Extracellular matrix, cartilages, tendons | Provide tensile strength and elasticity |
| Heparin | Intracellular component of mast cells | Anticoagulant |
Glycoconjugates
Glycoconjugates refer to carbohydrates covalently linked to proteins or lipids. They play crucial roles in various biological processes and are classified into different
types based on the relative proportion of carbohydrates and proteins or lipids.
Glycoproteins
Glycoproteins have a higher proportion of proteins compared to carbohydrates. They are involved in numerous biological functions, including cell recognition, immune response, and hormone regulation.
Proteoglycans
Proteoglycans have a higher proportion of carbohydrates compared to proteins. They consist of a core protein with attached glycosaminoglycan chains. Proteoglycans are abundant in the extracellular matrix and play essential roles in maintaining tissue structure, hydration, and cell signaling.
Glycolipids
Glycolipids are carbohydrates covalently attached to lipids, specifically glycosphingolipids and gangliosides. They are found in cell membranes and are involved in cell-cell recognition, signaling, and membrane stability.
Understanding the biochemistry of carbohydrates, including their structure, isomerism, glycosidic bonds, and different types of carbohydrates, provides valuable insights into their functions and roles in biological systems. These complex molecules play crucial roles in energy storage, structural support, cell signalling, and various other physiological processes.
