Carbohydrates | Principles of bioenergetics and fuel molecule metabolism | Bio/biochem

Carbohydrates are essential biomolecules that serve as energy sources and structural components in living organisms.

They are classified based on the number of sugar units they contain:

Monosaccharides are single sugar molecules
Disaccharides consist of two linked sugars
Polysaccharides are long chains of sugars.

Monosaccharide types and structures showing aldoses and ketoses

Glucose, with the chemical formula $C_{6} H_{12} O_{6}$ , is a vital energy source for humans. Other important monosaccharides include galactose, a component of lactose (milk sugar), and fructose, which is found in sucrose (common table sugar and fruit).

Although glucose, galactose, and fructose all share the same molecular formula ( $C_{6} H_{12} O_{6}$ ), they are structural isomers. Their distinct chemical behaviors arise from differences in the arrangement of functional groups around their asymmetric carbons, and notably, each of these sugars contains more than one chiral center.

Hexose isomers glucose, galactose, and fructose with structural differences

Nomenclature

Carbohydrates can be named using both systematic IUPAC nomenclature and common names, and their classification points to their diverse functions in biology.

Carbohydrate prefixes:

Deoxy = “without oxygen” has an $- H$ in place of an - $O H$ at a certain position.
D/L = absolute configuration = the prefixes D and L indicate the absolute configuration of a sugar. This designation is based on the stereochemistry (3D arrangement of atoms) of the chiral carbon farthest from the carbonyl group; if the hydroxyl group on this carbon is oriented similarly to that of D-glyceraldehyde, the sugar is designated as D, while the opposite configuration is labeled L.
$α$ / $β$ = anomeric configuration= arises when a sugar forms a cyclic structure. During cyclization, the carbonyl group reacts with a hydroxyl group to create a new chiral center called the anomeric carbon.
- The $α$ form has the hydroxyl group on the anomeric carbon positioned opposite to the CH₂OH group at the far end of the ring
- The $β$ form features the hydroxyl group on the same side as the $C H_{2} O H$ group.
- These distinctions in configuration are critical, as they influence the physical and chemical properties of the sugar, as well as its biological interactions.

Carbohydrate suffix: all sugars end in -ose.

Ring (cyclic) forms of monosaccharides

Many hexoses, such as glucose, predominantly exist in cyclic forms rather than as **open-chain molecules, **particularly when in an aqueous solution. The formation of these cyclic structures involves an intramolecular reaction that creates a hemiacetal (an alcohol and ether attached to the same carbon) or **hemiketal **(when an alcohol adds to a ketone), resulting in characteristic conformations (like chair or boat forms) that affect the stability and reactivity of the sugar.

Within these cyclic forms, subtle variations lead to the formation of epimers and anomers.

Epimers differ in configuration at just one specific carbon atom
Anomers are a special subset of epimers formed at the new chiral center (the anomeric carbon) during cyclization. These anomers are typically designated as alpha ( $α$ ) or beta ( $β$ ), depending on the orientation of the substituent relative to the ring.

Monosaccharide ring and linear forms including alpha and beta glucose

Disaccharides and polysaccharides
Glycosidic bonds join individual monosaccharides into larger carbohydrate molecules.

Sucrose (table sugar) is a disaccharide composed of $α$ -glucose and $β$ -fructose that are connected through their anomeric hydroxyl groups, forming acetal bonds.
Lactose is formed by linking $β$ -galactose to glucose (which can be either $α$ or $β$ ) via a 1-4 glycosidic bond.

Disaccharide formation showing glycosidic bond in sucrose between glucose and fructose

Maltose (malt sugar) is another disaccharide and results from a Dehydration reaction between two molecules of glucose.

Common disaccharides maltose and lactose

Starch is a polymer (polysaccharide made up of monosaccharides) composed of glucose units connected primarily by $α$ 1-4 glycosidic bonds, serving as the main energy storage molecule in plants.

Glycogen is structurally similar to starch but features additional $α$ 1-6 linkages that introduce branching, making it an efficient energy reserve in animals, primarily stored in the liver.

Reactions of monosaccharides

Acetal formation occurs when an extra hydroxyl group attacks a carbonyl carbon, which can link monosaccharides together into polysaccharides if the attacking – $O H$ originates from another sugar unit.
Mutarotation refers to the equilibrium between the $α$ and $β$ forms (anomers) of a sugar.
Under strong oxidation conditions, the aldehyde group and terminal hydroxyls are converted into carboxylic acids, while other hydroxyl groups are transformed into ketones; in extreme cases, such as during cellular respiration, all carbon atoms can be fully oxidized to $C O_{2}$ .
In contrast, mild oxidation is more selective: for example, the Tollens reagent specifically oxidizes the aldehyde group of aldoses to a carboxylic acid, and nitric acid oxidizes both the aldehyde and the terminal hydroxyl groups to carboxylic acids while leaving the other hydroxyl groups intact.
Reduction reactions can convert monosaccharides into polyalcohols by transforming carbonyl groups into additional hydroxyl groups.

Hydrolysis of the glycoside linkage

Another critical reaction in carbohydrate chemistry is the hydrolysis of the glycosidic linkage.
Hydrolysis, often catalyzed by glycosidase enzymes, breaks these bonds by adding a water molecule, thereby releasing the constituent sugar units.