Carbohydrates: Structure, Classification, Stereochemistry, and Metabolism

Carbohydrates

Introduction

Carbohydrates, also known as saccharides, are macromolecules abundant in living organisms. They serve as the principal source of energy and play structural and functional roles in cells and tissues. Plants synthesize carbohydrates via photosynthesis, while animals obtain them through their diet.

• General Formula: Most carbohydrates follow the empirical formula $C_n(H_2O)_n$.

• Functional Groups: Carbohydrates are polyhydroxyaldehydes or polyhydroxyketones, containing multiple hydroxyl groups and either an aldehyde or ketone group.

• Biological Importance: They are involved in energy metabolism, cell recognition, and structural integrity.

Classification of Carbohydrates

Main Types

Carbohydrates are classified based on their complexity and structure:

• Monosaccharides: Simple sugars (e.g., glucose, fructose, galactose, ribose, deoxyribose).

• Oligosaccharides: Short chains of monosaccharide units (e.g., disaccharides like lactose, sucrose, maltose).

• Polysaccharides: Long chains of monosaccharide units (e.g., starch, cellulose, glycogen, chitin).

• Glycoconjugates: Carbohydrates covalently linked to proteins or lipids (e.g., glycoproteins, glycolipids).

Monosaccharides are further classified as:

• Aldoses: Monosaccharides with an aldehyde group (e.g., glucose, galactose, ribose).

• Ketoses: Monosaccharides with a ketone group (e.g., fructose).

Table: Classification of Carbohydrates

Type	Examples	Function
Monosaccharides	Glucose, Fructose, Galactose	Energy source, metabolic intermediates
Disaccharides	Lactose, Sucrose, Maltose	Energy source, transport
Polysaccharides	Starch, Cellulose, Glycogen, Chitin	Energy storage, structural support
Glycoconjugates	Glycoproteins, Glycolipids	Cell recognition, signaling

Stereochemistry of Carbohydrates

Isomerism

Carbohydrates exhibit various forms of isomerism due to the presence of multiple chiral centers.

• Isomers: Compounds with the same molecular formula but different physical and chemical properties.

• Types of Isomerism:

  • Structural (Constitutional) Isomers: Differ in connectivity of atoms.

  • Stereoisomers: Same connectivity, different spatial arrangement.

  • Geometric Isomers: Differ in arrangement around double bonds.

  • Optical Isomers: Differ in the way they rotate plane-polarized light.

Optical Isomerism

• Enantiomers: Non-superimposable mirror images; differ in configuration at all chiral centers.

• Diastereomers: Isomers that are not mirror images; differ at one or more (but not all) chiral centers.

• Epimers: Diastereomers that differ at only one chiral center.

Example: D-glucose and D-mannose are epimers at C-2.

Chirality and D/L Notation

• Chiral Carbon: A carbon atom bonded to four different groups.

• D- and L- Configuration: Determined by the position of the hydroxyl group on the chiral carbon furthest from the carbonyl group in Fischer projection. D-sugars have the hydroxyl group on the right; L-sugars on the left.

• Most naturally occurring sugars in humans are D-isomers.

Number of Optical Isomers

• For aldoses: $2^{n-2}$, where n = number of carbons (since two carbons are not chiral).

• For ketoses: $2^{n-3}$, since three carbons are not chiral.

Monosaccharides: Structure and Properties

Linear and Cyclic Forms

Monosaccharides exist in equilibrium between open-chain (Fischer) and cyclic (Haworth) forms. Cyclization involves the formation of a hemiacetal (aldoses) or hemiketal (ketoses) linkage between the carbonyl group and a hydroxyl group.

• Anomeric Carbon: The carbon derived from the carbonyl group during cyclization; gives rise to two anomers (α and β).

• α-Anomer: The hydroxyl group on the anomeric carbon is trans to the CH2OH group.

• β-Anomer: The hydroxyl group on the anomeric carbon is cis to the CH2OH group.

Examples of Monosaccharides

• Glucose: Most abundant and physiologically important monosaccharide; also called dextrose due to its dextrorotatory property.

• Galactose: An aldohexose; forms lactose with glucose; important in glycoproteins and glycolipids.

• Fructose: A ketohexose; also called levulose; forms sucrose with glucose; has high sweetening power.

Disaccharides

Structure and Examples

Disaccharides are formed by condensation of two monosaccharides via a glycosidic bond, releasing a molecule of water.

• Lactose: Composed of glucose and galactose; found in milk; important for infant nutrition.

• Sucrose: Composed of glucose and fructose; found in table sugar; produced from sugar cane or beet.

• Maltose: Composed of two glucose units; produced by hydrolysis of starch and glycogen.

Table: Common Disaccharides

Name	Monosaccharide Units	Source
Lactose	Glucose + Galactose	Milk
Sucrose	Glucose + Fructose	Sugar cane, beet
Maltose	Glucose + Glucose	Starch hydrolysis

Polysaccharides

Structure and Function

Polysaccharides are long chains of monosaccharide units linked by glycosidic bonds. They serve as energy storage and structural components.

• Starch: Plant storage polysaccharide; consists of amylose (linear, α(1→4) linkages) and amylopectin (branched, α(1→6) linkages).

• Cellulose: Structural polysaccharide in plants; linear chains of β-D-glucose linked by β(1→4) bonds; indigestible by humans.

• Glycogen: Animal storage polysaccharide; highly branched chains of D-glucose with α(1→4) and α(1→6) linkages.

• Chitin: Structural polysaccharide in invertebrates; composed of N-acetylglucosamine units.

Glycosidic Bond Formation

Mechanism

A glycosidic bond forms between the hydroxyl group of one monosaccharide and the anomeric carbon of another, eliminating a molecule of water.

Transport of Hexoses

Transport Systems

• SGLTs (Sodium-Glucose Linked Transporters): Active transport proteins in the intestinal mucosa and renal tubules.

• GLUTs (Glucose Transporters): Facilitate diffusion of glucose across cell membranes in various tissues.

Table: Characteristics of Monosaccharide Transporters

Transporter	Location	Substrate	Function
SGLT1	Intestine, kidney	Glucose, galactose	Active transport
SGLT2	Kidney	Glucose	Reabsorption
GLUT1	All tissues	Glucose	Basal uptake
GLUT2	Liver, pancreas, intestine	Glucose, galactose, fructose	Bidirectional transport
GLUT4	Muscle, adipose tissue	Glucose	Insulin-dependent uptake

Dietary Fiber

Role and Benefits

• Energy Value: ~2 kcal/g

• Recommended Intake: 25-30 g/day for adults

• Health Benefits:

  • Prevents constipation, hemorrhoids, diverticulosis

  • Regulates blood glucose and insulin

  • Reduces risk of cardiovascular disease and some cancers

  • Promotes satiety and weight control

Metabolism of Carbohydrates

Digestion and Absorption

Polysaccharides are hydrolyzed by amylases in the mouth and pancreas, producing oligosaccharides and disaccharides, which are further broken down into monosaccharides for absorption.

Glycolysis

Glycolysis is the metabolic pathway that converts glucose into pyruvate, generating ATP.

• General Equation:

$\text{Glucose} + 2\,\text{ADP} + 2\,\text{Pi} + 2\,\text{NAD}^+ \rightarrow 2\,\text{Pyruvate} + 2\,\text{ATP} + 2\,\text{NADH} + 2\,\text{H}_2\text{O}$

• Anaerobic Glycolysis: Occurs without oxygen, producing lactate.

• Aerobic Glycolysis: Pyruvate enters the mitochondria for further oxidation.

Additional info:

• Carbohydrates are essential for cellular communication, especially in the immune system and blood group antigens (e.g., ABO system).

• Excess dietary sugar is linked to metabolic disorders such as diabetes and obesity.