Chapter 11: Carbohydrates – Structure, Isomerism, and Biological Roles

Carbohydrates

Introduction

Carbohydrates are one of the major classes of biomolecules, serving as energy sources, structural components, and mediators of cellular recognition. This chapter explores the structure, isomerism, and biological functions of carbohydrates, focusing on monosaccharides, their derivatives, and their roles in complex biomolecules.

Monosaccharides: The Simplest Carbohydrates

Definition and Structure

• Monosaccharides are the simplest carbohydrates, consisting of aldehydes or ketones with two or more hydroxyl groups.

• The smallest monosaccharides have three carbon atoms (trioses).

• Monosaccharides exist in many isomeric forms, contributing to their chemical diversity.

Examples: Dihydroxyacetone (a ketose), D-glyceraldehyde (an aldose), and L-glyceraldehyde (an aldose).

Isomeric Forms of Carbohydrates

Carbohydrates exhibit several types of isomerism, which affect their chemical and biological properties.

Isomer Type	Description	Example
Constitutional Isomers	Same molecular formula, different connectivity	Dihydroxyacetone vs. Glyceraldehyde
Stereoisomers	Same connectivity, different spatial arrangement	D-Glucose vs. L-Glucose
Enantiomers	Non-superimposable mirror images	D-Glyceraldehyde vs. L-Glyceraldehyde
Epimers	Isomers differing at one asymmetric carbon	D-Glucose vs. D-Mannose
Anomers	Isomers differing at the new asymmetric carbon formed on ring closure	α-D-Glucose vs. β-D-Glucose

Common Monosaccharides

• Aldoses contain an aldehyde group (e.g., D-glucose, D-mannose, D-galactose).

• Ketoses contain a ketone group (e.g., D-fructose).

• The configuration (D or L) is determined by the chiral carbon farthest from the carbonyl group.

Example: D-glucose is the most abundant monosaccharide in nature.

Cyclic Forms of Monosaccharides

Formation of Hemiacetals and Hemiketals

Monosaccharides can cyclize via intramolecular reactions:

• An aldehyde reacts with an alcohol to form a hemiacetal.

• A ketone reacts with an alcohol to form a hemiketal.

$\text{Aldehyde} + \text{Alcohol} \rightarrow \text{Hemiacetal}$ $\text{Ketone} + \text{Alcohol} \rightarrow \text{Hemiketal}$

Pyranose and Furanose Rings

• Pyranose: Six-membered ring formed by cyclization of an aldose (e.g., glucose).

• Furanose: Five-membered ring formed by cyclization of a ketose (e.g., fructose).

Example: D-glucose forms α- and β-D-glucopyranose; D-fructose forms α- and β-D-fructofuranose.

Anomers

• Cyclization creates a new chiral center at the anomeric carbon, resulting in anomers.

• α-anomer: Hydroxyl group at the anomeric carbon is below the plane of the ring.

• β-anomer: Hydroxyl group at the anomeric carbon is above the plane of the ring.

Reducing and Non-Reducing Sugars

Reducing Properties

• Monosaccharides with a free aldehyde or ketone group can act as reducing sugars.

• Glucose exists in equilibrium between α, β, and open-chain forms; the open-chain form can reduce oxidizing agents.

Example: Glucose reduces copper(II) ions in Benedict's test.

Biological Implications

• Glucose can react with amino groups in proteins, forming glycosylated proteins (e.g., hemoglobin A1c).

• Measurement of hemoglobin A1c is used to monitor long-term blood glucose control in diabetics.

Glycosidic Bonds and Modified Monosaccharides

Glycosidic Bond Formation

• An O-glycosidic bond forms between the anomeric carbon and an alcohol oxygen.

• An N-glycosidic bond forms between the anomeric carbon and an amine nitrogen.

Example: N-glycosidic bond links ribose to adenine in adenosine monophosphate (AMP).

Modified Monosaccharides

• Monosaccharides can be chemically modified (e.g., phosphorylation, acetylation).

• Phosphorylated sugars (e.g., glucose 6-phosphate) are key intermediates in metabolism.

$\text{Glucose} + \text{ATP} \rightarrow \text{Glucose 6-phosphate} + \text{ADP}$

Complex Carbohydrates: Disaccharides and Polysaccharides

Disaccharides

• Formed by linking two monosaccharides via O-glycosidic bonds.

• Maltose: Two glucose units linked by α-1,4-glycosidic bond.

• Sucrose: Glucose and fructose linked by α-1,2-glycosidic bond.

• Lactose: Galactose and glucose linked by β-1,4-glycosidic bond.

Polysaccharides

• Glycogen: Animal storage polysaccharide; glucose units linked by α-1,4-glycosidic bonds, branched by α-1,6-glycosidic bonds every ~12 units.

• Starch: Plant storage polysaccharide; amylose (linear, α-1,4) and amylopectin (branched, α-1,6 every ~30 units).

• Cellulose: Plant structural polysaccharide; glucose units linked by β-1,4-glycosidic bonds, forming straight chains and strong fibrils via hydrogen bonding.

Structural and Dietary Roles

• β-1,4 linkages (cellulose) favor straight chains for structural support.

• α-1,4 linkages (glycogen, starch) favor bent chains for energy storage.

• Insoluble fiber (cellulose) aids digestion by increasing intestinal transit.

• Soluble fiber (pectin) facilitates nutrient absorption.

Glycoproteins and Proteoglycans

Glycoproteins

• Proteins covalently linked to carbohydrates; play roles in cell recognition, signaling, and membrane structure.

• Carbohydrates are attached via N-linked (asparagine) or O-linked (serine/threonine) glycosidic bonds.

Proteoglycans

• Proteins attached to glycosaminoglycans (long, negatively charged polysaccharides).

• Major components of extracellular matrix and cartilage; provide structural support and lubrication.

Example: Aggrecan in cartilage cushions joints by releasing and rebinding water.

Mucins

• Glycoproteins with extensive carbohydrate chains; act as lubricants and protective barriers.

Lectins: Carbohydrate-Binding Proteins

Definition and Function

• Lectins are proteins that specifically bind carbohydrates on cell surfaces.

• Facilitate cell-cell interactions, tissue formation, and transmission of information.

Example: Influenza virus binds to sialic acid residues on host cell glycoproteins to initiate infection.

Biological Importance

• Lectin-carbohydrate interactions are crucial for immune response, pathogen recognition, and cell signaling.

• Pathogens such as Plasmodium falciparum (malaria) exploit glycan binding for host invasion.

Summary Table: Key Carbohydrate Types and Linkages

Carbohydrate Type	Linkage	Function
Monosaccharide	None	Energy source, metabolic intermediate
Disaccharide	O-glycosidic bond	Dietary sugar (e.g., sucrose, lactose)
Polysaccharide	α-1,4 or β-1,4 glycosidic bonds	Storage (glycogen, starch), structure (cellulose)
Glycoprotein	N-linked or O-linked glycosidic bond	Cell recognition, signaling
Proteoglycan	Protein-glycosaminoglycan linkage	Extracellular matrix, lubrication
Lectin	Non-covalent carbohydrate binding	Cell-cell interaction, pathogen recognition

Additional info: This guide expands on the original notes by providing definitions, examples, and context for each major carbohydrate class and their biological significance.