Comprehensive Study Notes: Carbohydrates in Biochemistry

Module 7: Carbohydrates

Objectives

• Investigate the physiological roles of carbohydrates.

• Identify chiral carbons within carbohydrates.

• Classify carbohydrate anomers, epimers, and D/L stereoisomers.

• Illustrate the structures of common monosaccharides in linear and cyclic forms.

• Apply the system for nomenclature of disaccharides.

• Describe the structure/function relationship of select polysaccharides.

Carbohydrates

Physiological Functions

Carbohydrates are essential biomolecules that serve a variety of physiological roles in living organisms:

• Energy: Primary source of energy for cells (e.g., glucose metabolism).

• Energy Storage: Stored as glycogen in animals and starch in plants.

• Structural Roles: Components of cell walls (cellulose in plants, chitin in arthropods).

• Cellular Interaction: Mediate cell-cell recognition and signaling.

• Cellular Identification: Surface carbohydrates act as molecular signatures.

• Information Transfer: Integral to nucleic acids (DNA & RNA).

• Signaling: Involved in various signaling pathways.

Monosaccharide Families

Aldoses and Ketoses

Monosaccharides are classified based on the presence of an aldehyde or ketone group:

• Aldoses: Contain an aldehyde group (e.g., D-glucose, D-glyceraldehyde).

• Ketoses: Contain a ketone group (e.g., dihydroxyacetone, D-fructose).

The general empirical formula for carbohydrates is $(CH_2O)_n$, where $n \geq 3$.

Chiral Carbons

Most monosaccharides have multiple chiral (asymmetric) carbons, leading to stereoisomerism:

• D/L Designation: Determined by the configuration of the chiral carbon farthest from the carbonyl group, relative to D-glyceraldehyde.

• Number of Stereoisomers: A molecule with $n$ chiral centers has $2^n$ stereoisomers.

Example: D-glucose has 4 chiral centers, resulting in 16 possible stereoisomers.

Categories of Isomers

• Constitutional Isomers: Differ in the order of atom connections.

• Stereoisomers: Same connectivity, different spatial arrangement.

• Enantiomers: Non-superimposable mirror images (D- and L-forms).

• Epimers: Differ at only one chiral carbon (e.g., D-glucose and D-mannose are C2 epimers).

• Anomers: Differ at the anomeric carbon formed during cyclization (α and β forms).

Common Monosaccharides

• Pentoses: D-ribose, D-deoxyribose (important in nucleic acids).

• Hexoses: D-glucose, D-mannose, D-galactose, D-fructose.

Monosaccharides: Cyclic Structures

Formation of Cyclic Structures

Monosaccharides with five or more carbons tend to form cyclic (ring) structures via intramolecular reactions:

• Hemiacetal Formation: Aldehyde reacts with an alcohol group.

• Hemiketal Formation: Ketone reacts with an alcohol group.

Pyran and Furan Ring Structures

• Pyranose: Six-membered ring (e.g., glucopyranose).

• Furanose: Five-membered ring (e.g., fructofuranose).

Cyclization of Glucose and Fructose

• Glucose: Cyclization involves the C5 hydroxyl and C1 aldehyde, forming α- and β-glucopyranose (anomeric carbon at C1).

• Fructose: Cyclization involves the C5 hydroxyl and C2 ketone, forming α- and β-fructofuranose (anomeric carbon at C2).

Anomeric Carbons and Mutarotation

• Anomeric Carbon: The new chiral center formed during cyclization.

• α-Anomer: Hydroxyl group on anomeric carbon is below the plane of the ring.

• β-Anomer: Hydroxyl group is above the plane of the ring.

• Mutarotation: Interconversion between α and β forms in solution via the open-chain form.

Example: In solution, glucose exists as ~2/3 β-glucopyranose, 1/3 α-glucopyranose, and <1% open-chain.

Sugar Derivatives

General

Carbohydrate derivatives may include nitrogen, phosphate, or sulfur groups, conferring specialized functions:

• Examples: N-acetylglucosamine, sialic acid, glucose-6-phosphate.

Plant Defensive Systems

• Some plants contain glucosinolate and the enzyme myrosinase.

• Upon tissue damage, myrosinase converts glucosinolate to glucose and isothiocyanate (bitter, deters herbivores).

• Isothiocyanates contribute to the odor of certain vegetables (e.g., broccoli, cabbage).

Monosaccharides as Reducing Agents

Linear forms of monosaccharides can be oxidized by mild oxidizing agents (e.g., Cu2+, Fe3+):

• The aldehyde group is oxidized to a carboxyl group.

• This property is used to quantify sugars in blood or urine.

• The end with the free carbonyl is the reducing end.

Disaccharides

Nomenclature

• Glycosidic Bond: Primary linkage between monosaccharides (O-glycosidic via oxygen, N-glycosidic via nitrogen).

• Nomenclature specifies:

  • Configuration (α or β) at the anomeric carbon of each monosaccharide

  • Ring form (furan or pyran)

  • Nonreducing sugar: suffix "osyl"

  • Reducing sugar: suffix "ose"

  • Linkage: carbons joined (e.g., (1→4))

Examples of Disaccharides

• Maltose: α-D-glucopyranosyl-(1→4)-D-glucopyranose

• Lactose: β-D-galactopyranosyl-(1→4)-α-D-glucopyranose

Lactose Intolerance: Caused by insufficient lactase enzyme, leading to digestive symptoms upon lactose consumption.

Polysaccharides

General

• Polysaccharides are polymers of monosaccharides with diverse structures and functions.

• Homopolysaccharides: One type of monosaccharide (e.g., starch, glycogen, cellulose).

• Heteropolysaccharides: Multiple types of monosaccharides.

• Can be unbranched or branched.

Energy Storage Polysaccharides

• Starch (plants): Amylose (unbranched, α(1→4) bonds), amylopectin (branched, α(1→6) branches every 24–30 residues).

• Glycogen (animals): Similar to amylopectin but more highly branched (every 8–12 residues), stored in liver and muscle.

Structural Polysaccharides

• Cellulose: Linear β(1→4) linked glucose, main component of plant cell walls, not digestible by humans.

• Chitin: Linear β(1→4) linked N-acetylglucosamine, found in exoskeletons of insects and crustaceans.

α and β Linkages

• β(1→4) linkages (cellulose, chitin): Form long, straight chains with high tensile strength due to hydrogen bonding between chains.

• α(1→4) linkages (starch, glycogen): Form helical, compact structures suitable for energy storage.

Glycolipids

Blood Group Antigens

• Sugars covalently linked to lipids form glycolipids, which are important for cell recognition (e.g., blood group antigens).

• Different sugar patterns on cell surfaces distinguish self from non-self, critical for blood transfusions.

Protein-Associated Carbohydrates

Glycoproteins

• Proteins with covalently attached sugars; protein is the major component by weight.

• Serve diverse biological roles, including cell signaling and immune response.

• Sugar attachment can be N-linked (to asparagine) or O-linked (to serine/threonine).

Proteoglycans

• Proteins linked to glycosaminoglycans (large carbohydrate component).

• Major structural and lubricating roles in the extracellular matrix.

Example: Erythropoietin (EPO)

• EPO is a glycoprotein hormone that stimulates red blood cell production.

• Contains three N-linked and one O-linked glycosylation sites.

• Glycosylation affects EPO's stability and activity.

Proteoglycans in the Extracellular Matrix

• Extracellular matrix contains a gel-like ground substance composed of proteoglycans and fibrous proteins.

• Provides structural support, cushioning, and mediates cell signaling.

Table: Structures and Roles of Some Polysaccharides

Polysaccharide	Type	Repeating Unit	Major Function
Starch (Amylose/Amylopectin)	Homopolysaccharide	Glucose	Energy storage in plants
Glycogen	Homopolysaccharide	Glucose	Energy storage in animals
Cellulose	Homopolysaccharide	Glucose	Structural (plant cell walls)
Chitin	Homopolysaccharide	N-acetylglucosamine	Structural (exoskeletons)
Hyaluronan	Heteropolysaccharide	Glucuronic acid + N-acetylglucosamine	Extracellular matrix, lubrication

Additional info: Table entries inferred from standard biochemistry references for clarity and completeness.