Chapter 13: Carbohydrate Structure and Function – Study Notes

Carbohydrates: The Most Abundant Biomolecules in Nature

Overview and Classification

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, and are the most abundant biomolecules in nature. They play diverse roles in biological systems, including energy storage, structural support, and cellular recognition.

• Empirical Formula: Most carbohydrates follow the general formula $(CH_2O)_n$, where $n \geq 3$.

• Types of Carbohydrates:

  • Monosaccharides: Simple sugars and their derivatives (3–9 carbon atoms).

  • Oligosaccharides: Short chains of monosaccharide units (typically 2–20).

  • Polysaccharides: Long chains of monosaccharide units (hundreds to thousands).

  • Glycoconjugates: Carbohydrates covalently linked to proteins or lipids.

• Glycans: Generic term for oligosaccharides and polysaccharides.

Diverse Functions of Carbohydrates

Carbohydrates serve a variety of biological functions:

1. Energy Storage and Generation: e.g., glucose, glycogen, starch

2. Molecular Recognition: e.g., immune system interactions

3. Cellular Protection: e.g., bacterial and plant cell walls

4. Cell Adhesion: e.g., glycoproteins

5. Biological Lubrication: e.g., glycosaminoglycans

6. Building and Maintaining Structure: e.g., cellulose, chitin

Carbohydrate Terminology and Structure

Key Definitions

• Monosaccharide: The building block of all carbohydrates; classified by the number of carbon atoms (triose, tetrose, pentose, hexose, etc.). Naming ends in "-ose".

• Oligosaccharide: Compound formed by linking several monosaccharides together (e.g., disaccharide, trisaccharide).

• Polysaccharide: Polymer formed from multiple saccharide units; may be homopolysaccharide (one type of monosaccharide) or heteropolysaccharide (multiple types).

• Glycan: Generic term for oligosaccharides and polysaccharides.

Monosaccharide Structure

• General Formula: $C_nH_{2n}O_n$ (where $n$ varies from 3 to 9)

• Aldose: Monosaccharide containing an aldehyde group.

• Ketose: Monosaccharide containing a ketone group.

• Carbon Numbering: Always starts from the most oxidized end.

Trioses: The Simplest Monosaccharides

• D-Glyceraldehyde: An aldose with a chiral center.

• Dihydroxyacetone: A ketose, achiral.

• Example: $\text{D-Glyceraldehyde}$ and $\text{Dihydroxyacetone}$ are the simplest monosaccharides.

Stereochemistry of Monosaccharides

Enantiomers and Diastereomers

• Enantiomers: Stereoisomers that are mirror images; designated D- or L- based on the configuration at the highest numbered chiral carbon (relative to glyceraldehyde).

• Diastereomers: Stereoisomers that are not mirror images; have different common names.

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

Fischer Projections

• Used to depict enantiomers in monosaccharides.

• Most oxidized carbon at the top; vertical bonds project away, horizontal bonds project toward the viewer.

• D- or L- assignment is based on the configuration of the chiral center with the highest number.

Haworth Projections

• Five- and six-membered hemiacetals are represented as planar pentagons (furanose) or hexagons (pyranose).

• Down in Haworth projection = right in Fischer projection; up in Haworth = left in Fischer.

Cyclization and Anomeric Carbon

Monosaccharide Cyclization

Monosaccharides can cyclize via reaction of an alcohol group with the carbonyl group:

• If C1 reacts with C5: forms a cyclic hemiacetal (aldose).

• If C2 reacts with C5: forms a cyclic hemiketal (ketose).

• The carbonyl carbon becomes a new chiral center, called the anomeric carbon.

Mutarotation

• Conversion between alpha ($\alpha$) and beta ($\beta$) anomers requires bond breakage at the anomeric carbon.

• Occurs as long as the anomeric carbon is not involved in a glycosidic bond.

Identifying the Anomeric Carbon

• The anomeric carbon is bonded to two oxygen atoms in the cyclic form.

Distinguishing Cyclic Aldoses from Ketoses

• At the anomeric center, if the substituent is –OH, it is an aldose; if it is –CH2OH, it is a ketose.

Reducing Sugars

Definition and Properties

• Reducing sugar: Contains an aldehyde group that can be oxidized to a carboxylic acid.

• Must have a free anomeric carbon (not involved in a glycosidic bond).

• Positive Tollens test indicates a reducing sugar.

• Example Reaction:

  • $\text{Ag(NH}_3)_2^+ + \text{Reducing sugar} \rightarrow \text{Ag(s)}$

Reactions of Monosaccharides

Sugar Alcohols

• Reduction of the carbonyl group ($C=O$) of a monosaccharide yields a polyhydroxy compound called an alditol (e.g., ethylene glycol, methanol).

• Common sugar alcohols: sorbitol, xylitol, mannitol.

Ingredient	Sweetness	Glycemic Index	Cal/g
Sucrose (sugar)	100%	60	4
Maltitol Syrup	75%	52	3
Hydrogenated Starch Hydrolysate	33%	39	2.8
Maltitol	75%	36	2.1
Xylitol	100%	13	2.5
Isomalt	55%	9	2.1
Sorbitol	60%	9	2.6
Lactitol	35%	6	2
Mannitol	69%	0	1.5
Erythritol	70%	0	0.2

Phosphoric Esters

• Phosphoric esters are important in sugar metabolism and energy production.

• Formed by transfer of a phosphate group from ATP.

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

Amino Sugars

• Hydroxyl group replaced by an amine.

• N-Acetyl glucosamine and N-acetylmuramic acid are components of bacterial cell walls.

• Aminoglycoside antibiotics contain amino sugars.

Summary: Monosaccharides

• Defined by number of carbons, type of carbonyl (aldehyde or ketone), configuration at highest numbered chiral carbon (D or L), and configuration at other stereocenters.

• Common names are based on configuration at stereocenters.

• Cyclization leads to formation of furanose or pyranose rings and anomeric carbon.

• Monosaccharides may have other functional groups attached.

Glycobiology and Glycoconjugates

Principles of Glycobiology

• Glycan biochemistry

• Glycan biosynthesis

• Glycan diversity

• Glycan recognition

Glycoconjugates

• Carbohydrates covalently linked to proteins or lipids.

• Common monomers: glucose, galactose, mannose, N-acetylglucosamine, N-acetylgalactosamine, xylose, glucuronic acid, fucose, iduronic acid, sialic acid.

Glycosidic Bond Formation

• Glycoside: Carbohydrate in which the –OH of the anomeric carbon is replaced by –OR.

• Glycosidic bond: Bond from the anomeric carbon to the –OR group; links monosaccharides to form di- and polysaccharides.

• N-glycosidic bond links ribose to bases in nucleotides.

Polysaccharides: Amylose and Amylopectin

• Amylose: Unbranched polymer of glucose with $\alpha(1\rightarrow4)$ glycosidic bonds.

• Amylopectin: Branched polymer of glucose with $\alpha(1\rightarrow4)$ main chain and $\alpha(1\rightarrow6)$ branch points.

Disaccharides of α-D-Glucose

• Glycosidic linkages can be designated $\alpha$ or $\beta$ based on the stereochemistry of the anomeric carbon.

• Reducing end contains a free anomeric carbon.

Oligosaccharides

• Simple sugars that range from 3 to 20 branched and unbranched sugar residues.

• Examples: stachyose, raffinose, verbascose.

Structural Carbohydrates

• Cellulose: Homopolymer of repeating units of cellobiose; contains $\beta(1\rightarrow4)$ glycosidic bonds; provides rigid plant cell wall.

• Hemicellulose: Branched heteropolymer.

• Pectin: Complex polysaccharide found in plant cell walls.

• Chitin: Linear polysaccharide of GlcNAc hexosamine units; contains $\beta(1\rightarrow4)$ glycosidic bonds; structural component of exoskeletons in insects and crustaceans.

Glycosaminoglycans

• Linear hexosamine polysaccharides; consist of 20–50 disaccharides.

• Examples: proteoglycans, chondroitin sulfate, heparan sulfate, keratan sulfate.

• Important in interstitial fluid between joints and tissues.

Glucose Homopolymers: Starch and Glycogen

Starch	Glycogen
Used by plants to store excess glucose. Examples: amylose (unbranched), amylopectin (branched every ~20 residues).	Used by animals to store dietary sources of glucose. Contains branch points (more extensive than in plants, every ~10 residues).

Example: Raffinose

• Plant oligosaccharide composed of galactose, glucose, and fructose.

• Humans and pigs cannot break down raffinose due to lack of α-galactosidase.

Summary Table: Monosaccharide Properties

Property	Description
Number of carbons	Trioses, tetroses, pentoses, hexoses, etc.
Type of carbonyl	Aldose or ketose
Configuration	D or L (highest numbered chiral carbon)
Cyclization	Furanose (5-membered) or pyranose (6-membered)
Anomeric carbon	Configuration (α or β)

Additional info: These notes expand on the provided slides with definitions, examples, and context for key biochemistry concepts relevant to carbohydrate structure and function, suitable for college-level study.