Carbohydrates Basics - Page 1

Elements and Formula

• Carbohydrates are composed of carbon (C), hydrogen (H), and oxygen (O) only.
• General empirical formula often written as $(CH<em>2O)</em>n$; common monosaccharide example: $C<em>6H</em>{12}O_6$.
• The two key functional-group indicators that help identify carbohydrates:
  • Aldehyde group at the end of the carbon skeleton → aldehyde-containing sugars are called aldoses.
  • Ketone group in the middle of the carbon skeleton → ketones-containing sugars are called ketoses.
• Most carbohydrates are polyhydroxy aldehydes or ketones (with multiple hydroxyl groups).

Functional Groups and Classification

• Aldose: contains an aldehyde group at the terminal carbon (the carbonyl group is at C1). Examples: $\text{glucose}, \text{galactose}, \ \text{ribose}$.
• Ketose: contains a ketone group (carbonyl in the middle of the chain). Examples: $\text{fructose}$.
• Classification by number of carbons: triose, tetrose, pentose, hexose, etc.
• Relationship to isomerism: many aldoses and ketoses have multiple stereoisomers, including D- and L- forms.

Ring Formation and Anomerism

• Do carbohydrates form ring structures? Yes.
• In aqueous solution, many monosaccharides cyclize via intramolecular reaction between a carbonyl group and a hydroxyl group to form a hemiacetal (from aldoses) or hemiketal (from ketoses).
• The resulting rings are typically called pyranoses (6-membered rings) or furanoses (5-membered rings).
• Anomeric carbon: the former carbonyl carbon that becomes part of the ring; its configuration determines α or β forms.
  • Alpha (α): OH on the anomeric carbon is trans to the CH2OH substituent.
  • Beta (β): OH on the anomeric carbon is cis to the CH2OH substituent.
• Examples:
  • Glucose can form α-D-glucopyranose or β-D-glucopyranose.
  • Fructose can form furanose rings (e.g., β-D-fructofuranose).

Glycosidic Bonds and Linkages

• Glycosidic bond definition: A covalent bond that links two monosaccharides together.
• Formation: via dehydration (condensation) reaction between a hydroxyl group of one sugar and the anomeric hydroxyl group of another sugar (or sometimes between non-anomeric hydroxyls under specific conditions).
• Key features:
  • Involves the anomeric carbon of one sugar.
  • Can be α- or β- depending on the orientation of the glycosidic linkage at the anomeric carbon.
  • Common linkages include $\alpha- (1 \to 4)$ and $\alpha- (1 \to 2)$, $\beta- (1 \to 4)$, $\beta- (1 \to 6)$, etc.
• Examples of disaccharides and their linkages:
  • Maltose: two glucose units with an $\alpha- (1 \to 4)$ glycosidic bond ⇒ $\text{Glc-} \alpha (1 \to 4)\text{-Glc}$.
  • Lactose: glucose and galactose with a $\beta- (1 \to 4)$ linkage ⇒ $\text{Gal-}\beta (1 \to 4)\text{-Glc}$.
  • Sucrose: glucose and fructose linked via $\alpha- (1 \to 2)$ glycosidic bond (glucose C1 to fructose C2).
• Enzymatic hydrolysis: specific enzymes (e.g., maltase, sucrase, lactase) cleave these bonds to release monosaccharides.

Common Carbohydrates and Examples

• Monosaccharides (simple sugars):
  • Hexoses: $\text{glucose}, \text{fructose}, \text{galactose}$ (glucose and galactose are aldoses or ketoses; glucose is an aldose, fructose is a ketose).
  • Pentoses: $\text{ribose}, \text{deoxyribose}$.
• Disaccharides:
  • Maltose: $\text{Glc-}\alpha(1\to4)\text{-Glc}$.
  • Lactose: $\text{Gal-}\beta(1\to4)\text{-Glc}$.
  • Sucrose: $\text{Glc-}\alpha(1\to2)\text{- Fru}$ (glucose-fructose linkage).
• Polysaccharides:
  • Starch (plants): mainly amylose ($\alpha- (1\to 4)$) and amylopectin ($\alpha- (1\to 4)$ with $\alpha- (1\to 6)$ branches).
  • Glycogen (animals): highly branched $\alpha- (1\to 4)$ and $\alpha- (1\to 6)$ linkages.
  • Cellulose (plants): $\beta- (1\to 4)$ linkages, leading to structural, fibrous material.

Biological Roles and Relevance

• Energy storage:
  • Starch in plants and glycogen in animals function as energy reserves.
• Structural roles:
  • Cellulose provides plant cell wall structure (β-1,4 linkages require different enzymes than humans possess).
  • Chitin provides exoskeletons in some arthropods (N-acetylglucosamine polymers).
• Biochemical recognition:
  • Glycoproteins and proteoglycans on cell surfaces involve carbohydrate moieties important for cell-cell recognition and signaling.
• Metabolic pathways:
  • Glycolysis begins with glucose metabolism to extract energy.

Key Terms and Quick Reference

• Carbohydrate: biomolecule composed of C, H, O; general formula $(CH<em>2O)</em>n$.
• Aldose: carbohydrate with an aldehyde group at the end of the chain.
• Ketose: carbohydrate with a ketone group in the middle of the chain.
• Monosaccharide: single sugar unit (e.g., glucose, fructose).
• Disaccharide: two monosaccharides linked by a glycosidic bond (e.g., maltose, lactose, sucrose).
• Glycosidic bond: covalent link between sugar units; formed via dehydration synthesis; hydrolyzed by specific enzymes.
• Anomer: α or β configuration at the anomeric carbon in cyclic forms.
• Pyranose: six-membered ring form of a sugar.
• Furanose: five-membered ring form of a sugar.
• Polysaccharide: long chains of monosaccharide units (starch, glycogen, cellulose, etc.).