HE

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 (CH2O)n ; common monosaccharide example: C6H{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 (CH2O)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.).