Lehninger Principles of Biochemistry, 6th Edition ( PDFDrive.com )(11) (1)

Carbohydrates Overview

  • Carbohydrates are the most abundant biomolecules on Earth.

  • Photosynthesis annually converts over 100 billion metric tons of CO2 and H2O into plant products like cellulose.

  • Carbohydrates, including sugars and starch, are a dietary staple globally.

  • Oxidation of carbohydrates is the primary energy-yielding pathway in most nonphotosynthetic cells.

Types of Carbohydrates

Major Classes

  1. Monosaccharides: Simple sugars; single polyhydroxy aldehyde or ketone unit.

    • Example: D-glucose is the most abundant natural monosaccharide.

  2. Oligosaccharides: Short chains (2-10) of monosaccharide units linked by glycosidic bonds.

    • Example: Sucrose (D-glucose + D-fructose) is a common disaccharide.

  3. Polysaccharides: Long chains (20+) of monosaccharide units.

    • Examples: Cellulose (linear), Glycogen (branched).

Monosaccharides and Their Characteristics

  • Definition: Polyhydroxy aldehydes or ketones with multiple chiral centers.

  • Sizes: Classified based on carbon count: tetroses (4C), pentoses (5C), hexoses (6C), heptoses (7C).

  • Isomerism: Enzymes recognize specific stereoisomers, making stereoisomerism biologically significant.

Stereochemistry of Monosaccharides

  • Monosaccharides except for dihydroxyacetone have at least one chiral carbon.

  • Fisher projection is used to represent sugar structures:

    • Horizontal bonds project toward the reader.

    • Vertical bonds project away.

  • Configuration is determined by the orientation of hydroxyl groups relative to the reference carbon.

Formation of Cyclic Structures

  • Aldoses and ketoses predominantly form cyclic structures in an aqueous solution due to reactions of alcohols with carbonyl groups.

  • Anomeric Carbon: The new chiral center formed during cyclic structure formation.

  • Anomers: Different configurations at the anomeric carbon (α and β forms).

Oligosaccharides and Glycosidic Bonds

  • Disaccharides: Formed by the glycosidic bond between two monosaccharides.

    • Example: Maltose (D-glucose + D-glucose) retains a free anomeric carbon, hence a reducing sugar.

  • Nomenclature: Disaccharides named conventionally indicating the structure, stereochemistry, and involved carbons.

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

  • Non-reducing sugars involve a glycosidic bond that keeps the anomeric carbon occupied.

Important Sugars and Their Functions

  • Lactose: A disaccharide that yields D-galactose and D-glucose upon hydrolysis; found in milk.

  • Sucrose: Nonreducing disaccharide (table sugar) formed from glucose and fructose.

  • Trehalose: Energy storage compound in insects, also nonreducing.

Modifications and Discoveries

  • Amino sugars result from replacing hydroxyl groups with amino groups (e.g., Glucosamine).

  • Acidic sugars contain carboxylate groups, yielding negative charges, play roles in biological processes.

  • Derivatives like N-acetylneuraminic acid are crucial in glycoproteins affecting cell recognition.

Metabolic Pathways and Realities

  • Glucose is essential for brain function; levels must be tightly regulated.

  • Understanding how carbohydrates affect health is crucial for treating conditions like diabetes.

  • Measurement methods include assessing glycated hemoglobin (GHB) levels for chronic glucose monitoring.

Summary of Key Points

  • Monosaccharides can exist in multiple stereochemical forms and are reducing sugars except when involved in glycosidic bonds.

  • Glycosidic bonds safeguard the anomeric carbon from oxidation, defining the classification of sugars as reducing or nonreducing.

  • Multiple names and structures exist for sugars, reflecting their biological relevance and interactions.