Biochemistry 4115 - Notes on Carbohydrates

Lecture Information

  • Course: Biochemistry 4115
  • Schedule:
    • Mon – Wed – Friday 11:15 – 12:05 PM
    • Wed 5:30 – 6:20 PM (Usually Led by Teaching Assistants)
  • Lecture Title: Carbohydrates
  • Date: September 17th, 2025
  • Instructor: Dr. Daniel J. Slade
  • Contact: dslade@vt.edu

Carbohydrates and Life

  • Carbohydrates are a versatile class of molecules with the general formula (CH₂O)ₙ.
  • Functions:
    • Major form of stored energy in organisms.
    • Metabolic precursors of virtually all other biomolecules.
    • Conjugates of carbohydrates with proteins and lipids perform diverse functions including cellular recognition processes crucial for growth and transformation.
  • Taste: Sugars' sweet taste is attributed to specific bond patterns between sugar molecules and taste receptor proteins in taste buds.
    • Example Study:
      • Fructose, glucose, and mannose form hydrogen bonds of different lengths; sweeter-tasting sugars form tighter, stronger bonds (Di Mino et al., J Phys Chem Lett. 2018;9(13)).

Characteristic Chemical Features of Carbohydrates

  • Asymmetric Centers: Presence of at least one and often two or more asymmetric (chiral) centers.
  • Structure: Can exist in both linear and ring forms.
  • Polymeric Structures: Capability to form polymeric structures through glycosidic bonds.
  • Hydrogen Bonding: Ability to form multiple hydrogen bonds with water or other environmental molecules.
  • Name Origin: The name "carbohydrate" derives from the basic molecular formula (CH₂O)ₙ, where n ≥ 3.

Classification of Carbohydrates

  • Monosaccharides:

    • Also known as simple sugars.
    • General formula: (CH₂O)ₙ.
    • Cannot be broken down into smaller sugars under mild conditions.

  • Oligosaccharides:

    • Name derived from Greek word "oligo" meaning "few."
    • Composed of 2 to 10 simple sugar residues.
    • Common examples include disaccharides and trisaccharides.
    • 4- to 6-sugar-unit oligosaccharides are typically covalently bound to other molecules, such as glycoproteins.

  • Polysaccharides:

    • Polymers made up of simple sugars and their derivatives.
    • Can be linear or branched, containing hundreds or thousands of monosaccharide units.
    • Molecular weights of polysaccharides can exceed 1 million.

Functional Groups in Carbohydrate Nomenclature

  • Aldoses and Ketoses:
    • Aldoses contain an aldehyde group (;text{CHO}) whereas ketoses have a ketone group (;text{C=O}).
  • Examples:
    • Aldose: Glyceraldehyde
    • Ketose: Dihydroxyacetone

Configuration of Monosaccharides

  • For monosaccharides with ≥ 2 asymmetric carbons, configuration is denoted by a "D" or "L":
    • D-Configuration: The hydroxyl group on the highest numbered asymmetric carbon is to the right in Fischer projection (e.g., D-glyceraldehyde).
    • L-Configuration: The hydroxyl group on the highest numbered asymmetric carbon is to the left in Fischer projection.
  • Note: D/L configuration indicates relation to glyceraldehyde, not the sign of optical rotation.
    • For precise optical rotation, D or L can be combined with (+) or (-) signs for clarity.

Dominance of D-forms in Nature

  • D-forms of monosaccharides and L-amino acids predominate in biological systems.
  • Stereospecificity of Enzymes: This preference is thought to result from early evolutionary choices that have been maintained through enzyme specificity.

Questions for Identifying Carbohydrate Type

  1. Is it an Aldose or Ketose?
  2. How many carbons are present?
  3. Is it a D- or L-configuration?

Stereoisomers in Carbohydrates

  • Enantiomers:
    • Monosaccharides that are mirror images of each other (D- & L-forms).
  • Diastereomers:
    • Isomers with opposite configurations at one or more chiral centers but are not mirror images.

Changes at Chiral Centers

  • Altering the highest numbered chiral center results in the formation of an enantiomer. - Changing the configuration at another chiral center generates a diastereomer.

Cyclization Reactions in Monosaccharides

  • Alcohols react with aldehydes to form hemiacetals.
    • Cyclization from Fischer to Haworth projections occurs; five- and six-membered rings are most stable.
    • Example: D-Glucose cyclizes to a pyran form.
  • Alcohols can also react with ketones to form hemiketals.
    • Example: D-Fructose converts into a furan form.

Ring Conformations

  • Haworth Projections:
    • Convenient for representing monosaccharide structures, but do not portray true conformations.
    • Chair vs. Boat Conformations:
    • Ring substituents can be equatorial (stabilizing) or axial (less stable).
    • Bulky groups favor equatorial positions for stability.
    • Chair conformations are more stable than boat conformations due to sterics.

Derivative Forms of Monosaccharides

  • Chemical and enzymatic reactions produce various derivatives from simple sugars.
    • Aldonic Acids: Derived from aldoses, capable of reducing oxidizing agents, thus termed reducing sugars.

Sugar Alcohols

  • Structures of sugar alcohols like D-Glucitol (sorbitol), D-Mannitol, and D-Xylitol are depicted.
  • Sugar alcohols are used as sweeteners in