6: Carbohydrate classification/stereochemistry

Section 1: Biomolecule Fundamentals

6: Carbohydrate Classification & Stereochemistry

Carbohydrates are polyhydroxy aldehydes or ketones, often termed "saccharides" or "sugars." They serve vital biological roles, including energy storage (starch, glycogen), structural support (cellulose, chitin), and cellular recognition (glycoproteins). Their general empirical formula is $(CH_2O)_n$, where $n \geq 3$.

Classification hinges on polymerization degree:

• Monosaccharides: Single sugar units; cannot be hydrolyzed further. Classified by functional group and carbon number:
  • Aldoses: Possess an aldehyde group (e.g., glyceraldehyde, glucose).
  • Ketoses: Possess a ketone group (e.g., dihydroxyacetone, fructose).
  • Chain length: Trioses (3C), tetroses (4C), pentoses (5C, e.g., ribose), hexoses (6C, e.g., glucose, galactose), heptoses (7C).
• Disaccharides: Two monosaccharides linked by a glycosidic bond (e.g., sucrose [glucose + fructose], lactose [galactose + glucose], maltose [glucose + glucose]).
• Oligosaccharides: Short chains (3-10 monosaccharides), often attached to proteins/lipids (glycoconjugates).
• Polysaccharides: Long chains (>10 units); homopolysaccharides (e.g., cellulose, glycogen – both glucose) or heteropolysaccharides (e.g., glycosaminoglycans).

Stereochemistry is fundamental due to chiral centers. A chiral carbon has four distinct substituents. Glyceraldehyde (an aldotriose) has one chiral center, existing as two enantiomers (mirror images): D-glyceraldehyde and L-glyceraldehyde. Most biologically relevant sugars are in the D-configuration, defined by the configuration of the highest-numbered chiral carbon (penultimate carbon) matching D-glyceraldehyde (OH group on the right in Fischer projection).

• Enantiomers: Non-superimposable mirror images (e.g., D-glucose and L-glucose).
• Diastereomers: Stereoisomers not mirror images.
• Epimers: Diastereomers differing at exactly one chiral center (e.g., D-glucose and D-galactose differ at C4; D-glucose and D-mannose differ at C2).

Monosaccharides $\geq$5 carbons predominantly exist as cyclic structures in solution. The carbonyl group reacts with a hydroxyl group, forming a hemiacetal (aldoses) or hemiketal (ketoses). This creates:

• Pyranose: Six-membered ring (resembling pyran; e.g., glucopyranose).
• Furanose: Five-membered ring (resembling furan; e.g., fructofuranose, ribofuranose).

Cyclization generates a new chiral center at the anomeric carbon (original carbonyl carbon), leading to anomers:

• $\alpha$-anomer: Anomeric OH opposite the CH$_2$OH group in the standard Haworth projection (trans configuration for D-sugars).
• $\beta$-anomer: Anomeric OH on the same side as the CH$_2$OH group (cis configuration for D-sugars).
  Anomers spontaneously interconvert in solution (mutarotation).