Carbohydrates: Definition and Classification

Study Notes

Carbohydrates are polyhydroxy-aldehydes, polyhydroxy-ketones, or their derivatives.
They are organic compounds composed of carbon, hydrogen, and oxygen, also known as saccharides.
Key functions include: primary energy source; structural and protective roles (mucopolysaccharides); lubrication (glycoproteins & proteoglycans); cell adhesion and recognition; components of glycoconjugates (glycoproteins or glycolipids); constituents of nucleic acids (DNA, RNA), coenzymes (NAD, FAD), regulatory molecules (cAMP, cGMP), and high-energy compounds (ATP, GTP).

According to the number of sugar units: monosaccharides (cannot be hydrolyzed further), disaccharides (two monosaccharides linked by a glycosidic bond), oligosaccharides (3-10 monosaccharides), and polysaccharides (more than 10 monosaccharides; can be linear or branched).
According to the active group and number of carbon atoms: Aldoses (containing an aldehyde group) and ketoses (containing a ketone group) are categorized into trioses (3C), tetroses (4C), pentoses (5C), and hexoses (6C). Examples include glyceraldehyde (aldotriose), dihydroxyacetone (ketotriose), erythrose, ribose, glucose, galactose, mannose, and fructose.

Stereoisomers: Molecules with the same chemical formula but different spatial arrangements. Examples include aldo-keto isomers, epimers (differing at one chiral carbon; e.g., glucose, galactose, and mannose), and enantiomers (mirror images; D and L isomers; e.g., D- and L-glyceraldehyde). D-isomers are physiologically predominant.
Anomers (α & β): Differ in the configuration around the anomeric carbon (the carbonyl carbon in the cyclic form). α-form has the OH group down, β-form has it up.
Optical Activity: The ability to rotate plane-polarized light; caused by asymmetric carbon atoms. All monosaccharides except dihydroxyacetone are optically active. D- and L-enantiomers rotate light in opposite directions; α and β anomers rotate in the same direction but to different degrees.

Reducing Character: Due to the presence of a free aldehyde or ketone group, all monosaccharides exhibit reducing properties.
Monosaccharide Derivatives: Include amino sugars (e.g., glucosamine from glucose, replacing an OH with an amino group), deoxy sugars (e.g., 2-deoxyribose from ribose, replacing an OH with H), and sugar acids (formed by oxidation; aldonic acids, uronic acids, and saccharic acids).
Sugar Alcohols: Formed by reduction of the aldehyde or ketone group to an alcohol group (e.g., sorbitol, mannitol).