glycosidic bond

Meaning: A glycosidic bond is the covalent C–O–C link between the anomeric carbon of a cyclic sugar and an oxygen of another molecule, usually a hydroxyl of another sugar. It forms by condensation (loss of H₂O) and is broken by hydrolysis. Chemically it is an acetal (or ketal), made when the hemiacetal OH at the anomeric carbon is replaced by OR. The name gives the anomeric configuration and the carbons joined: α-1,4 links C1 of one residue, with its OH pointing down, to C4 of the next. Nucleotides use an N-glycosidic bond between sugar and base.

On the MCAT: Expect disaccharides and polysaccharides: maltose (α-1,4), lactose (β-1,4), sucrose (α-1,β-2). Starch and glycogen use α-1,4 links with α-1,6 branches; cellulose uses β-1,4. Questions ask why humans digest starch but not cellulose (amylase acts on α linkages), or which sugar is reducing. Sucrose is nonreducing because both anomeric carbons are in the bond.

Common confusion: α and β describe the configuration at the anomeric carbon, not which carbons are joined. A sugar is reducing when its anomeric carbon is free; once that carbon is tied up in a glycosidic bond, it cannot open to an aldehyde.