glycosidic
Summary
A glycosidic bond joins a sugar's anomeric carbon to an oxygen (or, in nucleosides, a nitrogen) of another molecule, formed by condensation. Most testable: α versus β configuration decides enzyme specificity, so humans digest α-1,4 starch but not β-1,4 cellulose.
Detail
Meaning: A glycosidic bond is the covalent C–O–C link between the anomeric carbon of a sugar and the oxygen of a hydroxyl group on another molecule, usually another sugar. It forms by condensation, releasing H₂O, as the anomeric hemiacetal (or hemiketal) becomes an acetal (or ketal). Hydrolysis breaks it. Bonds are named by anomeric configuration and the carbons joined, such as α-1,4 or β-1,4. In nucleosides, C1′ of the sugar bonds to a base nitrogen in an N-glycosidic bond.
On the MCAT: Passages ask which enzyme digests a sugar or why humans digest starch but not cellulose. Amylase hydrolyzes α-1,4 links in starch and glycogen; α-1,6 links form the branches; cellulose has β-1,4 links that human enzymes do not cleave. Lactose is β-1,4 (galactose–glucose). Sucrose joins both anomeric carbons, so it is nonreducing. A sugar reduces Benedict's or Tollens' reagent only if a free anomeric carbon remains.
Common confusion: α and β describe the orientation of the bond at the anomeric carbon (α is down in a Haworth drawing of a D-sugar), not which carbons are joined; the numbers give that.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Gatto, Stryer)
- Organic Chemistry (Wade)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.