We have seen that when a second alcohol attacks a hemiacetal or hemiketal, the result is an acetal or ketal, with the glycosidic bonds in carbohydrates provided as an important example. But what if a hemiacetal is attacked not by a second alcohol, but by an amine?
What results in this case is a kind of ‘mixed acetal’ in which the carbon that was originally part of the aldehyde is bonded to one oxygen and one nitrogen. These are referred to by biochemists as N-glycosidic bonds. You may recognize them as the bonds that link DNA and RNA bases to the sugar-phosphate backbone:
The starting point for the synthesis of purine nucleotide triphosphates (ATP and GTP) is a phosphorylated derivative of the ribofuranose called phosphoribosylprophosphate (PRPP). In the first step of purine base synthesis, a molecule of ammonia attacks the anomeric carbon of PRPP, displacing the diphosphate group in a two-step (SN1) mechanism with an oxonium ion intermediate.
As you can see, the reaction proceeds with inversion of configuration. With the N-glycosidic bond in place, the rest of the DNA base is then built piece by piece.
In the breakdown of the RNA base uridine, the N-glycosidic bond is broken by an attacking phosphate nucleophile:
This reaction also takes place with inversion of configuration, and although it would seem reasonable to assume that it proceeds via an SN1-like mechanism, recent evidence (Biochemistry 2011, 50, 9158) suggests that the mechanism is actually SN2.