Nature's many mechanisms for the degradation of oligosaccharides.

Recent work on the mechanistic elucidation of the polysaccharide lyases, the [small alpha]-1,4-glucan lyases, and the Family 4 glycosidases have demonstrated that nature has evolved to use elimination steps for the degradation of oligosaccharides. The polysaccharide lyases (E.C. 4.2.2.-) have been shown to cleave uronic acid-containing polysaccharides via a stepwise E1cB mechanism. The mechanism of the alpha-1,4-glucan lyases (E.C. 4.2.2.13) is similar to the Family 31 glycosidases, forming a covalent glycosyl-enzyme intermediate, which is subsequently cleaved by an E1-like E2 mechanism. Meanwhile, the Family 4 glycosidases (E.C. 3.2.1.6) are suggested to undergo an oxidation-elimination-addition-reduction sequence. These three groups of enzymes are examples of stark contrast to the vast number of well-characterized glycosidases (E.C. 3.2.1.-), which utilize either the direct or double displacement mechanisms as proposed by Koshland over 50 years ago.