Structural and mechanistic classification of uronic acid-containing polysaccharide lyases

Polysaccharide lyases (PLs) have been assigned to 21 families based on their sequences, with similar to 50 singletons awaiting further classification. For 19 of these families, the structure of at least one protein is known. In this review, we have analyzed the available structural information and show that presently known PL families belong to six general folds. Only two general catalytic mechanisms have been observed among these PLs: (1) metal-assisted neutralization of the acidic group of the sugar next to the cleaved bond, with, rather unusually, arginine or lysine playing the role of Bronsted base and (2) neutralization of the acidic group on the sugar by a close approach of an amino or acidic group forcing its protonation and Tyr or Tyr-His acting as the Bronsted base and acid.