What are endo- and exo-acting chitinases?
Endo-acting enzymes randomly attack (suitable) glycosidic linkages somewhere in the chitin or chitosan chain, while exo-acting enzymes attack the molecules from their ends, typically hydrolyzing the penultimate bond starting at the reducing or the non-reducing end. Endo-acting enzymes reduce the degree of polymerization of their polymeric substrates quickly and, thus, reduce the viscosity of polymer solutions quickly. Oligomeric products do not appear quickly but only over time, first larger oligomers, later smaller ones. In contrast, exo-acting enzymes do not quickly reduce the viscosity of polymer solutions but produce small oligomers, typically only dimers, from the start. Thus, endo-acting enzymes would be preferred by organisms wanting to break a chitinous barrier such as a fungal cell wall or an insect carapax, while exo-acting enzymes would be preferred by organisms wanting to use a chitinous substrate as a nutrient. Processively acting endo-enzymes combine both aspects, quickly breaking the polymeric structure and also quickly providing small oligomers. Eventually, all of these enzymes tend to degrade their polymeric substrates into dimers and trimers which are then further degraded to monomers by glycosidases.