What are processively acting chitinases?
Processively acting enzymes do not immediately release the products of the reaction they catalyze but rather catalyze consecutive reactions of the same type, using (one of) its product(s) as a substrate. Processively acting chitinases successively cleave glycosidic bonds in a chitin/chitosan substrate, typically releasing dimeric products in the process. Processivity can occur in both endo- and exo-acting enzymes. If a processive chitinase acts on a partially acetylated chitosan as a substrate, it may encounter binding sites that it cannot cleave because of its subsite specificities. As an example, a GH18 chitinase requires a GlcNAc unit at the (-1) subsite for cleavage. If after a first cleavage, it moves ahead two units to attempt a second cleavage, it may encounter a GlcN unit at its catalytically crucial (-1) subsite so that it cannot perform the hydrolysis. The enzyme may then release its substrate, or it may move on another two units, in which case it would be a processive enzyme with the ability for non-productive binding. Such an enzyme initially releases even-numbered oligomers when acting on chitosan (which later, upon release and renewed binding at a different position, may be further cleaved to yield smaller oligomers, including odd-numbered ones).