What are processively acting chitinases?

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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).