What are chitin binding proteins/modules/domains?
There are different types of proteins or protein modules (also called protein domains) with the ability to bind to chitin, but without catalyzing chemical reactions. i.e. without enzymatic activity. Many chitinases contain one or several chitin binding modules, either or both N- and C-terminally to the catalytic domain. These chitin binding modules appear to increase the activity (and possibly processivity) of the enzymes on insoluble chitin substrates which are poor substrates to the catalytic domains alone. However, when working on soluble substrates such as partially acetylated chitosans, the presence of the binding module(s) tends to slow down the enzyme. Chitin binding modules are found in many different CBM families according to the CAZY classification of Carbohydrate Active enZYmes, including the distantly related families CBM5 and 12. Chitin binding proteins also occur alone, not attached to a catalytic domain, but their role is less clear. For one specific class (formerly CBM33, now AA10), lytic chitin monooxygenase activity has been shown. Another class of chitin binding proteins are chitin receptors and chitin specific lectins which have been described in different plants. There are also a surprising number of catalytically inactive 'chitinases' in the human genome, some of these proteins have been shown to bind to chitin but due to the mutation of an amino acid residue in the active site required for catalysis, they show no enzymatic activity. These chitin binding proteins have been implicated to be involved in a variety of physiological roles.