This domain is found in a number of bacterial chitinases and similar viral proteins. It is organised into a fibronectin III module domain-like fold, comprising only beta strands. Its function is not known, but it may be involved in interaction with the enzyme substrate, chitin [1,2]. It is separated by a hinge region from the catalytic domain (Pfam:PF00704); this hinge region is probably mobile, allowing the N-terminal domain to have different relative positions in solution [1].

1: Crystal structure of a bacterial chitinase at 2.3 A resolution. Perrakis A, Tews I, Dauter Z, Oppenheim AB, Chet I, Wilson KS, Vorgias CE; Structure 1994;2:1169-1180. PMID:7704527

2: Evolution of immunoglobulin-like modules in chitinases: their structural flexibility and functional implications. Perrakis A, Ouzounis C, Wilson KS; Fold Des 1997;2:291-294. PMID:9377712