The glycosyl hydrolases family 18 (GH18) [E1] is widely distributed in all kingdoms, including viruses, bacteria, plants, fungi and animals. The GH18 family contains hydrolytic enzymes with chitinase or endo-N-acetyl-β-D-glucosaminidase (ENGase) activity as well as chitinase like lectins (chi-lectins/proteins (CLPs). Chitinases (EC 3.2.1.14) are hydrolytic enzymes that cleave the β-1,4-bond releasing oligomeric, dimeric (chitobiose) or monomeric (N-actetylglucosamine, GlcNAc) products. ENGases (EC 3.2.1.96) hydrolyze the β-1,4 linkage in the chitobiose core of N-linked glycans from glycoproteins leaving one GlcNAc residue on the substrate. CLPs do not display chitinase activity but some of them have been reported to have specific functions and carbohydrate binding property. The catalytic domain of GH18s may be connected to one or several substrate binding modules (CBMs), which enhance binding of enzymes to insoluble substrates. Certain GH18s also contain peptide signals for localization such as an N-terminal secretion peptide, a C-terminal glycosyl-phosphatidylinositol (GPI) anchor signal for attachment to the plasma-membrane, or N- or O-linked glycosylation sites for oligosaccharide modifications [1,2,3,4,5,6,7,8,9].

The catalytic domain of GH18s has a common (β/α)8 triosephosphate isomerase (TIM)-barrel structure, which consists of a barrel-like framework made from eight internal parallel β-strands that are alternately connected by eight exterior α helices (see <PDB:4LGX>). The active site motif DxxDxDxE is essential for the activity of the GH18 catalytic domain. The Glu (E) in this motif acts as the catalytic proton donor, and the last Asp (D(3)) is supposed to contribute to the stabilization of the essential distortion of the substrate [3,4,5,6,7].

We used a region centered around the active site motif as a signature pattern and we have also developed a profile that covers the entire GH18 catalytic domain.