Human Chitotriosidase: Catalytic Domain or Carbohydrate Binding Module, Who’s Leading HCHT’s Biological Function

Abstract

Chitin is an important structural component of numerous fungal pathogens and parasitic nematodes. The human macrophage chitotriosidase (HCHT) is a chitinase that hydrolyses glycosidic bonds between the N-acetyl-D-glucosamine units of this biopolymer. HCHT belongs to the Glycoside Hydrolase (GH) superfamily and contains a well-characterized catalytic domain appended to a chitin-binding domain (ChBDCHIT1). Although its precise biological function remains unclear, HCHT has been described to be involved in innate immunity. In this study, the molecular basis for interaction with insoluble chitin as well as with soluble chito-oligosaccharides has been determined. The results suggest a new mechanism as a common binding mode for many Carbohydrate Binding Modules (CBMs). Furthermore, using a phylogenetic approach, we have analysed the modularity of HCHT and investigated the evolutionary paths of its catalytic and chitin binding domains. The phylogenetic analyses indicate that the ChBDCHIT1 domain dictates the biological function of HCHT and not its appended catalytic domain. This observation may also be a general feature of GHs. Altogether, our data have led us to postulate and discuss that HCHT acts as an immune catalyser.

Structure of the Cladosporium fulvum Avr4 effector in complex with (GlcNAc)6 reveals the ligand-binding mechanism and uncouples its intrinsic function from recognition by the Cf-4 resistance protein

“…Next to Avr4, the only information available on the ligand-binding mechanism of a CBM14 family member with a known structure is the ligand-free structure of ChBD of the human chitotriosidase CHIT1 (ChBD CHIT1 ) [ 12 , 13 ]. Cf Avr4 and ChBD CHIT1 share a similar overall fold but the residues that dictate the binding affinity of the two proteins for their ligand are different.…”

“…For instance, the residues in Cf Avr4 essential for binding (GlcNAc) 6 , Trp100 and Asp102, are replaced by cysteine (Cys462) and threonine (Thr464) in the ChBD CHIT1 structure ( S1D Fig ). In ChBD CHIT1 , residues Pro451, Leu454, and Trp465 that were identified as crucial for chitin binding [ 12 ] are conserved in Cf Avr4, aligning to Pro87, Leu90, and Tyr103, respectively. However, although Pro87 and Leu90 sit on the loop connecting the two β-sheets and point toward the ligand binding site suggesting that they will have a role in binding (GlcNAc) 6 , the P87A mutation does not have an effect on the binding thermodynamics of Cf Avr4, whereas Leu90 is not within binding distance to (GlcNAc) 6 , as the closest contact is 3.3Å between the terminal methyl of the sidechain of chain A and the C6 hydroxyl of GlcNAc-4 of chain B. Interestingly, ChBD CHIT1 binding assays showed that there was no thermodynamic effect on binding when increasing the degree of polymerization beyond the disaccharide of GlcNAc, whereas, Avr4 showed a greatly decreased K d and more negative Δ H upon an increase to the degree of polymerization of the oligosaccharide.…”

“…The predicted location of the ChBD in Pf Avr4 matches to that of Cf Avr4, which used NMR titration experiments to identify residues that may interact with chito-oligomers but was unable to resolve its three-dimensional structure [ 9 ]. Next to Pf Avr4, the only known structures of CBM14 members are those of tachycitin [ 10 ], a small antimicrobial protein from horseshow crab, Der p 23 [ 11 ], an allergen from the dust mite Dermatophagoides pteronyssinus , and the ChBD of the human chitotriosidase CHIT1 (ChBD CHIT1 ) [ 12 , 13 ]. All four structures share a common core containing two β-sheets in a distorted β-sandwich arrangement, a seemingly common fold among CBMs [ 14 ].…”

Expression of Chitotriosidase in Macrophages Modulates Atherosclerotic Plaque Formation in Hyperlipidemic Mice

“…However, studies have demonstrated that the DG42 gene in Xenopus and mouse models produces chitin-oligosaccharides as a precursor to HA chains (Meyer and Kreil, 1996;Semino et al, 1996). Also, due to the structural similarity of HA to its native substrate; chitin, the CHIT1 binding domain is capable of interactions with HA (Ujita et al, 2003;Crasson et al, 2017). It may be possible that chitin interactions with HA prevent its degradation, thereby promoting plaque stability.…”

NMR and Fluorescence Spectroscopies Reveal the Preorganized Binding Site in Family 14 Carbohydrate-Binding Module from Human Chitotriosidase

“…It has also been shown that removal or mutagenesis of its only tryptophan has a detrimental effect on the CBMs ability to bind chitin. 12,13 Moreover, CHIT1 with its CBM degrades chitin faster and much more efficiently than its isoform without the CBM. 14…”