The hidden face of the human macrophage chitotriosidase: taking a new look at this enzyme based on the biochemical and phylogenomic analysis of its chitin binding domain

Carbohydrates recognition is a critical process involved in numerous aspects of the cell biology such as inflammation, innate immune responses and proliferation. Chitin is an homopolysaccharide composed of β-1,4-linked N-acetylglucosamine (GlcNAc) units that is an abundant structural component of various infectious organisms like protozoans, nematodes and fungi. As there is no endogenous chitin produced by mammals, this polymer appeared to be a strategic target for innate immune agents which is why various carbohydrate binding proteins, associated or not with catalytic domains, are synthetized by plants and animals and are known to play a crucial role in innate immunity. The macrophage chitotriosidase (HCHT) is one of the three active chitinases synthetized by humans and has triggered significant attention recently due to its association with various inflammatory disorders. HCHT belongs to the Glycosyl Hydrolase family 18 (GH18) and is known to be involved in innate immunity. Nevertheless, its precise physiological function remains unclear. As numerous GHs, HCHT is a modular protein composed of a catalytic domain (GH18) associated to a Carbohydrate Binding Module (CBM) which is essential to hydrolyse crystalline chitin. If the catalytic domain GH18 is highly common in other GHs from animals, plants, fungi, bacteria, archea and viruses, its CBM (named ChBD) is much less conserved which makes the association between these two domains particularly intriguing. This work aims to demystify HCHT’s physiological function. Firstly, using competitive inhibition assays, we have highlighted the ability of ChBD to interact with chitooligosaccharides (GlcNAc1-2-4-6) which suggests that ChBD can potentially act as a lectin domain. Secondly, to better understand the molecular basis for chitin recognition, we have used homology modelling to build, with high confidence, the 3D structure model of ChBD. Based on this model, a specific set of residues has been selected for alanine scan mutagenesis which has allowed us to define the minimum chitin binding interface of the protein. Thirdly, Phylogenomic studies were performed to analyse the evolutionary history of the isolated catalytic and ChBD domains and understand how these domains were combined. Based on all these results, we discuss a new way of looking at HCHT where its ChBD would be the key determinant that has guided the catalytic domain from a basic metabolic function to a critical component of innate immunity in human. Finally, we propose a mechanism that explains how this enzyme could act at the molecular level to defend us against chitin-containing pathogens.