2018). which they specifically recognize glycosylated IgG antibodies. Mechanism of action of GHs and glycosynthases GHs, which can be separated into 161 families based on amino acid sequence similarity, as in the Carbohydrate-Active Enzymes Database, (CAZy; www.cazy.org), are enzymes that cleave glycosidic bonds. GH family 18 (GH18) is predominantly composed of chitinases (EC and endo–N-acetylglucosaminidases (ENGases) (EC, the latter of which contains EndoS, EndoS2 and other IgG-active endoglycosidases. Several other ENGases reside in GH85, but are not the focus of this review. Chitinases break down chitin, a linear polymer of -1,4-linked-N-acetylglucosamine, while ENGases hydrolyze the chitobiose (GlcNAc2) core of (SmChiB) suggest that this reaction intermediate is a neutral oxazoline with an oxazolinium ion formed on the pathway toward the reaction products (Coines et al. 2018). A second carboxylate residue (D2: e.g., D233 in EndoS, D184 in EndoS2) assists the oxazoline intermediate through a hydrogen bond, orienting it and enhancing the nucleophilicity of the acetamido group that attacks the anomeric center (Williams et al. 2002). In the second step of the reaction, the same BF-168 general acid/base residue from the first step now deprotonates an incoming water. This water molecule attacks the anomeric carbon, breaking the oxazoline ring and regenerating the sugar hemiacetal product with overall retention of stereochemistry (Figure ?(Figure1c)1c) (van Aalten et al. 2001). Before the product is released, GlcNAc (?1) can often be found in a skew-boat conformation, suggesting that this is a normal part of the catalytic cycle (Hsieh et al. 2010; Malecki et al. 2013; Speciale et al. 2014; Fadel et al. 2015; Ranok et al. 2015; Itoh et al. 2016, Klontz et al. 2019). In addition, other conserved residues in the GH18 ENGases BF-168 contribute to stabilize the reaction intermediates (e.g., Q250 and Y252 in EndoS2), while Y70 and T138 stabilize the charge on D182 (D1), and D182 keeps D184-E235 protonated in EndoS2 (Figure ?(Figure1c)1c) (Synstad et al. 2004). If, during the second step of the reaction, a sugar molecule replaces the role of water, a glycosidic linkage is created (Figure ?(Figure1d).1d). In this case, the reaction is referred to as transglycosylation. The GlcNAc (+1) in the active site is referred to as the acceptor, while the incoming BF-168 sugar is the donor. Most ENGases IFRD2 are capable of performing transglycosylation in addition to hydrolysis; however, transglycosylation is usually very inefficient because the product remains an excellent substrate for hydrolysis. To get appreciable accumulation of transglycosylation product, a large excess of donor is usually required. Transglycosylation efficiency is determined by the ratio between transglycosylation and hydrolysis rates for the enzyme. Increasing transglycosylation or decreasing hydrolysis both serve to increase the amount of product produced. To circumvent the necessity for large excesses of donor, Mackenzie et al. (1998) introduced an alternative approach in which they mutated a catalytic residue (in their case, the nucleophile). Another key breakthrough in the field was the identification of (EndoCCN180H), exhibited transglycosylation activity. Similar to design strategies applied to EndoD (Fan et al. 2012) and EndoM (Umekawa et al. 2008), this mutation targets the residue responsible for assisting oxazoline complex formation. Here, as well, transglycosylation can be performed using high concentrations of SGP as a donor substrate (Manabe et al. 2018). Structural basis of glycan specificity by IgG processing enzymes EndoS, encoded by the gene, was first reported in 2001 from serotype M1 (Collin and Olsn 2001). EndoS2, encoded by the gene, was discovered over a decade later in a serotype M49 strain (Sj?gren et al..