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The members of family 16 are active on β-1,4 or β-1,3 glycosidic bonds in various glucans and galactans. A wide diversity of glycoside hydrolases active on plant and marine polysaccharides are found in GH16, including:
endo-xyloglucanases (EC 3.2.1.151, a.k.a. xyloglucan endo-hydrolases, XEHs, in plants [2]).
Notably, some members of GH16 are predominant transglycosylases. These include the plant xyloglucan:xyloglucosyltransferases (EC 2.4.1.207, a.k.a. xyloglucan endo-transglycosylases, XETs) [2] and yeast chitin/beta-glucan crosslinking enzymes Crh1 and Crh2 [3, 4, 5]. Some invertebrate GH16 proteins have lost their catalytic amino acids and are involved in immune response activation through the Toll pathway upon binding of β-1,3 glucan. The role of the GH16 domain in this immune response has not been fully elucidated [6].
Several of the activities observed for GH16 members are delineated into individual sequence-based subfamilies, while other polyspecific subfamilies capture a range of activities [7].
Diverse polysaccharides cleaved by GH16 enzymes
Agar β-D-Galp-(1,4)-α-L-3,6-anhydro-Galp-1,3-β-D-Galp-(1,4)-α-L-3,6-anhydro-Galp
Members of GH16 enzymes are retaining enzymes, as first shown by NMR [8] on an endo-1,3-1,4-β-D-glucan 4-glucanohydrolase from Bacillus licheniformis. As such, they utilize a covalent glycosyl-enzyme intermediate, which is broken-down by glycosyl transfer [9, 10] to water or a carbohydrate acceptor substrate in glycoside hydrolases or transglycosylases, respectively.
Catalytic Residues
The catalytic nucleophile of GH16 enzymes was first proposed using a non-specific epoxyalkyl β-glycoside inhibitor and identification of the site of covalent labelling using ESI-MS and Edman degradation on an endo-1,3-1,4-β-D-glucan 4-glucanohydrolase from Bacillus amyloliquefaciens [11]. This was subsequently verified by azide rescue of the E134A mutant of a Bacillus licheniformis 1,3-1,4-β-D-glucan 4-glucanohydrolase resulting in an α-glycosyl azide from the β-glycoside substrate [12]. The general acid/base residue was identified by making the E138A site-directed mutant of the Bacillus licheniformis 1,3-1,4-β-D-glucan 4-glucanohydrolase together with kinetic analysis and azide rescue, which resulted in a β-glycosyl azide product [12]. These structurally conserved catalytic residues have been confirmed in a number of other GH16 members, including plant XETs and XEHs [13, 14], and yeast Crh1 and Crh2 [5].
The mechanistic analysis of bacterial mixed-linkage endo-glucanases has been expertly reviewed in the broader context of GH16 [15].
Three-dimensional structures
Proteins in GH16 share a β-jelly-roll fold in which two β-sheets align in a curved, sandwich-like manner and present a cleft-shaped active-site bounded by loops extending from the β-strands. The first solved 3D structure was a hybrid protein of licheninase M from Paenibacillus macerans and BglA from Bacillus amyloliquefaciens (PDB 1byh) in 1992 [16]. Many three-dimensional structures have been solved of family 16 members of archeal, bacterial, and eukaryotic origin (see http://www.cazy.org/GH16_structure.html for an updated list). Of these, the first eukaryotic 3D structure was the xyloglucan endo-transglycosylase PttXET16-34 from Populus tremula×tremuloides (PDB 1umz) [17] and the first archeal 3D structure was a endo-1,3-β-glucanase Lam16 from Pyrococcus furiosus (PDB 2vy0) [18].
The structural diversity of GH16 members across sequence-related subfamilies has been reviewed in detail [7].
Evolution of GH16
Figure 1. Proposed evolution of GH16 (click to enlarge).
GH16 is a member of clan GH-B together with GH7; both families share the β-jellyroll fold. The different specificities of GH16 are proposed to have evolved from an ancestral β-1,3-glucanase [19]. This proposal was first elaborated using a structure-based phylogeny approach, which suggested that an early branching event lead to the evolution of the bacterial κ-carrageenases and the β-agarases, while a later branching event lead to the bacterial licheninases and the plant XETs [20] (Figure 1). GH16 has more recently been divided into subfamilies within CAZy, the corresponding phylogenetic analysis of which supports this overall evolutionary trajectory [7].
Particularly notable, the GH16 active-site residues are located in-train on one beta-strand at the center of the substrate binding cleft. Depending upon the phylogenetic clade, this beta-strand features one of two topologies. The beta-bulge motif, which has the consensus sequence EXDXXE, is more frequent in GH16 compared to the regular beta-strand with the consensus sequence EXDXE (the catalytic nucleophile is the first glutamate and the catalytic acid/base is the second, with a proposed "helper" asparate in-between [15]). Due to the predominance of the beta-bulge motif and its presence as the only motif in GH7, Michel et al. proposed that the beta-bulge is the ancestral motif, which subsequently gave rise to the regular beta-strand of extant plant XETs and bacterial licheninases [20].
Within plant lineages, similar structure-based phylogenetic approaches have suggested that XEHs evolved subsequently to XEHs within the xyloglucan endo-transglycosylase/hydrolase (XTH) gene family [2, 21]. The identification of a group of bifunctional GH16 glycoside hydrolases, which is active on both mixed-linkage beta-glucan and xyloglucan, provides additional support for the close evolutionary relationship of XETs and licheninases [22, 23, 24].
Family firsts
First stereochemistry determination
Bacillus licheniformis 1,3-1,4-β-D-glucan 4-glucanohydrolase by NMR [8].
Suggested in Bacillus amyloliquefaciens 1,3-1,4-β-D-glucan 4-glucanohydrolase via non-specific epoxyalkyl β-glycoside labeling [11]. Later verified by azide rescue of inactivated mutants [12].
Bacillus licheniformis 1,3-1,4-β-D-glucan 4-glucanohydrolase, first suggested by sequence homology and mutational studies [25]. This was later verified by azide rescue of inactivated mutants [12].
First 3-D structure
A hybrid licheninase (Bacillus amyloliquefaciens and Paenibacillus macerans) by X-ray crystallography (PDB 1byh) [16].
Agar
κ-carrageenan
Porphyran
β-1,3-glucan
Keratan sulphate
β-1,3-galactan
β-1,4/1,3-glucan
β-1,4-galactan
Xyloglucan
