Glycoside Hydrolase Family 35

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Glycoside Hydrolase Family GH35
Clan	GH-A
Mechanism	retaining
Active site residues	known
CAZy DB link
http://www.cazy.org/GH35.html

Substrate specificities

The majority of glycoside hydrolases of GH35 are β-galactosidases (EC 3.2.1.23). GH35 enzymes have been isolated from microorganisms such as fungi, bacteria and yeasts, as well as higher organisms such as plants, animals, and human cells. These β-galactosidases catalyse the hydrolysis of terminal non-reducing β-D-galactose residues in, for example, lactose (1,4-O-β-D-galactopyranosyl-D-glucose), oligosaccharides, glycolipids, and glycoproteins. Various GH35 β-galactosidases demonstrate specificity towards β-1,3-, β-1,6- or β-1,4-galactosidic linkages [1, 2, 3], and are often most active under acidic conditions [4, 5, 6]. As with many other CAZy families [7, 8, 9], GH35 members tend to be represented by multi-gene families in plants [3, 10, 11, 12, 13]. Moreover, plant GH35 β-galactosidases have be divided into two classes: members of the first are capable of hydrolyzing pectic β-1,4-galactans, while those of the second can specifically cleave β-1,3- and β-1,6-galactosyl linkages of arabinogalactan proteins [14].

In addition to β-galactosidases, GH35 also contains a limited number of archeal exo-β-glucosaminidases (EC 3.2.1.165) [15, 16]. Such enzymes hydrolyze chitosan or chitosan oligosaccharides to remove successive D-glucosamine residues from non-reducing termini.

Kinetics and Mechanism

Beta-galactosidases of GH35 catalyze the hydrolysis of terminal β-galactosyl residues via a classical Koshland retaining mechanism, which leads to net retention of the β-anomeric configuration of the released galactose molecule. The stereochemistry of the reaction was first shown by NMR for the human β-galactosidase precursor [4] and has been subsequently confirmed by other investigators for microbial and plant enzymes [1, 5] .

Catalytic Residues

The catalytic residues for family 35 were first predicted on the basis of hydrophobic cluster analysis of proteins of similar protein fold [17]. Experimentally, the glutamic acid residue 268 was first identified as the catalytic nucleophile in human lysosomal β-galactosidase precursor using a slow substrate, 2,4-dinitrophenyl 2-deoxy-2-fluoro-β-D-galactopyranoside, which allowed trapping of a covalent glycosyl-enzyme intermediate and subsequent peptide mapping [18]. This approach was repeated for two bacterial β-galactosidases from Xanthomonas manihotis and Bacillus circulans [19]. The general acid/base residue was inferred to be Glu200 from structural studies of a Penicillium sp. β-galactosidase [20]. Recent structural studies (vide infra) revealed two different conformations of the general acid/base residue in the β-galactosidase of Trichoderma reesei [21].

Three-dimensional structures

The first 3D-structures of a GH35 enzyme, those of a β-galactosidase from Pencillium sp. (Psp-β-gal) in native (PDB 1tg7) and product-complexed (PDB 1xc6) forms, were reported in 2004 at 1.90 Å and 2.10 Å resolution, respectively [20]. The structure of a β-galactosidase from Bacteriodes thetaiotamicron (Btm-β-gal) was subsequently reported by the New York Structural GenomiX Research Consortium in 2008 at 2.15 Å resolution (PDB 3d3a). In 2010, an atomic (1.2 Å) resolution crystal structure of a Trichoderma reesei (Hypocrea jecorina) β-galactosidase (Tr-β-gal, PDB 3og2) was reported, together with complex structures with galactose, IPTG and PETG at 1.5, 1.75 and 1.4 Å resolutions, respectively (PDB codes 3ogr, 3ogs, and 3ogv, respectively) [21].

GH35 enzymes belong to Clan GH-A, and thus have an (α/β)₈ (TIM) barrel as the catalytic domain, in which two glutamic acid residues act as the general acid-base and nucleophilic catalysts. These residues are located in strands 4 and 7 of the barrel.

The comparison of the native structures of Psp-β-gal, Tr-β-gal and Btmβ-gal reveals two things (Figure 1): Firstly, Btm-β-gal consists of three distinct domains, whereas Psp-β-gal and Tr-β-gal consist of five and six domains, respectively. The second and third domains of Btm-β-gal are quite similar with the fourth and fifth domains of Psp-β-gal, and with the fifth and sixth domains of Tr-β-gal. Secondly, major structural differences between Psp-β-gal and Tr-β-gal are in the conformations of the loop regions. Although the crystal structures of Psp-β-gal and Tr-β-gal are similar, the interpretation of the structure of Tr-β-gal is somewhat different from that presented earlier for Psp-β-gal: Rojas et al. considered Psp-β-gal to be composed of five distinct structural domains. The overall structure is built around the first, TIM barrel, domain. Domain 2 is an all β-sheet domain containing an immunoglobulin-like subdomain, Domain 3 is based on a Greek-key β-sandwich, and Domains 4 and 5 are jelly rolls [20]. In contrast, Maksimainen et al. concluded the domain 2 includes two different domains and thus the Tr-β-gal and Psp-β-gal structures both form six similar domains [21].

The superimposition of the active sites of the GH35 β-galactosidases shows a remarkable similarity. In addition to the catalytic residues, the active sites of the GH35 β-galactosidases contain many identical residues (Figure 1B). Based on the galactose-bound crystallographic models of Psp-β-gal and Tr-β-gal, a single galactose molecule is bound to the active site of the GH35 enzyme in the chair conformation in the β-anomeric configuration.

Additionally, Maksimainen et al. have described conformational changes in two loop regions of the active site of Tr-β-gal, that implicates a conformational selection mechanism for the enzyme (Figure 2). Unlike the induced fit theory, which assumes that the initial interaction between a protein and its binding partner induces a conformational change in the protein through a stepwise process, the conformational selection theory is based on the assumption that the unbound protein exists as an ensemble of conformations in dynamic equilibrium. Interaction between a weakly populated, higher-energy conformation and a binding partner causes the equilibrium to move in favor of the selected conformation [22, 23]. This can be seen in the structures of Tr-β-gal: the open and closed conformation are both favorable in the native structure and the closed conformation becomes more favorable in the complex structures. Furthermore, The acid/base catalyst Glu200 has two different conformations in the IPTG and PETG complex structures that clearly affects the pK_a value of this residue and thus the catalytic mechanism of the enzyme [21].

Structure images

Figure 1. Comparison of the native structures of GH35 β-galactosidases. A. Global structures, B. Active sites. Psp-β-gal (PDB code 1tg7), Tr-β-gal (PDB code 3og2) and Btm-β-gal (PDB code 3d3a) are colored in green, brown and blue, respectively.

Figure 2. Illustration of the conformational selection mechanism observed in Tr-β-gal [21].

Family Firsts

First stereochemistry determination

Retaining stereochemical outcome for human β-galactosidase precursor by NMR [4]

First catalytic nucleophile identification

Human β-galactosidase precursor by 2-fluorogalactose labeling [18].

First general acid/base residue identification

Penicillium sp. β-galactosidase by structural identification [20].

First 3-D structure

Penicillium sp. β-galactosidase [20].

References

All Medline abstracts: PubMed