|CATH domain||Related DB codes (homologues)|
|18.104.22.168 : TIM Barrel||S00202,S00210,S00748,S00906,S00907,S00911,S00912,S00915,M00134,M00160,D00479,S00204,S00205,S00207,S00203,S00208,S00209,S00211,S00213,S00214,M00113,T00307,D00165,D00166,D00169,D00176,D00501,D00502,D00503,D00844,D00861,D00864,M00026,M00112,M00193,M00346,T00057,T00062,T00063,T00066,T00067|
|Activity||Hydrolysis of terminal non-reducing beta-D- galactose residues in beta-D-galactosides.|
|Compound table: links to PDB-related databases & PoSSuM|
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|References for Catalytic Mechanism|
|References||Sections||No. of steps in catalysis|
|Authors||Jenkins J, Lo Leggio L, Harris G, Pickersgill R|
|Title||Beta-glucosidase, beta-galactosidase, family A cellulases, family F xylanases and two barley glycanases form a superfamily of enzymes with 8-fold beta/alpha architecture and with two conserved glutamates near the carboxy-terminal ends of beta-strands four and|
|Authors||Moracci M, Capalbo L, Ciaramella M, Rossi M|
|Title||Identification of two glutamic acid residues essential for catalysis in the beta-glycosidase from the thermoacidophilic archaeon Sulfolobus solfataricus.|
|Comments||X-ray crystallography (2.6 angstroms).|
|Journal||J Mol Biol|
|Authors||Aguilar CF, Sanderson I, Moracci M, Ciaramella M, Nucci R, Rossi M, Pearl LH|
|Title||Crystal structure of the beta-glycosidase from the hyperthermophilic archeon Sulfolobus solfataricus: resilience as a key factor in thermostability.|
|Journal||Curr Opin Struct Biol|
|Authors||White A, Rose DR|
|Title||Mechanism of catalysis by retaining beta-glycosyl hydrolases.|
|Authors||Chi YI, Martinez-Cruz LA, Jancarik J, Swanson RV, Robertson DE, Kim SH|
|Title||Crystal structure of the beta-glycosidase from the hyperthermophile Thermosphaera aggregans: insights into its activity and thermostability.|
|Authors||Corbett K, Fordham-Skelton AP, Gatehouse JA, Davis BG|
|Title||Tailoring the substrate specificity of the beta-glycosidase from the thermophilic archaeon Sulfolobus solfataricus.|
|This enzyme belongs to the family-1 of glycosidase enzymes, a member family of 4/7 superfamily, which has got the catalytic residues at the C-terminal ends of beta-4 and beta-7 on the (alpha/beta)8 barrel fold .|
The liteature , mentioned that the conserved glucamic acid residue at C-terminal end of beta-7 (Glu387 of 1gow) is the catalytic nucleophile, whilst another conserved residue at C-terminal end of beta-4 (Glu206) might function as a general acid in the first step, in which the covalent intermediate would be formed between the nucleophilic residue and the substrate, and as a general base for a nearby water in the second step, where the intermediate will be hydrolyzed by this water. However, the paper  suggested that a conserved histidine residue (His150) at beta-3 might play a role as the general base in the second step, by activating the water, although this residue is not conserved among all the families of 4/7-superfamily.
The literature  described general aspects of the catalytic mechanism of retaining beta-glycosyl hydrolases. Accoriding to the paper, the mechanism can be described as follows:
(1) Saccharide binds in a "twisted-boat" conformation.
(2) The beta-1,4 linkage is broken, leading to the formation of a transition state with a slight positive charge at the anomeric carbon, in a "half-chair" conformation, which develops a oxocarbenium-ion-like character.
(3) An approach of the ionic species to the catalytic nucleophile (Glu387 in 1gow) leads to the formation of a covalent intermediate of inverted alpha-configuration in a so-called chair conformation. The aglycon is released and a water molecule diffuses into the vicinity of the acidic residue as a general base.
(4) The covalent intermediate reactivates through an oxocarbenium-ion-like transition state. The general base (Glu206 or His150 in 1gow) abstracts a proton from the incoming water, which in turn carries out a nucleophilic attack on the C1 atom of the residual saccharide.
Moreover, comparing the structural data with that of family-10 enzyme, xylanase (E.C. 22.214.171.124) (D00479 in EzCatDB), Tyr322 (of 1gow) might stabilize the leaving nucleophile, Glu387 in deglycosylation. On the other hand, Tyr322 might modulate the activity of the nucleophile, according to the data of the other family enzyme, beta-glucosidase (E.C. 126.96.36.199) (S00205 in EzCatDB).