|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,S00206,S00207,S00203,S00208,S00211,S00213,S00214,M00113,T00307,D00165,D00166,D00169,D00176,D00501,D00502,D00503,D00844,D00861,D00864,M00026,M00112,M00193,M00346,T00057,T00062,T00063,T00066,T00067|
|Protein name||Lichenase-2||licheninaselichenasebeta-(1->4)-D-glucan 4-glucanohydrolase1,31,4-beta-glucan endohydrolase1,31,4-beta-glucan 4-glucanohydrolase1,3-1,4-beta-D-glucan 4-glucanohydrolase|
|Synonyms||EC 22.214.171.124Lichenase IIEndo-beta-1,3-1,4 glucanase IIEndo-beta-1,3-1,4 glucanase II) ((1->3,1->4)-beta-glucanase isoenzyme EII|
|CAZy||GH17 (Glycoside Hydrolase Family)|
|Activity||Hydrolysis of (1->4)-beta-D-glucosidic linkages in beta-D-glucans containing (1->3)- and (1->4)-bonds.|
|References for Catalytic Mechanism|
|References||Sections||No. of steps in catalysis|
|Comments||crystallization, preliminary X-ray diffraction analysis (1.8 angstroms)|
|Journal||J Mol Biol|
|Authors||Chen L, Garrett TJ, Varghese JN, Fincher GB, Hoj PB|
|Title||Crystallization and preliminary X-ray analysis of (1,3)- and (1,3;1,4)-beta-D-glucanases from germinating barley.|
|Comments||catalytic amino acids, evolution|
|Journal||J Biol Chem|
|Authors||Chen L, Fincher GB, Hoj PB|
|Title||Evolution of polysaccharide hydrolase substrate specificity. Catalytic amino acids are conserved in barley 1,3-1,4- and 1,3-beta-glucanases.|
|Comments||X-ray crystallography (2.2 angstroms).|
|Journal||Proc Natl Acad Sci USA|
|Authors||Varghese JN, Garrett TPJ, Colman PM, Chen L, Hoej PB, Fincher GB|
|Title||Three-dimensional structures of two plant beta-glucan endohydrolases with distinct substrate specificities.|
|Journal||Biochim Biophys Acta|
|Authors||Chen L, Sadek M, Stone BA, Brownlee RT, Fincher GB, Hoj PB|
|Title||Stereochemical course of glucan hydrolysis by barley (1-->3)- and (1-->3, 1-->4)-beta-glucanases.|
|Journal||Curr Opin Struct Biol|
|Authors||White A, Rose DR|
|Title||Mechanism of catalysis by retaining beta-glycosyl hydrolases.|
|Comments||X-ray crystallography (2.0 angstroms).|
|Journal||J Biol Chem|
|Authors||Mueller JJ, Thomsen KK, Heinemann U|
|Title||Crystal structure of barley 1,3-1,4-beta-glucanase at 2.0-A resolution and comparison with Bacillus 1,3-1,4-beta-glucanase.|
|Journal||J Biol Chem|
|Authors||Hrmova M, Imai T, Rutten SJ, Fairweather JK, Pelosi L, Bulone V, Driguez H, Fincher GB|
|Title||Mutated varley (1,3)-beta-D-glucan endohydrolases synthesize crystalline (1,3)-beta-D-glucans.|
|This enzyme belongs to the family-17 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.|
Glu232 has been reported to be the nucleophilic residue of this enzyme . This paper  also suggested that Glu288 is likely to be the catalytic acid. However, another paper  suggested that Glu94 is most likely to be the catalytic acid/base. Comparing with other 4/7 superfamily enzymes, Glu288 is too distant from the nucleophile, Glu232. Thus, Glu94 must be the acid/base.
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 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 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 xylanase (E.C. 126.96.36.199) (D00479 in EzCatDB), Tyr170 might stabilize the leaving nucleophile, Glu232 in deglycosylation. On the other hand, Tyr170 might modulate the activity of the nucleophile, according to the data of the other homologous enzyme, beta-glucosidase (E.C. 188.8.131.52) (S00205 in EzCatDB).