Identification of two functionally different classes of exocellulases.
X-RAY CRYSTALLOGRAPHY (1.9 ANGSTROMS) OF 47-651
Structure and mechanism of endo/exocellulase E4 from Thermomonospora fusca.
Roles of the catalytic domain and two cellulose binding domains of Thermomonospora fusca E4 in cellulose hydrolysis.
Kinetic studies of Thermobifida fusca Cel9A active site mutant enzymes.
This enzyme belongs to the glycosidase family-9. Although this enzyme binds two calcium ions, they are not involved in catalysis at all. According to the literature [2], this enzyme hydrolyzes cellulose with inversion of configuration at the anomeric carbon. The reaction mechanism of this enzyme requires a general acid to protonate the glycosidic oxygen and a general base to extract a proton from a nucleophilic water which attacks the anomeric carbon. The catalytic mechanism seems to be similar to that of beta-amylase (D00166 in EzCatDB). The reaction proceeds as follows (see [2] & [4]): (0) Tyr206 modulates the pKa of Asp55, whereas His125 modulates the pKa of Asp58. (1) Glu424 acts as a general acid to protonate the leaving group, the glycosidic oxygen, forming an oxocarbonium ion in the transition state. (SN1-like reaction) (2) Both Asp55 and Asp58 act as general bases to deprotonate the nucleophilic water. (Here, Asp58 seems more likely to be the base.) (3) The activated water makes a nucleophilic attack on the anomeric carbon to complete the reaction.