Structure of an endoglucanase from termite, Nasutitermes takasagoensis.

Contrary to conventional wisdom, it has been shown recently that termites do not necessarily depend on symbiotic bacteria to process cellulose. They secrete their own cellulases, mainly endo-beta-1,4-glucanase and beta-1,4-glucosidase. Here, the first structure of an endogenous endoglucanase from the higher termite Nasutitermes takasagoensis (NtEgl) is reported at 1.40 A resolution. NtEgl has the general folding of an (alpha/alpha)(6) barrel, which is a common folding pattern for glycosyl hydrolase family 9. Three-dimensional structural analysis shows that the conserved Glu412 is the catalytic acid/base residue and the conserved Asp54 or Asp57 is the base. The enzyme has a Ca(2+)-binding site near its substrate-binding cleft. Comparison between the structure of the Ca(2+)-free enzyme produced by reducing the pH of the soaked crystal from 5.6 (the pH of optimum enzyme activity) to 2.5 with that of the Ca(2+)-bound enzyme did not show significant differences in the locations of the C(alpha) atoms. The main differences are in the conformation of the residue side chains ligating the Ca(2+) ion. The overall structure of NtEgl at pH 6.5 is similar to that at pH 5.6. The major change observed was in the conformation of the side chain of the catalytic acid/base Glu412, which rotates from a hydrophobic cavity to a relatively hydrophilic environment. This side-chain displacement may decrease the enzyme activity at higher pH.

Determination of the structure of an endoglucanase from Aspergillus niger and its mode of inhibition by palladium chloride.

The fungus Aspergillus niger is a main source of industrial cellulase. beta-1,4-Endoglucanase is the major component of cellulase from A. niger. In spite of widespread applications, little is known about the structure of this enzyme. Here, the structure of beta-1,4-endoglucanase from A. niger (EglA) was determined at 2.1 A resolution. Although there is a low sequence identity between EglA and CelB2, another member of family 12, the three-dimensional structures of their core regions are quite similar. The structural differences are mostly found in the loop regions, where CelB2 has an extra beta-sheet (beta-sheet C) at the non-reducing end of the binding cleft of the native enzyme. Incubation of EglA with PdCl(2) irreversibly inhibits the EglA activity. Structural studies of the enzyme-palladium complex show that three Pd(2+) ions bind to each EglA molecule. One of the Pd(2+) ions forms a coordinate covalent bond with Met118 S(delta) and the nucleophilic Glu116 O(epsilon1) at the active site of the enzyme. The other two Pd(2+) ions bind on the surface of the protein. Binding of Pd(2+) ions to EglA does not change the general conformation of the backbone of the protein significantly. Based on this structural study, one can conclude that the palladium ion directly binds to and blocks the active site of EglA and thus inactivates the enzyme.