cellulase.htm

A model of termite cellulase presented to the science teachers at A&M Consolidated High School; February, 2003

These photos demonstrate the docking a model of cellulose (glucose tetramer= cellulobiose dimer) to the active site of termite cellulase. Mr. John Tollett (A&M Consolidated High School) assisted with the construction of the models.


ligands	native (empty) active site	complex

The Structure of an Endoglucanse from Termite, Nasutitermes takasagoensis.

Shahram Khademi, Linda A. Guarino, Hirofumi Watanabe, Gaku Tokuda, and Edgar F. Meyer
(2002) Acta Cryst., D58, 653-659.

Summary
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-b-1,4-glucanase and b-1,4-glucosidase.
Here, the first structure of an endogenous endoglucanase from the higher termite, Nasutitermes takasagoensis, (NtEgl) is reported at 1.55 Å resolution. NtEgl has the general folding of an (a/a)6 barrel, which is a common folding pattern for glycosyl hydrolases family 9.
The 3D structural analysis shows that conserved Glu 412 is the catalytic acid/base residue and conserved Asp 54 or Asp 57 is the base.
The enzyme has a Ca2+ binding site near its substrate binding cleft.
The comparison between the structure of Ca2+-free enzyme, produced by reducing the pH of the soaked crystal from 5.6 (the optimum pH for enzyme) to 2.5, with that of the Ca2+-bound enzyme did not show significant differences in locations of a-carbon atoms.
The main differences are in the conformation of the residues ligating the Ca2+ ion.
The overall structure of NtEgl at pH 6.5 is similar to that of pH 5.6.
The major change observed was in the conformation of the side chain of the catalytic acid/base Glu 412 which rotates from a hydrophobic cavity to a relatively hydrophilic environment.
This side chain displacement may decrease enzyme activity at higher pH.

Research was supported by the Robert A. Welch Foundation (A-328), the US National Science Foundation (DBI-0242318) and the State of Texas Advanced Technology Program (5170317-1999)