A multinuclear NMR study of the active site of an endoglucanase from a strain of Bacillus. Use of Trp residues as structural probes.

In the hydrolytic reaction catalyzed by an endoglucanase from a Bacillus strain (endoglucanase K), 2 of 12 Trp residues, Trp174 and Trp243, are responsible for binding of the substrate and/or for the catalysis (Kawaminami, S., Ozaki, K., Sumitomo, N., Hayashi, Y., Ito, S., Shimada, I., and Arata, Y. (1994) J. Biol. Chem. 269, 28752-28756). Here we report results of a stable isotope-aided NMR analysis of the active site of endoglucanase K, using Trp174 and Trp243 as structural probes. Hydrogen-deuterium exchange experiments performed for the NH protons of main and side chains of Trp residues revealed that Trp174 and Trp243 are located in the hydrophilic and hydrophobic microenvironments in the active site, respectively. We also carried out pH titration experiments for indole C2 proton resonances of Trp residues and measured the pH dependence of specific activities for wild-type endoglucanase K and its mutants in which Glu or Asp residues are replaced with their respective amide forms. On the basis of the results obtained from the present study, we conclude that (a) Glu130 and Asp191, which are in spatial proximity to Trp174 and Trp243 in the active site, play a crucial role in the enzymatic activity; (b) Glu130 and Asp191 interact with each other in the active site, leading to an increase in the pKa values to 5.5 for both amino acid residues; and (c) the pKa values of Glu130 and Asp191 would lead to an unusually narrow pH-activity profile of the endoglucanase K.