Identification of glutamic acid 105 at the active site of Bacillus amyloliquefaciens 1,3-1,4-beta-D-glucan 4-glucanohydrolase using epoxide-based inhibitors

Bacillus amyloliquefaciens 1,3-1,4-beta-D-glucan 4-glucanohydrolase (EC 3.2.1.73) was modified by the mechanism-based, affinity-labeling reagent [14C](3,4)-epoxybutyl beta-D-cellobioside. Following partial inactivation a completely inactivated enzyme preparation containing 1.1 mol of covalently bound inhibitor/mol of protein was obtained by chromatography on a cellulosic matrix. The inactivated enzyme was digested with endoproteinase Glu-C and radioactive peptides purified by reversed-phase high performance liquid chromatography (HPLC). The affinity label was esterified exclusively to the gamma-carboxylate of Glu105 in the sequence Gly-Thr-Pro-Trp-Asp-Glu-Ile-Asp-Ile-Glu109. The sequence motif Glu-(Ile/Leu)-Asp-Ile is found in many glucanases and xylanases and may therefore serve to identify the catalytic nucleophile in beta-glycanases, which otherwise exhibit a low degree of sequence identity. The esterification of Glu105 by the affinity label abolished endoproteinase Glu-C-mediated hydrolysis of the Glu-Ile106 peptide bond. Identification of phenylthiohydantoin-Glu105 during automated sequence analysis was not possible unless the affinity label was liberated by prior base hydrolysis. These observations formed the basis for the development of a highly sensitive approach for the identification of catalytic carboxylates in polysaccharide hydrolases employing non-radioactive inhibitors, comparative HPLC mapping, electrospray mass spectrometry, and Edman degradation.