The Dual Carboxymethyl Cellulase and Gelatinase Activities of a Newly Isolated Protein from Brevibacillus agri ST15c10 Confer Reciprocal Regulations in Substrate Utilization

A protein showing endoglucanase-peptidase activity was prepared from a newly isolated bacterium (ST15c10). We identified ST15c10 as Brevibacillus agri based on electron-microscopic images and its 16S-rDNA sequence (GenBank accession No. HM446043), which exhibits 98.9% sequence identity to B. agri (KZ17)/B. formosus (DSM-9885T)/B. brevis. The enzyme was purified to homogeneity and gave a single peak during high-performance liquid chromatography on a Seralose 6B-150 gel-matrix/C-18 column. MALDI-TOF mass-spectrometry and bioinformatics studies revealed significant similarity to M42-aminopeptidases/endoglucanases of the CelM family. These enzymes are found in all Brevibacillus strains for which the genome sequence is known. ST15c10 grows optimally on carboxymethyl cellulose (CMC)-gelatin (40°C/pH 8-9), and also shows strong growth/carboxymethyl cellulase (CMCase) activity in submerged bagasse fermentation. The purified enzyme also functions as endoglucanase with solid bagasse/rice straw. Its CMCase activity (optimal at pH 5.6 and 60°C/K_m = 35.5 µM/V_max = 1,024U) was visualized by zymography on a CMC-polyacrylamide gel, which provided a strong band of approximately 70 kDa. The purified enzyme also showed strong peptidase (gelatinase) activity (pH 7.2/40°C during zymography on 6-12% gelatin/1% gelatin-PAGE (at approx. 70 kDa). The CMCase activity is inhibited by the metal ions Mn/Cu/Fe/Co (50%), Hg/KMnO₄ (100%), and by glucose or lactose (50-75%; all at 1 mM). The observed dose/time-dependent inhibition by Hg ions could be prevented with 2-mercaptoethanol. A comparison of the B. agri endoglucanase-aminopeptidase (ELK43520; 350 aa) with other members of the M42-family revealed the conservation of active-site residues Cys256/Cys260, which were previously identified as metal-binding sites. Regulation of the endoglucanase activity probably occurs via metal binding-triggered changes in the redox state of the enzyme. Studies on this type of enzyme are of high importance for basic scientific and industrial research.
J Mol Microbiol Biotechnol 2017;27:319-331

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