A thermoacidophilic endoglucanase (CelB) from Alicyclobacillus acidocaldarius displays high sequence similarity to arabinofuranosidases belonging to family 51 of glycoside hydrolases

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Abstract
A 100‐kDa protein with endoglucanase activity was purified from Triton X‐100 extract of cells of the thermoacidophilic Gram‐positive bacterium Alicyclobacillus acidocaldarius. The enzyme exhibited activity towards carboxy methyl cellulose and oat spelt xylan with pH and temperature optima of 4 and 80 °C, respectively. Cloning and nucleotide sequence analysis of the corresponding gene (celB) revealed an ORF encoding a preprotein of 959 amino acids which is consistent with an extracellular localization. Purified recombinant CelB and a variant lacking the C‐terminal 203 amino acid residues (CelBtrunc) displayed similar enzymatic properties as the wild‐type protein. Analysis of product formation suggested an endo mode of action. Remarkable stability was observed at pH values between 1 and 7 and 60% of activity were retained after incubation for 1 h at 80 °C. CelB displayed homology to members of glycoside hydrolase family 51, being only the second entry with activity typical of an endoglucanase but lacking activity on p‐nitrophenyl‐α‐l‐arabinofuranoside (pNPAraf). Highest sequence similarity was found towards the other endoglucanase F from Fibrobacter succinogenes (EGF), forming a distinct group in the phylogenetic tree of this family. Analysis of the amino acid composition of the catalytic domains demonstrated that CelB contains fewer charged amino acids than its neutrophilic counterparts, which is in line with adaptation to low pH. Wild‐type and full‐length recombinant CelB were soluble only in Triton X‐100. In contrast, CelBtrunc was completely water soluble, suggesting a role of the C‐terminal region in cell association. This C‐terminal hydrophobic region displayed local sequence similarities to an α‐amylase from the same organism.

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