Structure of the β‐glucosidase gene bglA of Clostridium thermocellum Sequence analysis reveals a superfamily of cellulases and β‐glycosidases including human lactase/phlorizin hydrolase

The nucleotide sequence of the Clostridium thermocellum gene bglA, coding for the thermostable β‐glucosidase A, has been determined. The coding region of 1344 bp was identified by comparison with the N‐terminal amino acid squence of recombinant β‐glucosidase A purified from Escherichia coli. The deduced amino acid sequence corresponds to a protein of 51482 Da. The coding region is flanked by putative promoter and transcription terminator sequences. The protein is unrelated to β‐glucosidase B of C. thermocellum, but has a high level of similarity with other bacterial β‐glucosidases and phospho‐β‐glucosidases. Similarity is also observed with the β‐galactosidase of the archaebacterium Sulfolobus solfataricus. Unexpectedly, it was found that human lactasephlorizin hydrolase contains three copies of a sequence closely related to C. thermocellumβ‐glucosidase A (up to 40% sequence identity). These diverse β‐glucosidases can therefore be grouped into an enzyme family (BGA) of common structural design. Sequence comparison by hydrophobic cluster analysis revealed that all BGA enzymes share a well conserved region which is homologous to the catalytic domain of the widely distributed cellulase family A. A distinctive feature of this domain is the sequence motif His – Ans‐Glu‐Pro in which the catalytic residues His and Glu are separated by 35–55 amino acid residues. The cellulase family A and the β‐glucosidase family BGA might thus be considered as members of a protein super‐family comprising β‐glucanases and β‐glycosidases from all three primary kingdoms of living organisms.