Characterization of a β-N-acetylhexosaminidase and a β-N-acetylglucosaminidase/β-glucosidase from Cellulomonas fimi

The Gram-positive soil bacterium Cellulomonas fimi is shown to produce at least two intracellular beta-N-acetylglucosaminidases, a family 20 beta-N-acetylhexosaminidase (Hex20), and a novel family 3-beta-N-acetylglucosaminidase/beta-glucosidase (Nag3), through screening of a genomic expression library, cloning of genes and analysis of their sequences. Nag3 exhibits broad substrate specificity for substituents at the C2 position of the glycone: k(cat)/K-m values at 25 degrees C were 0.066 s(-1).mM(-1) and 0.076 s(-1).mM(-1) for 4'-nitrophenyl beta-N-acetyl-D-glucosaminide and 4'-nitrophenyl beta-D-glucoside, respectively. The first glycosidase with this broad specificity to be described, Nag3, suggests an interesting evolutionary link between beta-N-acetylglucosaminidases and beta-glucosidases of family 3. Reaction by a double-displacement mechanism was confirmed for Nag3 through the identification of a glycosyl-enzyme species trapped with the slow substrate 2',4'-dinitrophenyl 2-deoxy-2-fluoro-beta-D-glucopyranoside. Hex20 requires the acetamido group at C2 of the substrate, being unable to cleave beta-glucosides, since its mechanism involves an oxazolinium ion intermediate. However, it is broad in its specificity for the D-glucosyl/D-galactosyl configuration of the glycone: K-m and k(cat) values were 53 mu M and 482.3 s(-1) for 4'-nitrophenyl beta-N-acetyl-D-glucosaminide and 66 mu M and 129.1 s(-1) for 4'-nitrophenyl beta-N-acetyl-D-galactosaminide.