Cloning and characterization of Thermotoga maritima β-glucuronidase

The putative β-glucuronidase from Thermotoga maritima, comprising 563 amino acid residues conjugated with a Hisx6 tag, was cloned and expressed in Escherichia coli. The enzyme has a moderately broad specificity, hydrolysing a range of p-nitrophenyl glycoside substrates, but has greatest activity on p-nitrophenyl β-d-glucosiduronic acid (kcat = 68 s−1, kcat/KM =  4.5 × 105 M−1 s−1). The enzyme also shows a relatively broad pH-dependence with activity from pH 4.5 to 7.5 and a maximum at pH 6.5. As expected the enzyme is stable towards heat denaturation, with a half life of 3 h at 85 °C, in contrast to the mesophilic E. coli enzyme, which has a half life of 2.6 h at 50 °C. The identity of the catalytic nucleophile was confirmed as Glu476 within the sequence VTEFGAD by trapping the glycosyl–enzyme intermediate using the mechanism-based inactivator, 2-deoxy-2-fluoro-β-d-glucosyluronic acid fluoride and identifying the labeled peptide in peptic digests by HPLC–MS/MS methodologies. Consistent with this, the Glu476Ala mutant was shown to be hydrolytically inactive. The acid/base catalyst was confirmed as Glu383 by generation and kinetic analysis of enzyme mutants modified at that position, Glu383Ala and Glu383Gln. The demonstration of activity rescue by azide is consistent with the proposed role for this residue. This enzyme therefore appears suitable for use in enzymatic oligosaccharide synthesis in either the transglycosylation mode or by use of glycosynthase and thioglycoligase approaches.

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