4-Nitrophenyl-α-D-glucuronide

A putative α-glucosidase belonging to glycosyl hydrolase family 4 of Thermotoga maritima (TM0752) was expressed in Escherichia coli and it was found that the recombinant protein (Agu4B) was a p-nitrophenyl α-D-glucuronopyranoside hydrolyzing α-glucuronidase, not α-glucosidase. It did not hydrolyze 4-O-methyl-D-glucuronoxylan or its fragment oligosaccharides. Agu4B was thermostable with an optimum temperature of 80°C. It strictly required Mn²⁺ and thiol compounds for its activity. The presence of NAD⁺ slightly activated the enzyme. The amino acid sequence of Agu4B showed higher identity with Agu4A (another α-glucuronidase of T. maritima, 61%) than with AglA (α-glucosidase of T. maritima, 48%).

The substrate specificity of a p-nitrophenyl α-D-glucopyranosyl-uronic acid-hydrolyzing enzyme (PNP-GAase) isolated from snail acetone powder has been investigated with various substrates, such as P-nitrophenyl α-D-glucopyranosyluronic acid (PNP-GA), 2-O-α-D-glucopyranosyluronic acid-D-xylose (GA-2X), 2-O-(4-O-methyl-α-D-glucopyranosyluronic acid)-D-xylose (MeGA-2X), and O-α-D-glucopyranosyluronic acid-α-D-glucopyranosiduronic acid (GA-GA). The K_m (mM) and V_max (µmol of glucuronic acid formed/mg of enzyme protein/min) toward these substrates were as follows; 0.13 and 3.21 for PNP-GA, 0.33 and 0.089 for GA-2X, 17.6 and 0.094 for MeGA-2X, and 0.36 and 0.015 for GA-GA, respectively. The results indicate that the PNP-GAase specifically hydrolyzed PNP-GA, however, the enzyme had broad substrate specificity.

In the first systematic study of the enzyme β-glucuronidase, Masamune showed that oxkidney preparations did not hydrolyse (-)-menthyl α-D-glucuronide, and this compound was also not hydrolysed by mouse-liver β-glucuronidase and by a snail preparation rich in β-glucuronidase and in other simple glycosidases. Nevertheless, although it is not so ubiquitous as the β-glucuronide residue, there is evidence for a naturally occurring α-conjugated glucuronic acid, for example in wheat straw. It has been inferred that uridine diphosphoglucuronic acid also has the α-configuration.