Alpha-glucuronidase involved in the hydrolysis of xylan, a major structural heterogeneous polysaccharide found in plant biomass representing the second most abundant polysaccharide in the biosphere, after cellulose. Releases 4-O-methylglucuronic acid from xylan. an alpha-D-glucuronoside + H2O = an alcohol + D-glucuronate Optimum pH is 4.5-6.0. Optimum temperature is 70 degrees Celsius. By xylan and xylose but not by glucuronic acid. Belongs to the glycosyl hydrolase 67 family.