Alpha-galacturonidase able to catalyze the hydrolysis of the chromogenic substrate p-nitrophenyl-alpha-D-galacturonic acid (pNPalphaGalUA), and of the probable natural substrate alpha-1,4-di-galacturonate (GalUA(2)). Can neither hydrolyze pNPbetaGalUA, nor the stereoisomeric pNPalphaGlcUA. Does not display alpha- or beta-glucosidase activity as it fails to hydrolyze melibiose, raffinose, lactose and the chromogenic analogs, pNPalphaGal and pNPbetaGal. Cannot use the following compounds as substrates: pNP-N-acetyl-alpha- and beta-D-galactosaminide, pNP-N-acetyl-alpha- and beta-D-glucosaminide, pNP-alpha-L- and beta-L-arabinopyranoside, pNP-alpha- and beta-D-glucuronide, pNP-alpha- and beta-D-glucopyranoside, pNP-alpha- and beta-D-glucopyranoside 6-phosphate, pNP-alpha-D-galactopyranoside 6-phosphate and oNP-beta-D-galactopyranoside 6-phosphate. [(1->4)-alpha-D-galacturonosyl](n) + H2O = [(1->4)-alpha-D-galacturonosyl](n-1) + alpha-D-galacturonate Binds 1 Mn(2+) ion per subunit. Although a Mg(2+) ion is seen in the structure, galacturonidase activity was shown using manganese (PubMed:23416295). kcat is 3.6 sec(-1) with pNPalphaGalUA as substrate (at about 37 degrees Celsius). Homotetramer. Belongs to the glycosyl hydrolase 4 family.