Catalyzes the first step in maceration and soft-rotting of plant tissue. [(1->4)-alpha-D-galacturonosyl methyl ester](n) + n H2O = [(1->4)-alpha-D-galacturonosyl](n) + n H(+) + n methanol Glycan metabolism; pectin degradation; 2-dehydro-3-deoxy-D-gluconate from pectin: step 1/5. Monomer. Upon overexpression also accumulates in the periplasm. Decreased growth on methylated oligogalacturides; double pemA-pemB deletions grow very poorly on methylated oligogalacturides. Belongs to the pectinesterase family.