Endo-polygalacturonase (EC 3.2.1.15, pectin depolymerase, pectolase, pectin hydrolase, and poly-α-1,4-galacturonide glycanohydrolase; systematic name (1→4)-α-D-galacturonan glycanohydrolase (endo-cleaving)) is an enzyme that hydrolyzes the α-1,4 glycosidic bonds between galacturonic acid residues: (1,4-α-D-galacturonosyl)n+m + H2O = (1,4-α-D-galacturonosyl)n + (1,4-α-D-galacturonosyl)m Polygalacturonan, whose major component is galacturonic acid, is a significant carbohydrate component of the pectin network that comprises plant cell walls. Therefore, the activity of the endogenous plant PGs works to soften and sweeten fruit during the ripening process. Similarly, phytopathogens use PGs as a means to weaken the pectin network, so that digestive enzymes can be excreted into the plant host to acquire nutrients. Structure This enzyme's multiple parallel β sheets form a helical shape that is called a β helix. This highly stable structure, thanks to numerous hydrogen bonds and disulfide bonds between strands, is a common characteristic of enzymes involved in the degradation of pectin. The interior of the β helix is hydrophobic. X-ray crystallography has been used to determine the three-dimensional structure of several PGs in different organisms. Fungal PGs from Colletotrichum lupini, Aspergillus aculeatus, and Aspergillus niger (PG1 and PG2) have been crystallized. The PGs from bacteria like Erwinia carotovora and Bacillus subtilis have also been crystallized. The active site of Fusarium moniliforme PG comprises six charged amino acid residues: H188, R267, and K269 are involved in substrate binding, D212 (a general acid) is responsible for proton donation to the glycosydic oxygen, and D213 and D191 activate H2O for a nucleophilic attack. Mechanism Polygalacturonase is a pectinase, an enzyme that degrades pectin by hydrolyzing the O-glycosyl bonds in pectin's polygalacturonan network, resulting in α-1,4-polygalacturonic residues. The rate of hydrolysis is dependent on polysaccharide chain length. Low rates of hydrolysis are associated with very short chains (e.g. digalacturonic acid) and very long chains. Exo- vs Endo-polygalacturonases Exo- and Endo-polygalacturonases utilize different hydrolytic modes of action. Endo-polygalacturonases hydrolyze in a random fashion along the polygalacturonan network. This method results in oligogalacturonides. Exo-polygalacturonases hydrolyze at the non-reducing end of the polymer, generating a monosaccharide galacturonic acid. Occasionally, organisms employ both methods. In addition to different modes of action, polygalacturonases polymorphism allows fungal polygalacturonases to more effectively degrade a wider range of plant tissues. PG variety in optimal pH, substrate specificity, and other factors are likely helpful for phytopathogenic organisms like fungi. Agricultural relevance Due to the applicability of this enzyme's activity on agricultural productivity and commercial success, much of the research on