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Polygalacturonase
Polygalacturonase (), also known as pectin depolymerase, PG, ''pectolase'', ''pectin hydrolase'', and ''poly-alpha-1,4-galacturonide glycanohydrolase'', is an enzyme that hydrolyzes the alpha-1,4 glycosidic bonds between galacturonic acid residues. Polygalacturonan, whose major component is galacturonic acid,〔(【引用サイトリンク】url=http://www.lexic.us/definition-of/polygalacturonan )〕 is a significant carbohydrate component of the pectin network that comprises plant cell walls.〔Jones, T. M., Anderson, A. J., and Albersheim, P. (1972) Hostpathogen interactions IV, Studies on the polysaccharide-degrading enzymes secreted by Fusarium oxysporum f. sp. lycopersici, Physiol. Plant Pathol. 2, 153-166.〕 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 beta sheets form a helical shape that is called a beta 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 beta 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'',〔D. Bonivento, D. Pontiggia, A.D. Matteo, J. Fernandez-Recio, G. Salvi, D. Tsernoglou, F. Cervone, G.D. Lorenzo, L. Federic, “Crystal Structure of the Endopolygalacturonase from the Phytopathogenic Fungus ''Colletotrichum lupini'' and its Interaction with Polygalacturonase-Inhibiting Proteins”. Proteins, 70, 294, 2008〕 ''Aspergillus aculeatus'',〔 and ''Aspergillus niger'' (PG1〔G.Van Pouderoyen, H.J. Snijder, J.A. Benen, B.W. Dijkstra, “Structural Insights Into the Processivity of Endopolygalacturonase I from ''Aspergillus niger''”. Febs Lett., 554, 462, 2003〕 and PG2〔Y. Van Santen, J.A. Benen, K.H. Schroter, K.H. Kalk, S. Armand, J. Visser, B.W. Dijkstra, “1.68-A Crystal Structure of Endopolygalacturonase II from ''Aspergillus niger'' and Identification of 3 Active Site Residues by Site-Directed Mutagenesis”. J.Biol.Chem., 274, 30474, 1999〕) have been crystallized. The PGs from bacteria like ''Erwinia carotovora''〔R. Pickersgill, D. Smith, K. Worboys, J. Jenkins, “Crystal Structure of Polygalacturonase from ''Erwinia carotovora'' Ssp. carotovora”. J. Biol. Chem., 273, 24660, 1998〕 and ''Bacillus subtilis''〔 have also been crystallized. Because of the significant role PGs play in agriculture and industry, computational molecular modeling has been applied to generate theoretical structures of other important PGs.〔http://www.pg-pgip.info〕 The experimentally determined crystal structures are used as templates for the computational threading process.
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.〔
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