because they secrete PGs in their saliva that digest plant tissue. They employ both exo- and endo-PGs. Inhibition Phytopathogenic fungi expose plant cell walls to cell wall degrading enzymes (CWDEs) like PGs. In response, most plants have natural inhibitor proteins that slow the hydrolytic activity of PG. These inhibitors also prompt long chain oligogalacturonide accumulation in order to encourage a defense mechanism against the attack. The polygalacturonase inhibitor proteins (PGIPs) are leucine-rich repeat proteins that have been reported to demonstrate both non-competitive and competitive inhibition of PGs. The active site of PG interacts with a pocket containing multiple polar amino acids in Phaseolus vulgaris PGIP2. The inhibitor prevents substrate binding by occupying the active site, resulting in competitive inhibition. The crystal structures for PGIP and PGIP2 have been determined for the bean P. vulgaris. The charged and polar residues that interact with the PG active site have been identified in P. vulgaris as D131, S133, T155, D157, T180, and D203. Using PGIP2 as a template, the theoretical structures of other PGIPs have been determined for some other common crops. References External links PG & PGIP Structural Database Pectinase Database EC 3.2.1