The hydrolysis of plant proteins ingested by animals, fungi and bacteria is catalysed by a variety of proteases. A major mechanism of plant defense involves elaboration of a variety of defensive protease inhibitor proteins. Such protease inhibitors can be either constitutive or inducible by wounding or pathogen infection. Proteases are classified as aspartic proteases, cysteine proteases, metalloproteases and serine proteases based on the critical involvement of aspartate, cysteine, divalent metal ions or serine, respectively, in the catalytic mechanism. Proteases are variously involved in digestion of ingested protein, blood clotting, protein processing, apoptosis, cell cycle regulation, protein turnover, inflammation and angiogenesis required for tissue remodelling and, pathologically, for tumour growth. Indeed proteases are involved in a variety of pathological states in man including cancer, inflammatory disease, Alzheimer`s disease and viral and protozoal infection. Accordingly plant protease inhibitor proteins are of interest not only for transgenic applications for plant crop defense against insects and fungal pathogens but also for potential pharmaceutical applications. A number of plant protease inhibitor proteins have been isolated that are specific for aspartic proteases and a variety of phytocystatins inhibit particular animal, plant and microbial cysteine proteases. A variety of plant peptides inhibit the metalloprotease angiotensin converting enzyme and several metallocarboxypeptidase inhibitor proteins have been isolated from plants and characterized. A large number of serine protease inhibitor proteins have been purified from plants and structurally characterized. The major classes of plant serine protease inhibitor proteins include the Bowman-Birk, Kunitz, potato inhibitor I, potato inhibitor II and squash families and the ragi/barley family of bifunctional α-amylase-and trypsin-inhibitory proteins. In addition some proteins homologous to the plant antifungal proteins of the defensin, napin, lipid transfer protein and thaumatin classes have been shown to be inhibitors of serine proteases.
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