Plant Protease Inhibitors and their Interactions with Insect Gut Proteinases

Enzymes hydrolysing peptide bonds have some overlapping terms, these include, proteases, proteinases and peptidases (Barrett et al. 1998). The Nomenclature Committee of the International Union of Biochemistry and Molecular Biology (NC-IUBMB 1992) recommended peptidase as the general term for enzymes hydrolyzing peptide bonds, which is further divided into exopeptidases, which catalyse the cleavage of one or a few amino acids from N-/C-terminus, and endopeptidases, which cleave the internal peptide bonds of polypeptides. The term “protease” includes both exopeptidases and endopeptidases while “proteinase” designates only endopeptidases (Barrett et al. 1998). Proteolytic enzymes are extensively found in plants, animals and in microorganisms (Kenny 1999) with a major role involved in every aspect of their physiology and development. Proteases are highly specific to their substrate, and the specificity depends on the localization of the substrate and the proteolytic enzyme, and structural and chemical properties at the active site of the enzyme.Their mode of action varies among all families and groups of proteases. Some of them work individually, some work in cascades in cooperation with other proteases and some form complexes constituting an active proteolytic machine. In plants, various roles of proteolytic enzymes involves: removal of misfolded, modified, and/or mistargeted proteins; supply of amino acids during translation; maturation of zymogens and peptide hormones by partial cleavages; control of metabolism and homeostasis by altering the levels of key enzymes and regulatory proteins; and the cleavage of targeted signals from proteins prior to their final integration into organelles (Vierstra 1996). In insects, proteolysis allows digestion of wide range of food diet mediated by concerted action of several proteases and several of them such as trypsin, chymotrypsin, aminopeptidase, etc., have been characterized from a vast variety of insect pests till now (Anwar and Saleemuddin 2002; Sanatan et al. 2013; Akbar et al. 2017). The insect attack on plants triggers the production of a series of secondary metabolites; definsins, thionines, lectins, and protease inhibitors which altogether constitute the defensive armoury of plants (Buchmanan et al. 2002). Plant protease inhibitors are proteinacious in nature and inhibit insect gut proteases by binding tightly to the active site, forming an essentially irreversible complex. The inability to utilize ingested protein and to recycle digestive enzymes results in critical amino acid deficiency, which affects the growth, development and survival of the herbivore (Chougule et al. 2008). In this chapter, we aim to summarize the interactions between insect midgut proteases and the plant protease inhibitors induced as a result of insect attack