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Genomics and Applied Biology
, 2012, Vol.3 No.2 8-21
http://gab.sophiapublisher.com
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PIs have broad-spectrum of biological activity, which
includes suppression of pathogenic nematodes
(Williamson and Hussey, 1996), inhibition of spore
germination and mycelium growth (Dunaevskii et al.,
1997) and hampering the growth of pathogenic fungi
(Joshi et al., 1998; Bhattacharjee and Prasad, 2005). In
addition, PIs also contribute to better and enhanced
nutritional quality of grains, as they are rich in
cysteine and lysine (Ryan, 1989, Bhattacharjee et al.,
2006). PIs are specific for each of four mechanistic
classes of proteolytic enzymes, classified as serine,
cysteine, aspartic and metallo-protease based on the
active amino acid in their reaction center. In general,
PIs are competitive inhibitors, bind to the active site
of the enzyme. For instance serine PIs bind to serine
proteases, which include trypsin, chymotrypsin,
elastase, subtilisin and thrombin. Serpins (serine
protease inhibitors or classified inhibitor family I4)
are the largest and most broadly distributed
superfamily of protease inhibitors (Rawlings et al.,
2004). Serpin-like genes have been identified in
animals, plants, bacteria, and some viruses (Gettins,
2002). Most serpins are irreversible inhibitors of
serine proteases of the chymotrypsin family, although
some have evolved to inhibit other types of serine
proteases, and a few are also able to inhibit cysteine
protease (Schick et al., 1998, McGowan et al., 2006,
Vercammen et al., 2006, Ong et al., 2007, Roberts and
Hejgaard, 2008). Furthermore, some serpins have the
ability to form complexes with very divergent
proteases (Huntington, 2006). Serpins are involved in
a number of fundamental biological processes, and a
role in the protection of storage tissue against insects
and pathogens has been proposed for plant serpins
(Dahl et al., 1996; Rasmussen et al., 1996).
1 Proteinase inhibitors in plants
Plants have developed defense systems to combat
various pathogens throughout their life cycle, from the
seed stage until senescence, and it is particularly
important to keep embryo freefrom infection. There
are several embryonic defense mechanisms detected,
including the production of plant lectins and
pathogen-relatedproteins, in response to pathogen or
insect attack (Ye et al., 2001; Guiderdoni et al., 2002).
Serine proteinase inhibitors are expressedin
developing seeds and are thought to play an important
rolein inhibiting trypsin andchymotrypsin of external
origin. Two major serine class of proteinase inhibitor
shave been studied extensivelyin plants: Kunitz
inhibitors and Bowman-Birk inhibitors (Ryan, 1990).
Proteinase inhibitors of high molecular weight with
low cystein content are termed as Kunitz type (Odani
and Ikeneka, 1973). Bowman-Birk inhibitors (BBIs)
are cystein rich proteins of about 8 kD to 16 kD with
disulfide bonds. Serine proteinase inhibitors are
universal and most studied class of proteinase
inhibitors in plant kingdom (Haq et al., 2004, Mello et
al., 2002). Although they are present in lower
concentration in vegetative tissues, are primarily
localized to storage tissues such as seeds and tubers
rich in storage proteins.
Plant cystatins or phytocystatins are the second most
studied class of inhibitors from plants, viz, cowpea,
potato, cabbage, ragweed, carrot, papaya, apple fruit,
avocado, chestnut, and Job’s tears. Seed cystatins have
been reported from wide range of crops including
sunflower, rice, wheat, maize, soybean, and sugarcane
(Kuroda et al., 2001; Yozaura et al., 2002; Connors et al.,
2002). Squash inhibitor, member of highly potent
canonical serine proteinase inhibitors with typical knottin
fold, was isolated and characterized (Chiche et al., 2004).
2 Families of proteinase inhibitors and their
distribution
A comprehensive system of classification has been
proposed for facilitating the exchange, storage and
retrieval of information about this group of proteins
(Rawlings et al., 2004). On the basis of
three-dimensional structures, 31 families are assigned
to 26 clans. The term “Clan” is to designate a single
evolutionary line of inhibitors defined by single type of
protein fold. Leo et al (2002) developed PLANT –PIs
database to facilitate retrieval of information on plant
protease inhibitors and related genes (Table 1).
Christeller and Laing (2005) identified eight families of
serine proteinase inhibitors (Table 2), which matched
previously identified eight families in MEROPS.
Soybean trypsin inhibitor was the first PI to be
isolated and characterized. Since then many PIs have
been characterized that are widely distributed
throughout the plant kingdom (Konarev et al., 2004).
Most of the plant PIs characterized are from
Gramineae, Poaceae, Leguminosae, Fabaceae, and
Solanaceae families (Brzin and Kidric, 1995). PIs are
usually found in storage organs, such as seeds and