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  • br Conflict of interest br Acknowledgements This work was su


    Conflict of interest
    Acknowledgements This work was supported by the Biotechnology Research Center, China Three Gorges University, Yichang, China (No. 2016KBC05), the National Natural Science Foundation of China (No. 31370600 and 31300508), the Zhejiang Science and Technology Major Program on Agricultural New Variety Breeding (No. 2016C02056-1), the National Nonprofit Institute Research Grant of CAF (No. RISF2014010 and CAFYBB2014QB014).
    Introduction For the fast decades, plant proteases are involved in many aspects of plant physiology and their development [1]. They play a pivotal role in various processes inside the plant system such as protein turnover, degradation of misfolded proteins, senescence and the proteasome pathway [2]. Cysteine proteases from plants are also involved in intra and extracellular processes such as development and ripening of fruits, nutritional reserve, degradation of storage protein in germinating seeds, activation of proenzymes and degradation of defective proteins. Proteases are also to some extent are responsible for the post-translational modification of proteins by limiting the proteolysis at highly specific sites [3]. A great diversity of cellular processes, photo inhibition in the chloroplast, defense mechanisms, programmed cell death and photo morphogenesis in the developing seedling is governed by proteases [4]. Proteases are thus involved in all aspects of the plant life Volasertib ranging from mobilization of storage proteins during seed germination to the initiation of cell death and senescence programs [3]. An earlier study on proteases also include from the latex of several plant families such as Asteraceae [5], Caricaceae [6], Moraceae [7], Asclepiadaceae [8], Apocynaceae [9] and Euphorbiaceae [10]. Most plant derived proteases have been classified as cysteine proteases and more rarely as aspartic proteases [11]. Proteolytic enzymes derived from plants are very attractive since they can be active over a wide range of temperature and pH [12]. A number of industrial processes involve the breakdown of proteins by proteases, some of which are extracted from plants. Cysteine proteases (Cp) are widely distributed among living organisms, found in both prokaryotes and eukaryotes (e.g. bacteria, parasites, plants, invertebrates and vertebrates) [13], [14]. The catalytic mechanism of these enzymes involves a cysteine group in the active site. Cp comprise a family of enzymes, consisting of papain and related plant proteases such as chymopapain, caricain, bromelain, actinidin, ficin, aleurain and the mammalian lysosomal cathepsins [15]. Most plant Cp belongs to the papain family, including those of Asclepiadaceae, the milkweed family [19]. High proteolytic activity has been reported in crude enzyme preparations of latex of different species of this family. Inhibition analysis, where specific protease inhibitors are employed to identify catalytic groups within the active centre of the protease, suggesting that these proteases belonged to the cysteine type [8]. Proteases like asclepain from Asclepias curassavica[8], A. syriaca [18], A.glaucescens[19], A.fruticosa [20], calotropin from Calotropis gigantea[21], procerain from Calotropis procera[22], araujain from Araujia hortorum[23], actinidin from Actinidia chinesis[24], funastrain from Funastrum clausum[25] and philibertain from Philibertia gilliessi[26] have been isolated and characterized. Based on the review, the intra molecular disulphide bridges are presumably responsible for the functional stability of Kunitz type protease inhibitors in the presence of physical and chemical denaturants such as temperature, pH and reducing agents [27]. Extreme pH conditions will alter the structure of the inhibitor such that they no longer bind with the enzymes or with their substrates. Under strong acidic or alkaline conditions, the proteinaceous inhibitors get denatured and as a consequence they lose their activity partially or completely [28].