br Acknowledgements br Introduction Microbial enzymes

Acknowledgements
Introduction Microbial enzymes have gained interest for their widespread uses in industries and medicine owing to their stability, catalytic activity, and ease of production and optimization than plant and animal enzymes (Singh et al., 2016). Microbial aminopeptidases (amino-acyl-peptide hydrolase, EC 3.4.11) are protease enzymes that hydrolyze N-terminus peptide bonds of proteins and polypeptides. They are considered as one of the earliest proteases discovered. These enzymes are seen extensively found in both prokaryotes and eukaryotes and a huge number of aminopeptidases of microbial origin have been characterized (Gonzales and Baudouy, 1996, Kunji et al., 1996, Christensen et al., 1999, Sanz, 2007, Barrett and Rawling, 2004, Nampoothiri et al., 2005). They are located in different sub cellular compartments including the cytoplasm, lysosomes and membranes, and can also be secreted into the extracellular medium. They catalyze the cleavage of single hydroxycarboxylic acid receptors from the amino terminus of proteins and thus have routinely been used for various purposes (Motyan et al., 2013).
Classification of aminopeptidases There are different systems of classification for aminopeptidases. The more general classification parameters include substrate specificity, cellular location, catalytic function, requirement for cofactors, and pH optimum (Taylor, 1993a, Taylor, 1993b). Aminopeptidases have been classified on the basis of their substrate specificity (broad or narrow), catalytic mechanism (metallo, cysteine, and serine peptidases), cellular location (membrane, cytosolic and microsomal) and molecular structure (Gonzales and Baudouy, 1996, Kunji et al., 1996, Christensen et al., 1999, Sanz, 2007, Barrett and Rawling, 2004).
Microbial aminopeptidases Aminopeptidase production from a variety of bacteria, fungi and actinomycetes has been reported by several workers. Aminopeptidase producers has been isolated from soil or spoiled food materials that contain protein rich complexes. A large number of aminopeptidases of microbial origin have been characterized (Gonzales and Baudouy, 1996, Jones, 1991, Kunji et al., 1996, Christensen et al., 1999, Sanz and Toldra, 2002, Sanz et al., 2002; Barret et al., 2004; Nampoothiri et al., 2005, Savijoki et al., 2006). Both constitutive and induced expression at different phases or temperatures of growth, have been demonstrated. It has also been shown that the expression of certain aminopeptidases from lactic acid bacteria is induced when the culture medium is enriched with peptides. Such observations agree with the presumptive role of these enzymes in nitrogen nutrition in these bacteria (Zheng et al., 2005). A few members of substrate specific microbial aminopeptidases and their producers are summarized in Table 1. Some of well-characterized aminopeptidases of microbial origin are bacterial leucyl aminopeptidase (LAP, EC 3.4.11.10), methionine aminopeptidase (MAP, EC 3.4.11.18), proline aminopeptidase, (EC 3.4.11.5), and Aminopeptidase P (PAP, EC 3.4.11.9), aminopeptidases from lactic acid bacteria such as PepC, PepA, PepY, PepN etc (Hossain et al., 2011). The aminopeptidase activities of microbes, mainly bacteria are localized in the cytoplasm, on membranes, associated with the cell envelope or secreted into the extracellular media. Studies on the bacterial aminopeptidase system have been carried out over the past three decades and are significant in fundamental and biotechnological domains. Several of microbial aminopeptidases have been purified and characterized biochemically over the past three decades. Many of the genes encoding these enzymes have also been cloned and characterized (Gonzales and Baudouy, 1996).
Improved enzyme characteristics
Future perspectives
Conclusions
Acknowledgements AN thank the Department of Science and Technology (DST), New Delhi, India for INSPIRE fellowship and we acknowledge grant from Council of Scientific and Industrial Research (CSIR – INDIA) network projects (CSIR NWP-006 and, CSC 130 (NaPAHA), for aminopeptidase research.