Aminopeptidases exist widely in prokaryotic and eukaryotic microbial species, which can selectively catalyze the cleavage of the N-terminal amino acid residues from peptides and proteins. APs are associated with many human diseases and play an important role in a wide range of biological processes. The research to elucidate the catalytic mechanisms of APs is significant for medicine and pharmacology. APs have great application in various fields because of their broad substrate specificity, strict enantioselectivity, and high thermal stability. Besides being applied to synthesize pharmaceutical compounds and acting as versatile chiral building blocks, they are mainly applied in 
the food industry. For instance, APs play an important role in debittering protein hydrolysate and increasing the content of free amino acids in protein hydrolysate. They are also used as additives in condiments, such as soy sauce and seasoning blends, to enhance the flavor and the nutritional value of foods. Based on the homology modeling and structure analysis of BSAP, we found that there was a protease-associated Gomisin-D domain in the non-catalytic region. The PA domain is about 150 amino acids long, containing two a-helices and seven b-sheets. Because this domain is found in several distinct Procyanidin-B1 classes of proteases, it was named PA domain. Besides the different protease families, this domain was also found in two classes of plant transmembrane proteins. However, the role of PA domain remains somewhat elusive. It has been speculated that PA domain was involved in protein-protein interaction. In M28 family, the PA domain was found in only a few APs and the role of PA domain in APs has not been reported yet. Herein we found that the PA domain showed much more flexibility than any other regions in BSAP by using molecular dynamics simulation. The structure of the hypothetical deleted form of BSAPDPA was modeled and analyzed using MDS, and it was predicted that deletion of this flexible domain can enhance the structure stability. In vivo, the enzymatic reaction is an important biological process that can serve a wide variety of purposes. Inappropriate activity of enzymes can be deleterious to the cell or the organism that produces them. Thus, many enzymes contain conserved domains that can serve as an auto-inhibitor, a substrate-targeting domain or a regulatory domain, allowing catalysis events to occur only in the correct subcellular compartment, at the correct time and with the correct substrates. However, in the industrial application of the enzymes, sometimes these regulatory domains were unnecessary or even detrimental to the catalytic process. In order to satisfy the industrial requirement, it was an efficient approach to reconstruct the enzymes via deletion of these redundant regions. It has been reported that truncation of the cellulose binding domain of glucanase improved its thermal stability. Recently, Xiangtao Liu et al reported that N-terminal truncation of a maleate cis-trans isomerase resulted in a highly active enzyme for the biocatalytic production of fumaric acid. The research about changing optimum pH of an invertase by N-terminal truncation was also reported.