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The '''precursor of ASP''' is composed of 624 amino acids. It contains a signal peptide of 24 amino acids, a catalytic domain, similar to that of subtilisin, and a P domain.

The '''maturation of ASP''' is achieved by [http://figshare.com/articles/Close-up_view_of_the_interaction_site_of_ASP_S336A_with_ORF2_/5496337/1 ORF2]. This protein plays the role of an external chaperone and is necessary for the construction of the stable ASP. Indeed, ASP doesn’t contain any propeptide that is involved in the proper folding of the protein. This is a major difference with an other protein, close to ASP : Kex2 ([[1r64]]) <ref>Structural Basis for Action of the External Chaperone for a Propeptide-deficient Serine Protease from Aeromonas sobria. Kobayashi H et al. Biol. Chem. 290(17):11130-43 (2015)</ref>

The '''ORF2 protein''' is composed of 152 amino-acids coded by the orf2 gene of 456 base pairs. The N-terminal extension and the C-terminal tail of the protein are implicated in the maturation of ASP. In fact, a complex ASP-ORF2 is formed. <ref>PMID:17951986</ref> This association requires a specific organization of ASP in the space. The <scene name='82/829344/The_p-domain/2'>P-domain</scene> of ASP doesn’t bind to ORF2 but the sixth residue from the C-terminus domain of ORF2 interacts with the non-mature ASP. In the complex, the active site of ASP is blocked. This protects the protein from degradation by others.

The '''structure of ASP''' is very similar to that of Kex2 ([[1r64]]), but it has a unique extra-occluding region close to its active site within the subtilisin domains. This unique extra-occluding region could serve as an useful target site to facilitate the development of new antisepsis drugs.

 

Moreover, we can find three <scene name='82/829344/Calcium_binding_sites/2'>Ca2+ Binding Sites</scene> in the ASP Structure (Ca1, Ca2 and Ca3). <scene name='82/829344/Ca1_et_ca2/4'>Ca1 and Ca2</scene> are situated in the N-terminal domain, while <scene name='82/829344/Ca3/3'>Ca3</scene> is situated in the C-terminal domain. These findings were assigned to ASP based on electron density, counter charges, and coordination. But contrary to Kex2 ([[1r64]]), ASP doesn't contain any Ca2+ binding sites near its catalytic site.

 

A schematic representation of the domains of the protein can be observed : [http://www.degruyter.com/viewimg/j/bchm.2017.398.issue-10/hsz-2016-0344/hsz-2016-0344.xml?img=graphic/j_hsz-2016-0344_fig_001.jpg '''secondary structure of ASP''']. We can see that Kex2 has the propeptide (in yellow) that is absent in ASP. The occluding subdomains in the C-terminal region of ASP are shown in dark blue.

'''The Subtilisin Domain:''' It contains ten helices (alpha 1 to 10) and twelve chains (beta 1 to 10 and béta 13 to 14). The N-terminal domain of ASP seems to be like the catalytic domain of Kex2 ([[1r64]]), which is similar to those of subtilisin and other subtilisin-related proteases. This ASP catalytic site contains <scene name='82/829344/Catalytic_triad/2'>the catalytic triad</scene> Asp78, His115, and Ser336 residues characteristic of subtilisins. In addition, four loops (L) protrude from the N-terminal subtilisin domain of ASP : Gly3– Pro26 (<scene name='82/829344/L1/2'>L1</scene>), Asn221–Phe241 (<scene name='82/829344/L2/2'>L2</scene>), Gly300–Cys326 (<scene name='82/829344/L3/2'>L3</scene>), and Gln-377–Glu-397 (<scene name='82/829344/L4/2'>L4</scene>). L1, L2, and L3 have random coil structure, whereas L4 forms a hairpin that protrudes toward the P-domain. Moreover, two <scene name='82/829344/Disulfide_bridges/2'>disulfide bridges</scene> are formed between Cys4 and Cys24 in L1 and between Cys301 and Cys326 in L3, which stabilize those loops.

'''The P-domain:''' The core of the P-domain in ASP contains eight beta-strands (beta 16 18 23 and 26). The <scene name='82/829344/Extra_occluding_region/2'>extra occluding-region</scene> is comprised of two parts, <scene name='82/829344/Pl1/4'>pL1</scene> (Gly521–Thr525, beta 5, 6, and 12) and <scene name='82/829344/Pl2/4'>pL2</scene> (Gly557–Asn578,  béta 25), and it is situated close to <scene name='82/829344/Catalytic_triad/2'>the catalytic triad</scene> Asp78,His115,and Ser336.  

'''The catalytic triad:''' The [http://en.wikipedia.org/wiki/Catalytic_triad catalytic triad] of ASP is composed of '''Asp78''', '''His115''' and '''Ser336'''. These amino acids are the base is the active site of the protein, where the mode of action of the serine protease takes place. A peptide can be inserted in the space of the active site. There, the amino acids of <scene name='82/829344/Catalytic_triad/2'>the catalytic triad</scene> will interact together and the mechanism will lead to a cut in the polypeptide.  

 

This triad can be observed in a 2D representation of the protein : [http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2785694/figure/F3/ '''catalytic triad of ASP''']

 

A schematic representation of the mechanism with the involved amino acids can be found under the following link : [http://fr.wikipedia.org/wiki/Fichier:Serine_protease_mechanism_by_snellios.png '''mechanism of the reaction''' ]