## 3 Results and Discussion
A PDB file was generated from SWISS-MODEL, which was then, visualized using Chimera 1.8.1 [19]. The largest deviations from the porcine structure occurred in the loop regions, in particular residues 626-627 of the spPEP (black) compared to residues 632-636 of the porcine PEP (grey) and residues 567-578 of the spPEP (black) compared to residues 570-681 of the porcine structure. Other minor deviations were identified using the alignment assessment tool in Chimera; these are indicated in black, as well. The conserved catalytic residues (spPEP/porcine), Ser553/554, Asp642/641 and His681/680, align at the same location in both structures (Figure 3B). Despite having only 50% identity in amino acid sequence, the modeling suggests that the structure is highly conserved, questioning why and how the spPEP recognizes large and small substrates compared to other PEPs in this family, which, to date, have only been shown to recognize substrates less than 3 kD in size. ### Structure Analysis of spPEP Based on Amino Acid Sequence  
The amino acid sequence was submitted to homology modeling using SWISS-MODEL [17,18]. The modeling consisted of comparing the submitted sequence to the available structures in the PDB database. The structure for the porcine brain PEP (PDB: 2 XDW) with an inhibitor peptide bound was selected to be the most conserved sequence and structure despite being distantly removed from the spPEP phylogenetically. As shown in Figure 3, the spPEP aligned 99% to that of the porcine brain PEP. A PDB file was generated from SWISS-MODEL, which was then, visualized using Chimera 1.8.1 [19]. The largest deviations from the porcine structure occurred in the loop regions, in particular residues 626-627 of the spPEP (black) compared to residues 632-636 of the porcine PEP (grey) and residues 567-578 of the spPEP (black) compared to residues 570-681 of the porcine structure. Other minor deviations were identified using the alignment assessment tool in Chimera; these are indicated in black, as well. The conserved catalytic residues (spPEP/porcine), Ser553/554, Asp642/641 and His681/680, align at the same location in both structures (Figure 3B). Despite having only 50% identity in amino acid sequence, the modeling suggests that the structure is highly conserved, questioning why and how the spPEP recognizes large and small substrates compared to other PEPs in this family, which, to date, have only been shown to recognize substrates less than 3 kD in size. ### Structure Analysis of spPEP Based on Amino Acid Sequence  
The amino acid sequence was submitted to homology modeling using SWISS-MODEL [17,18]. The modeling consisted of comparing the submitted sequence to the available structures in the PDB database. The structure for the porcine brain PEP (PDB: 2 XDW) with an inhibitor peptide bound was selected to be the most conserved sequence and structure despite being distantly removed from the spPEP phylogenetically. As shown in Figure 3, the spPEP aligned 99% to that of the porcine brain PEP. A PDB file was generated from SWISS-MODEL, which was then, visualized using Chimera 1.8.1 [19]. The largest deviations from the porcine structure occurred in the loop regions, in particular residues 626-627 of the spPEP (black) compared to residues 632-636 of the porcine PEP (grey) and residues 567-578 of the spPEP (black) compared to residues 570-681 of the porcine structure. Other minor deviations were identified using the alignment assessment tool in Chimera; these are indicated in black, as well. The conserved catalytic residues (spPEP/porcine), Ser553/554, Asp642/641 and His681/680, align at the same location in both structures (Figure 3B). Despite having only 50% identity in amino acid sequence, the modeling suggests that the structure is highly conserved, questioning why and how the spPEP recognizes large and small substrates compared to other PEPs in this family, which, to date, have only been shown to recognize substrates less than 3 kD in size.