FIGURE 4.

Cationic trypsinogen Ca²⁺-binding loop targeted for cleavage by CTRC. A, the Ca²⁺-binding loop of human cationic trypsinogen contains multiple acidic residues (orange), with additional acidic residues located in nearby neighboring loops. The bound Ca²⁺ is shown in purple; the Leu⁸¹-Glu⁸² peptide bond represents the preferred cleavage site targeted by CTRC. Structure coordinates are from PDB code 2RA3 (55). B, the electrostatic surface potential calculated for cationic trypsin reveals a negative charge surrounding the site of bound Ca²⁺, indicated by the large black arrow. Molecular surfaces were generated using the Molecular Surface module of Schrodinger 2012 as described under “Experimental Procedures”; the electrostatic potential color ramp was set from a minimum of −0.35 to a maximum of 0.15. C, Ca²⁺-binding loop cleavage sequence of cationic trypsinogen is shown modeled into the active site cleft of CTRC. The hydrophobic side chains of Ile⁷⁸ and Leu⁸¹ occupy the complementary S₄ and S₁ subsites, respectively. Acidic residues are shown in orange; Glu⁷⁹ at the P₂ position and Glu⁸⁵ at the P₄′ position are near CTRC regions of positive surface potential.