FIGURE 5.

Structural modeling of CTRC-substrate interactions. A, trypsinogen Ca²⁺-binding loop substrate positioning relative to nearby CTRC residues illustrates H-bonds formed between enzyme and substrate as black dotted lines. None of substrate acidic residues Glu⁷⁹, Glu⁸², or Glu⁸⁵ lie in close enough proximity to the CTRC basic side chains to form direct salt bridges. B, a similar view showing a model of trypsinogen activation peptide cleavage sequence bound to CTRC. This shorter alternative substrate forms one fewer non-primed side hydrogen bond with the enzyme, and lacks the hydrophobic stabilization that would be provided by a P₄ substrate residue. However, a direct salt bridge is formed between P₂′ residue Asp²⁰ and CTRC Arg¹⁴³. C and D, minimal differences are present in models of trypsinogen activation peptide (C) and p.A16V mutant (D) bound to CTRC. Where the Ala¹⁶ N-terminal amine is predicted to hydrogen bond with the CTRC Gly²¹⁶ carbonyl (C), rotation of the Val¹⁶ (magenta) eliminates this bond but the bulkier side chain forms additional hydrophobic contacts with the Pro¹⁷ ring and the β-carbon of Arg^217A (D).