Fig. 2.

Structural determinants of chymotrypsin C-mediated trypsin degradation. (A) Ribbon diagram of human cationic trypsin [Protein Data Bank ID code 1TRN; chain B of the crystallographic dimer shown here (21)] with the Leu⁸¹ (red) and Arg¹²² (blue) side chains indicated. The calcium-binding loop is denoted by the Ca²⁺ symbol. The catalytic triad consisting of His⁶³ (His⁵⁷ in the conventional chymotrypsin numbering); Asp¹⁰⁷ (chymotrypsin no. Asp¹⁰²) and Ser²⁰⁰ (chymotrypsin no. Ser¹⁹⁵) are shown in green. Note that Asp¹⁰⁷ is located on the yellow peptide segment, which is released upon cleavage of the Leu⁸¹-Glu⁸² and Arg¹²²-Val¹²³ peptide bonds. See text for details. The image was rendered using DeepView/Swiss-PdbViewer version 3.7. (B) Primary structure of human cationic trypsin. Individual amino acids are represented by circles. The catalytic triad is highlighted in green, Leu⁸¹ is in red, and Arg¹²² is in blue. The five disulfide bridges and the interactions between the calcium ion and amino acids within the calcium binding loop are indicated. Note that both the Leu⁸¹-Glu⁸² and the Arg¹²²-Val¹²³ peptide bonds are located in a long peptide segment not stabilized by disulfide bonds. The yellow section corresponds to the yellow peptide in A.