Figure 18.

(A) Stereo view of the m-shaped loop of hepatitis C NS3 protease (PDB structure 4KTC). The loop (magenta) is stabilized by a third crest (circled in yellow), together with the H-bond network shown in the figure. (B) Stereo view of the m-shaped loop of chymotrypsin (PDB structure 4CHA). The loop (green) is stabilized by a disulfide bond in the rising stem (circled in magenta), together with H-bonds between backbone groups, and between Asp194 and the protonated N-terminal Ile16. (C) The S1 subpocket is continuously accessible in NS3 protease, consistent with the lower desolvation cost of the Cys/Thr P1 side chains of its cognate substrates. (D) Stereo view of the S1 subpocket of chymotrypsin, which is continuously accessible, consistent with lower desolvation cost of the aromatic P1 side chains of its cognate substrates.