Effects of the 90s loop mutations on interactions with TL-3. Comparisons of the TL-3 complexes of wild-type FIV protease (green) and 12X protease (yellow) reveals conformational differences at the P1/P1' position of the inhibitor. The mutation of residue 98 from Isoleucine to Proline and residue 99 from Glutamine to Valine in the 12X mutant protease allows the formation of packing contacts with the P1/P1' position of TL-3, causing the P1/P1' phenyl ring to shift toward the side chain of Proline 98 by 2.0Å and rotate by 21° about the χ1 torsion angle. These movements are facilitated by other mutations in the 90s loop and active site core (see Fig. 4).