The structure of the D158E mutant of caricain (previously known as pap
aya protease omega) in complex with E-64 has been determined at 2.0 An
gstrom resolution (overall R factor 19.3%). The structure reveals that
the substituted glutamate makes the same pattern of hydrogen bonds as
the aspartate in native caricain, This was not anticipated since in t
he native structure there is insufficient room to accommodate the glut
amate side chain. The glutamate is accommodated in the mutant by a loc
al expansion of the structure demonstrating that small structural chan
ges are responsible for the change in activity.