The recently determined crystal structures of two aspartic proteinase zymog
ens, prophytepsin from barley and proplasmepsin II from the malarial parasi
te Plasmodium falciparum, have provided new insights into zymogen inactivat
ion. Prophytepsin shows a variation of the mechanism of inhibition used by
the well-known gastric aspartic proteinase zymogens, whereas proplasmepsin
II uses a completely new mode of inactivation.