Chroloplast Nucleoids DNA-binding Protease and Nucellin, pepsin-like aspartic proteases from plants.
This family contains pepsin like aspartic proteases from plants including Chloroplast Nucleoids DNA-binding Protease and Nucellin. Chloroplast Nucleoids DNA-binding Protease catalyzes the degradation of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) in senescent leaves of tobacco and Nucellins are important regulators of nucellar cell's progressive degradation after ovule fertilization. Structurally, aspartic proteases are bilobal enzymes, each lobe contributing a catalytic Asp residue, with an extended active site cleft localized between the two lobes of the molecule. The N- and C-terminal domains, although structurally related by a 2-fold axis, have only limited sequence homology except the vicinity of the active site. This suggests that the enzymes evolved by an ancient duplication event. The enzymes specifically cleave bonds in peptides which have at least six residues in length with hydrophobic residues in both the P1 and P1' positions. The active site is located at the groove formed by the two lobes, with an extended loop projecting over the cleft to form an 11-residue flap, which encloses substrates and inhibitors in the active site. Specificity is determined by nearest-neighbor hydrophobic residues surrounding the catalytic aspartates, and by three residues in the flap. The enzymes are mostly secreted from cells as inactive proenzymes that activate autocatalytically at acidic pH.