Aspartic proteases are a catalytic type of proteaseenzymes that use an activated water moleculebound to one or more aspartate residues for catalysis of their peptide substrates. In general, they have two highly conserved aspartates in the active site and are optimally active at acidicpH. Nearly all known aspartyl proteases are inhibited by pepstatin. Aspartic endopeptidases of vertebrate, fungal and retroviral origin have been characterised. More recently, aspartic endopeptidases associated with the processing of bacterial type 4 prepilin and archaean preflagellin have been described. Eukaryotic aspartic proteases include pepsins, cathepsins, and renins. They have a two-domain structure, arising from ancestral duplication. Retroviral and retrotransposon proteases are much smaller and appear to be homologous to a single domain of the eukaryotic aspartyl proteases. Each domain contributes a catalytic Asp residue, with an extended active site cleft localized between the two lobes of the molecule. One lobe has probably evolved from the other through a gene duplication event in the distant past. In modern-day enzymes, although the three-dimensional structures are very similar, the amino acid sequences are more divergent, except for the catalytic sitemotif, which is very conserved. The presence and position of disulfide bridges are other conserved features of aspartic peptidases.
Catalytic mechanism
Aspartyl proteases are a highly specific family of proteases – they tend to cleave dipeptide bonds that have hydrophobic residues as well as a beta-methylene group. Unlike serine or cysteine proteases these proteases do not form a covalent intermediate during cleavage. Proteolysis therefore occurs in a single step. While a number of different mechanisms for aspartyl proteases have been proposed, the most widely accepted is a general acid-base mechanism involving coordination of a water molecule between the two highly conserved aspartate residues. One aspartate activates the water by abstracting a proton, enabling the water to perform a nucleophilic attack on the carbonyl carbon of the substrate scissile bond, generating a tetrahedraloxyanion intermediate stabilized by hydrogen-bonding with the second aspartic acid. Rearrangement of this intermediate leads to protonation of the scissile amide which results in the splitting of the substrate peptide into two product peptides.
Many eukaryotic aspartic endopeptidases are synthesised with signal and propeptides. The animal pepsin-like endopeptidase propeptides form a distinct family of propeptides, which contain a conserved motif approximately 30 residues long. In pepsinogen A, the first 11 residues of the mature pepsinsequence are displaced by residues of the propeptide. The propeptide contains two helices that block the active site cleft, in particular the conserved Asp11 residue, in pepsin, hydrogen bonds to a conserved Arg residue in the propeptide. This hydrogen bondstabilises the propeptide conformation and is probably responsible for triggering the conversion of pepsinogen to pepsin under acidic conditions.
