Aspartic endopeptidases 3.4.23. of vertebrate, fungal and retroviral origin have been characterised [1]. More recently, aspartic endopeptidases associated with the processing of bacterial type 4 prepilin [2] and archaean preflagellin have been described [3, 4].

Structurally, aspartic endopeptidases are bilobal enzymes, each lobe contributing a catalytic Asp residue, with an extended active site cleft localised 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 site motif, which is very conserved. The presence and position of disulphide bridges are other conserved features of aspartic peptidases.All or most aspartate peptidases are endopeptidases. These enzymes have been assigned into clans (proteins which are evolutionary related), and further sub-divided into families, largely on the basis of their tertiary structure.

These aspartate proteases all contain a common closed beta barrel structure, which includes pepsin, cathepsin, chymosin, beta-secretase, plasmepsin, plant acid proteases and retroviral proteases [5, 6].

1. The aspartic proteases. Scand. J. Clin. Lab. Invest. Suppl. 210, 5-22
2. The type 4 prepilin peptidases comprise a novel family of aspartic acid proteases. J. Biol. Chem. 275, 1502-10
3. Archaeal flagella, bacterial flagella and type IV pili: a comparison of genes and posttranslational modifications. J. Mol. Microbiol. Biotechnol. 11, 167-91
4. Cleavage of preflagellins by an aspartic acid signal peptidase is essential for flagellation in the archaeon Methanococcus voltae. Mol. Microbiol. 50, 1339-47
5. Structure and mechanism of the pepsin-like family of aspartic peptidases. Chem. Rev. 102, 4431-58
6. Structure and function of plant aspartic proteinases. Eur. J. Biochem. 271, 2067-75

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