Abstract
The superfamily of acid proteases has two catalytic aspartates for proteolysis of their peptide substrates. Here, we show a minimal structural scaffold, the structural catalytic core (SCC), which is conserved within each family of acid proteases, but varies between families, and thus can serve as a structural marker of four individual protease families. The SCC is a dimer of several structural blocks, such as the DD-link, D-loop, and G-loop, around two catalytic aspartates in each protease subunit or an individual chain. A dimer made of two (D-loop + DD-link) structural elements makes a DD-zone, and the D-loop + G-loop combination makes a psi-loop. These structural markers are useful for protein comparison, structure identification, protein family separation, and protein engineering.