Figure 4.

The 'fireman's grip', a stereotypical rigid network structure involving the Asp-Thr-Gly signature sequence in (a) the classical aspartic peptidases and (b) the retroviral proteases. Amino acids are identified by three-letter codes. In each case, the catalytic aspartic acid residue (Asp) is hydrogen-bonded to the backbone NH group of the glycine (Gly) two amino acids further along in the sequence. In addition, the OH groups of threonine (Thr) are hydrogen-bonded to two points on the opposite domain or monomer, the backbone NH group (blue) of the threonine and to the carbonyl oxygen (red) of the residue before the catalytic aspartic acid.