Structure of pig lysosomal carboxypeptidase A (CTSA). According to its overall fold, this serine protease belongs to the SC clan.
The catalytic site consists of a triad of residues in a charge relay system, from Asp400 to His457 to Ser178.
The charge relay system increases the nucleophilicity of the terminal oxygen in the catalytic serine. Thus, the aspartate side chain interacts with the gamma hydrogen of the histidine. This interaction aligns the epsilon nitrogen of the histidine so that it forms a hydrogen bond with the beta hydrogen from the serine. You can click the render window and press '4' to see these interactions as translucid bars.



  • Structures are rendered as ribbons. Catalytic and catalytic metal-complexing residues are shown as neons. Inhibitors are displayed as space filling spheres.

  • Carbons are shown in light gray, hydrogens in cyan, oxygens in red, nitrogens in blue, and sulfurs in yellow.

  • Left-click and drag to rotate the protease. Right-click and drag up/down to zoom in/out. Click with both buttons and drag to move the protease. Use the upper bar to get different views and for display options. For a close-up of the catalytic site, choose the cat_site view.

  • Once you have clicked inside the structure window, you can use the keyboard to hide/show objects:

    '1' toggles the ribbon.
    '2' toggles the inhibitor.
    '3' toggles the catalytic residues.

  • You can download the pdf file here

  • To display and manipulate the structure, you need at least Adobe Acrobat Reader v7.0. For best results, install the last version of Acrobat Reader.

  • To hide/show elements in the structure, first you have to click inside the render window.