PMID: 17881000

 

    Legend: Sugar

Title : Structural basis of the zinc inhibition of human tissue kallikrein 5

Abstract :
  1. Human kallikrein 5 5 (hK5 ) is a member of the tissue kallikrein family of serine peptidases
  2. It has trypsin-like substrate specificity, is inhibited by metal ions, and is abundantly expressed in human skin, where it is believed to play a central role in desquamation
  3. To further understand the interaction of hK5 with substrates and metal ions, active recombinant hK5 was crystallized in complex with the tripeptidyl aldehyde inhibitor leupeptin , and structures at 2.3 A resolution were obtained with and without Zn2+
  4. While the overall structure and the specificity of S1 pocket for basic side-chains were similar to that of hK4 , a closely related family member, both differed in their interaction with Zn2+
  5. Unlike hK4 , the 75-loop of hK5 is not structured to bind a Zn2+
  6. Instead, Zn2+ binds adjacent to the active site , becoming coordinated by the imidazole rings of His99 and His96 not present in hK4
  7. This zinc binding is accompanied by a large shift in the backbone conformation of the 99-loop and by large movements of both His side-chains
  8. Modeling studies show that in the absence of bound leupeptin , Zn2+ is likely further coordinated by the imidazolyl side-chain of the catalytic His57 which can, similar to equivalent His57 imidazole groups in the related rat kallikrein proteinase tonin and in an engineered metal-binding rat trypsin , rotate out of its triad position to provide the third co-ordination site of the bound Zn2+, rendering Zn2+-bound hK5 inactive
  9. In solution, this mode of binding likely occurs in the presence of free and substrate saturated hK5 , as kinetic analyses of Zn2+ inhibition indicate a non-competitive mechanism
  10. Supporting the His57 re-orientation, Zn2+ does not fully inhibit hK5 hydrolysis of tripeptidyl substrates containing a P2-His residue
  11. The P2 and His57 imidazole groups would lie next to each other in the enzyme-substrate complex, indicating that incomplete inhibition is due to competition between both imidazole groups for Zn2+
  12. The His96-99-57 triad is thus suggested to be responsible for the Zn2+-mediated inhibition of hK5 catalysis