On the rational design of renin inhibitors: x-ray studies of aspartic proteinases complexed with transition-state analogs

The acceleration of the rates of specific reactions by enzymes is attributed to the stabilization of the transition state at the catalytic center. As a consequence, inhibitors that partially mimic the transition state bind more tightly than the equivalent substrate(s), and such transition-state analogues are being designed and tested for clinical use. Renin, an aspartic proteinase produced in the juxtaglomerula cells of the kidneys, catalyzes removal of the decapeptide angiotensin I (AI) from the N-terminus of angiotensinogen. The conversion of AI to the octapeptide angiotensin II (AII) is catalyzed by a carboxydipeptidase, angiotensin converting enzyme (ACE). Renin is a highly specific enzyme: it cleaves only the 10-11 bond in angiotensinogen. The minimum sequence of substrate still hydrolyzed by renin at a measurable rate is the 6-13 octapeptide. However, this cleavage is sufficiently slow to enable the octapeptide to act as a weak competitive inhibitor of the enzyme in vitro, with an IC/sub 50/ of 0.2 mM. The crystal structures of several aspartic proteinases have been solved by X-ray diffraction, revealing a common bilobal structure with a large cleft between the N- and C-terminal domains. The two essential carboxylates of Asp-32 and Asp-215 are within hydrogen-bonding distance and are approximately coplanar due to the restraints of a hydrogen-bonding network involving residues of the two highly conserved loops that contain the two essential aspartates. Modeling studies based on the homology of renin with other aspartic proteinases have shown that renins may assume tertiary structures that are similar to those of other aspartic proteinases.