Renin Inhibitor - Binding and Structure Activity Relationship of Renin Inhibitors

Binding and Structure Activity Relationship of Renin Inhibitors

The renin molecule is a monospecific enzyme that belongs to the aspartic protease family. Its structure is complex and consists of two homologous lobes that fold mainly in a β-sheet conformation. Between the two lobes, deep within the enzyme, resides the active site, and its catalytic activity is due to two aspartic acid residues (Asp32 and Asp 215, one from each lobe in the renin molecule). A flexible flap made from amino acids formed in a β-hairpin closes the active site by covering the cleft. The renin molecule contains both hydrophobic and hydrophilic amino acids. The hydrophilic ones tend to be on the outside of the molecule, while the hydrophobic ones tend to be more on the inside and form the active site, a large hydrophobic cavity that can accommodate a ligand with at least seven residues. The principal connection between a ligand and the enzyme is by hydrogen bonding. The residues are named after their places in the ligand, the residues closest to the cleavage site are named P1 and P1′ and they bind into the S1 and S1′ pockets, respectively. There are four S pockets, and three S′ pockets (table 1). The pockets alternate on either side of the backbone in the ligand. This alternation affects the orientation of the pockets, making the S3 and S1 pockets arrange together and the S2 pocket close to both S4 and S1′ pockets. Evidence suggests the closely arranged S1 and S3 pockets merge to form a spacious superpocket. Ligands that fill the superpocket have greater potency than those which do not, occupying increases potency 200-fold. These ligands can be structurally diverse and form van der Waals bonds to the surface of the superpocket. From the S3 pocket stretches a binding site distinct for renin, the S3sp subpocket. The S3sp subpocket can accommodate both hydrophobic and polar residues, the pocket can accommodate three water molecules, but has also lipophilic nature. The S3sp subpocket is not conformationally flexible, so the residues occupying the pocket must have certain characteristics. They can not be sterically demanding and must have reasonably high number of rotatable bonds and be able to connect with hydrogen bonds. The S2 pocket is large, bipartite and hydrophobic, but can accommodate both hydrophobic and polar ligands. This diversity of possible polarity offers the P2 residue opportunity of variation in its connection to the enzyme. The S3-S1 and the S3sp subpockets have been the main target of drug design, but recent discoveries have indicated other sites of interest. Interactions to the pockets on the S′ site have been proven to be critical for affinity, especially the S1′ and S2′, and in vitro tests have indicated the interaction with the flap region could be important to affinity.

Interaction with both aspartic acids in the active site results in a higher affinity. Higher affinity also results by occupying more active site pockets. However, some pockets contribute more to the affinity than others. A hydrophobic interaction with the S3sp subpocket, S1 and S3 contribute to higher potency and affinity. By having a large and aromatic residue in P3 increases inhibitory activity. Occupation of the S3sp subpocket can increase potency by 50-fold and results in tight binding.

Example of binding to the renin inhibitor: Aliskiren is a peptide-like renin inhibitor and, unlike most, it is rather hydrophilic. It blocks the catalytic function of the enzyme by occupying the S3 to S2′ pockets, except the S2 pocket. Aliskiren also binds to the S3sp subpocket and because that pocket is distinct for renin, aliskiren does not inhibit other aspartic proteases, such as cathepsin D and pepsin. The side chain of aliskiren binds the S3sp subpocket ideally, and leads to its quality as an inhibitor of human renin. The hydroxyl group in aliskiren forms a hydrogen bond with both oxygen atoms of the Asp32. The amine group forms a hydrogen bond with the carboxylic acid group of Gly217 and the oxygen atom of the Asp32. The methoxy group on the aromatic ring fills the S3 pocket and may possibly form a hydrogen bond with a secondary amine group of Tyr14. The amide group forms a hydrogen bond with a secondary amine group of Ser76. The S1 and S1′ pockets are occupied by the two propyl groups in positions P1 and P1′. The terminal amide in position P2′ anchors the amide tail in the active site by forming a hydrogen bond with Arg74 in the S2′ pocket.