Function and structure of bradykinin receptor 2 for drug discovery

Abstract

Type 2 bradykinin receptor (B2R) is an essential G protein-coupled receptor (GPCR) that regulates the cardiovascular system as a vasodepressor. Dysfunction of B2R is also closely related to cancers and hereditary angioedema (HAE). Although several B2R agonists and antagonists have been developed, icatibant is the only B2R antagonist clinically used for treating HAE. The recently determined structures of B2R have provided molecular insights into the functions and regulation of B2R, which shed light on structure-based drug design for the treatment of B2R-related diseases. In this review, we summarize the structure and function of B2R in relation to drug discovery and discuss future research directions to elucidate the remaining unknown functions of B2R dimerization.

Keywords: G protein-coupled receptor; drug discovery; functions; structures; type 2 bradykinin receptor.

Fig. 1. The renin–angiotensin system and kallikrein–kinin system.

In endothelial cells, upon bradykinin binding, GDP of the Gα_q subunit is replaced by GTP, accompanying with the dissociation of Gα_q and Gβγ subunits. Gα_q activates PLC and calcium mobilization, and the increasing level of Ca²⁺ enhances the activities of eNOS and PLA₂, which promote the release of NO and prostaglandins, respectively, and lead to vasodilation. Angiotensin II could either bind to AT₁R to elevate the blood pressure, induce pro-inflammatory and pro-fibrosis activities, or bind AT₂R to reduce the blood pressure, perform anti-inflammation and anti-fibrosis activities (PCP prolyl carboxypeptidase, APA aminopeptidases A, PK plasma kallikrein, TK tissue kallikrein, DAG diacyl glycerol, PKC protein kinase C, MAPK mitogen-activated protein kinase).

Fig. 2. Heterodimers of B2R with AT₁R, APJ, and κ-OR.

AT₁R-B2R heterodimer enhances the Ca²⁺-induced hypertension in preeclampsia and blocks the β-arrestin-mediated B2R internalization. APJ-B2R and κ-OR-B2R heterodimers promote cell proliferation through PLC-ERK1/2-eNOS and cAMP-PKA signaling pathways, respectively.

Fig. 3. B2R-G_q-bradykinin and B2R-G_q-kallidin structures.

a Overall structures of bradykinin-bound B2R-G_q complex (PDB ID: 7F6H, B2R, green; bradykinin, violet; Gα, salmon; Gβ, cyan; Gγ, yellow) and kallidin-bound B2R-G_q complex (PDB ID: 7F6I, B2R, violet; kallidin, green). b ECL2 of B2R exhibits a β-sheet, with two disulfide bonds formed between C130-C211 and C47-C304. c At the intracellular side, ICL2 adopts a short α-helical conformation and three cholesterols (CHOL 1–3) lie in clefts between TM2–TM4, TM3–TM4, and TM6–TM7. d Hydrogen bonds (dash lines) formed between ECL1 and ECL2. e Anion trap formed by aspartic acids and glutamates locates at the entrance of ligand-binding pocket.

Fig. 4. Key residues participated in B2R ligand-binding and G_q coupling.

a Binding poses of bradykinin (violet) and kallidin (green) in the pocket, key hydrogen bonds are formed between P^3B/P^4K-I213^ECL2 and G^4B/G^5K-R196^4.64. Hydrogen bonds are labeled as blue (bradykinin) and red (kallidin) dash lines (bradykinin-bound B2R, PDB ID: 7F6H, green; kallidin-bound B2R, PDB ID: 7F6I, violet). b, c Key hydrogen bonds (b) and hydrophobic interactions (c) involved in G_q coupling (Gα_q, salmon). d Conformations of toggle switch and PIF motif in kallidin-bound B2R structure.

Fig. 5. Structural comparison between the binding pockets of B1R and B2R.

a Structural comparison between B2R-bradykinin complex (PDB ID: 7F6H) and B1R-des-Arg¹⁰-kallidin complex (PDB ID: 7EIB). b–e Ionic interactions between B1R and F^9DK as well as steric hindrance between ligand pocket of B1R and R^9B determine the preference of des-Arg¹⁰-kallidin for B1R and bradykinin for B2R. (B1R, yellow-orange; B2R, green; des-Arg¹⁰-kallidin, cyan; bradykinin, violet).

Fig. 6. The third generation of B2R antagonists.

Chemical structures of WIN64338, FR 173657, Bradyzide, Anatibant, Fasitibant, and JSM10292.