Exploring the putative mechanism of allosteric modulations by mixed-action kappa/mu opioid receptor bitopic modulators

Abstract

The modulation and selectivity mechanisms of seven mixed-action kappa opioid receptor (KOR)/mu opioid receptor (MOR) bitopic modulators were explored. Molecular modeling results indicated that the 'message' moiety of seven bitopic modulators shared the same binding mode with the orthosteric site of the KOR and MOR, whereas the 'address' moiety bound with different subdomains of the allosteric site of the KOR and MOR. The 'address' moiety of seven bitopic modulators bound to different subdomains of the allosteric site of the KOR and MOR may exhibit distinguishable allosteric modulations to the binding affinity and/or efficacy of the 'message' moiety. Moreover, the 3-hydroxy group on the phenolic moiety of the seven bitopic modulators induced selectivity to the KOR over the MOR.

Keywords: allosteric modulation mechanism; kappa opioid receptor (KOR)/mu opioid receptor (MOR); mixed-action KOR/MOR ligand; molecular dynamics simulations; psychostimulant abuse and addiction; selectivity.

Figure 1.. Chemical structures of β-FNA, NFF, 42B, NBP, 3-dHNBP, NCP and 3-dHNCP with atom notations.

The ‘message’ moiety, linker and ‘address’ moiety in the structures are colored red, blue and magenta, respectively. β-FNA: β-funaltrexamine; NFF: Nalfurafine.

Figure 2.. The RMSD and RMSF of the 14 complexes relative to their respective starting structures.

The RMSD of the backbone atoms of the proteins (A) and ligands (B), together with the RMSF of the backbone atoms of the β-FNA_MOR^inactive, β-FNA_KOR^active, NFF_MOR^active, NFF_KOR^active, 42B_MOR^active, 42B_KOR^active, NBP_MOR^active, NBP_KOR^active, 3-dHNBP_MOR^inactive, 3-dHNBP_KOR^active, NCP_MOR^active, NCP_KOR^active, 3-dHNCP_MOR^inactive and 3-dHNCP_KOR^active complexes (C). β-FNA: β-funaltrexamine; KOR: Kappa opioid receptor; MOR: Mu opioid receptor; NFF: Nalfurafine; RMSD: Root-mean-square deviation; RMSF: Root-mean-square fluctuation.

Figure 3.. The binding modes of the seven ligands in the KOR after MD simulations.

(A) β-FNA_KOR^active, (B) NFF_KOR^active, (C) 42B_KOR^active, (D) NBP_KOR^active, (E) 3-dHNBP_KOR^active, (F) NCP_KOR^active and (G) 3-dHNCP_KOR^active. Active KOR shown as a cartoon model in light pink. β-FNA, NFF, 42B, NBP, 3-dHNBP, NCP and 3-dHNCP shown as stick-and-ball models. Key amino acid residues shown as stick models. Carbon atoms: β-FNA in white, NFF in magenta, 42B in cyan, NBP in orange, 3-dHNBP in green, NCP in pink, 3-dHNCP in dark cyan, key amino acid residues of the KOR in light cyan. Subdomain ABD2 of the allosteric site shown as a surface model in light orange. The red arrow in (A) represents the movement orientation of the ‘address’ moiety of β-FNA in the β-FNA_KOR^active complex. β-FNA: β-funaltrexamine; KOR: Kappa opioid receptor; NFF: Nalfurafine.

Figure 4.. Energy decomposition analyses of the seven ligands in the KOR.

(A) binding free energy, (B) van der Waals interaction and (C) net electrostatic interaction decomposition analyses of β-FNA_KOR^active, NFF_KOR^active, 42B_KOR^active, NBP_KOR^active, 3-dHNBP_KOR^active, NCP_KOR^active and 3-dHNCP_KOR^active complexes. β-FNA: β-funaltrexamine; KOR: Kappa opioid receptor; NFF: Nalfurafine.

Figure 5.. The binding modes of the seven ligands in the MOR after MD simulations.

(A) β-FNA_MOR^inactive, (B) 3dHNBP_MOR^inactive, (C) 3dHNCP_MOR^inactive, (D) NFF_MOR^active, (E) 42B_MOR^active, (F) NBP_MOR^active and (G) NCP_MOR^active complexes after MD simulations. Inactive and active MORs shown as cartoon models in light yellow and light blue, respectively. β-FNA, NFF, 42B, NBP, 3-dHNBP, NCP and 3-dHNCP shown as stick-and-ball models. Key amino acid residues shown as stick models. Carbon atoms: β-FNA in white, NFF in magenta, 42B in cyan, NBP in orange, 3-dHNBP in green, NCP in pink, 3-dHNCP in dark cyan, key amino acid residues of the MOR in dark violet. Subdomains ABD1 and ABD2 of the allosteric site shown as surface models in lemon and light orange, respectively. β-FNA: β-funaltrexamine; MOR: Mu opioid receptor; NFF: Nalfurafine.

Figure 6.. Energy decomposition analyses of the seven ligands in the MOR.

(A & B) binding free energy, (C & D) van der Waals interaction and (E & F) net electrostatic interaction decomposition analyses of β-FNA_MOR^inactive, 3dHNBP_MOR^inactive, 3dHNCP_MOR^inactive, NFF_MOR^active, 42B_MOR^active, NBP_MOR^active and NCP_MOR^active complexes. β-FNA: β-funaltrexamine; MOR: Mu opioid receptor; NFF: Nalfurafine.