Binding of imidazole, 1-methylimidazole and 4-nitroimidazole to yeast cytochrome c peroxidase (CcP) and the distal histidine mutant, CcP(H52L)

Imidazolium-based zwitterionic liquid-modified PEG-PLGA nanoparticles as a potential intravenous drug delivery carrier

Zwitterionic-based systems offer promise as next-generation drug delivery biomaterials capable of enhancing nanoparticle (NP) stimuli-responsiveness, biorecognition, and biocompatibility. Further, imidazole-functionalized amphiphilic zwitterions are able to readily bind to various biological macromolecules, enabling antifouling properties for enhanced drug delivery efficacy and bio-targeting. Herein, we describe structurally tuned zwitterionic imidazole-based ionic liquid (ZIL)-coated PEG-PLGA nanoparticles made with sonicated nanoprecipitation. Upon ZIL surface modification, the hydrodynamic radius increased by nearly 20 nm, and the surface charge significantly shifted closer to neutral. 1H NMR spectra suggests that the amount of ZIL on the nanoparticle surface is controlled by the structure of the ZIL and that the assembly occurs as a result of non-covalent interactions of ZIL-coated nanoparticle with the polymer surface. These nanoparticle-zwitterionic liquid (ZIL) constructs demonstrate selective affinity towards red blood cells in whole mouse blood and show relatively low human hemolysis at ∼5%. Additionally, we observe higher nanoparticle accumulation of ZIL-NPs compared with unmodified NP controls in human triple-negative breast cancer cells (MDA-MB-231). Furthermore, although the ZIL shows similar protein adsorption by SDS-PAGE, LC-MS/MS protein analysis data demonstrate a difference in the relative abundance and depletion of proteins in mouse and human serum. Hence, we show that ZIL-coated nanoparticles provide a new potential platform to enhance RBC-based drug delivery systems for cancer treatments.

Azole-Based Indoleamine 2,3-Dioxygenase 1 (IDO1) Inhibitors

The heme enzyme indoleamine 2,3-dioxygenase 1 (IDO1) plays an essential role in immunity, neuronal function, and aging through catalysis of the rate-limiting step in the kynurenine pathway of tryptophan metabolism. Many IDO1 inhibitors with different chemotypes have been developed, mainly targeted for use in anti-cancer immunotherapy. Lead optimization of direct heme iron-binding inhibitors has proven difficult due to the remarkable selectivity and sensitivity of the heme-ligand interactions. Here, we present experimental data for a set of closely related small azole compounds with more than 4 orders of magnitude differences in their inhibitory activities, ranging from millimolar to nanomolar levels. We investigate and rationalize their activities based on structural data, molecular dynamics simulations, and density functional theory calculations. Our results not only expand the presently known four confirmed chemotypes of sub-micromolar heme binding IDO1 inhibitors by two additional scaffolds but also provide a model to predict the activities of novel scaffolds.

Ligand accessibility to heme cytochrome b5 coordinating sphere and enzymatic activity enhancement upon tyrosine ionization

Recently, we observed that at extreme alkaline pH, cytochrome b₅ (Cb₅) acquires a peroxidase-like activity upon formation of a low spin hemichrome associated with a non-native state. A functional characterization of Cb₅, in a wide pH range, shows that oxygenase/peroxidase activities are stimulated in alkaline media, and a correlation between tyrosine ionization and the attained enzymatic activities was noticed, associated with an altered heme spin state, when compared to acidic pH values at which the heme group is released. In these conditions, a competitive assay between imidazole binding and Cb₅ endogenous heme ligands revealed the appearance of a binding site for this exogenous ligand that promotes a heme group exposure to the solvent upon ligation. Our results shed light on the mechanism behind Cb₅ oxygenase/peroxidase activity stimulation in alkaline media and reveal a role of tyrosinate anion enhancing Cb₅ enzymatic activities on the distorted protein before maximum protein unfolding.

pH dependence of cyanide and imidazole binding to the heme domains of Sinorhizobium meliloti and Bradyrhizobium japonicum FixL

Equilibrium and kinetic properties of cyanide and imidazole binding to the heme domains of Sinorhizobium meliloti and Bradyrhizobium japonicum FixL (SmFixLH and BjFixLH) have been investigated between pH5 and 11. KD determinations were made at integral pH values, with the strongest binding at pH9 for both ligands. KD for the cyanide complexes of BjFixLH and SmFixLH is 0.15±0.09 and 0.50±0.20μM, respectively, and 0.70±0.01mM for imido-BjFixLH. The association rate constants are pH dependent with maximum values of 443±8 and 252±61M(-1)s(-1) for cyano complexes of BjFixLH and SmFixLH and (5.0±0.3)×10(4) and (7.0±1.4)×10(4)M(-1)s(-1) for the imidazole complexes. The dissociation rate constants are essentially independent of pH above pH5; (1.2±0.3)×10(-4) and (1.7±0.3)×10(-4)s(-1) for the cyano complexes of BjFixLH and SmFixLH, and (73±19) and (77±14) s(-1) for the imidazole complexes. Two ionizable groups in FixLH affect the rate of ligand binding. The more acidic group, identified as the heme 6 propionic acid, has a pKa of 7.6±0.2 in BjFixLH and 6.8±0.2 in SmFixLH. The second ionization is due to formation of hydroxy-FixLH with pKa values of 9.64±0.05 for BjFixLH and 9.61±0.05 for SmFixLH. Imidazole binding is limited by the rate of heme pocket opening with maximum observed values of 680 and 1270s(-1) for BjFixLH and SmFixLH, respectively.

Apolar distal pocket mutants of yeast cytochrome c peroxidase: Binding of imidazole, 1-methylimidazole and 4-nitroimidazole to the triAla, triVal, and triLeu variants

Imidazole binding to three apolar distal heme pocket mutants of yeast cytochrome c peroxidase (CcP) has been investigated between pH4 and 8. The three CcP variants have Arg-48, Trp-51, and His-52 mutated to either all alanine, CcP(triAla), all valine, CcP(triVal), or all leucine residues, CcP(triLeu). The imidazole binding curves for all three mutants are biphasic indicating that each of the mutants exists in at least two conformational states with different affinities for imidazole. At pH7, the high-affinity conformations of the three CcP mutants bind imidazole between 3.8 and 4.7 orders of magnitude stronger than that of wild-type CcP while the low-affinity conformations have binding affinities about 2.5 orders of magnitude larger than wild-type CcP. Imidazole binding to the three CcP mutants is pH dependent with the strongest binding observed at high pH. Apparent pK(a) values for the transition in binding vary between 5.6 and 7.5 for the high-affinity conformations and between 6.2 and 6.8 for the low-affinity conformations of the CcP triple mutants. The kinetics of imidazole binding are also biphasic. The fast phase of imidazole binding to CcP(triAla) and CcP(triLeu) is linearly dependent on the imidazole concentration while the slow phase is independent of imidazole concentration. Both phases of imidazole binding to CcP(triVal) have a hyperbolic dependence on the imidazole concentration. The apparent association rate constants vary between 30 and 170 M(-1)s(-1) while the apparent dissociation rate constants vary between 0.05 and 0.43 s(-1). The CcP triple mutants have higher binding affinities for 1-methylimidazole and 4-nitroimidazole than does wild-type CcP.