Structural evidence for a functionally relevant second camphor binding site in P450cam: model for substrate entry into a P450 active site.

P450cam has long served as a prototype for the cytochrome P450 (CYP) gene family. But, little is known about how substrate enters its active site pocket, and how access is achieved in a way that minimizes exposure of the reactive heme. We hypothesize that P450cam may first bind substrate transiently near the mobile F-G helix that covers the active site pocket. Such a two-step binding process is kinetically required if P450cam rarely populates an open conformation-as suggested by previous literature and the inability to obtain a crystal structure of P450cam in an open conformation. Such a mechanism would minimize exposure of the heme by allowing P450cam to stay in a closed conformation as long as possible, since only brief flexing into an open conformation would be required to allow substrate entry. To test this model, we have attempted to dock a second camphor molecule into the crystal structure of camphor-bound P450cam. The docking identified only one potential entry site pocket, a well-defined cavity on the F-helix side of the F-G flap, 16 A from the heme iron. Location of this entry site pocket is consistent with our NMR T1 relaxation-based measurements of distances for a camphor that binds in fast exchange (active site camphor is known to bind in slow exchange). Presence of a second camphor binding site is also confirmed with [(1)H-(13)C] HSQC titrations of (13)CH3-threonine labeled P450cam. To confirm that camphor can bind outside of the active site pocket, (13)CH3-S-pyridine was bound to the heme iron to physically block the active site, and to serve as an NMR chemical shift probe. Titration of this P450cam-pyridine complex confirms that camphor can bind to a site outside the active site pocket, with an estimated Kd of 43 microM. The two-site binding model that is proposed based on these data is analogous to that recently proposed for CYP3A4, and is consistent with recent crystal structures of P450cam bound to tethered-substrates, which force a partially opened conformation.

Homology-modeled ligand-binding domains of zebrafish estrogen receptors alpha, beta1, and beta2: from in silico to in vivo studies of estrogen interactions in Danio rerio as a model system.

Homology models were constructed for the ligand-binding domains of zebrafish estrogen receptors (zfERs) alpha, beta(1), and beta(2). Estradiol-binding sites are nearly identical in zfERs and their human homologs, suggesting that zebrafish will serve as a good model system for studying human ER-binding drugs. Conversely, studies of endocrine disruptor effects on zebrafish will benefit from the wealth of data available on xenoestrogen interactions with human ERs. Compounds flagged by the Interagency Coordinating Committee on the Validation of Alternative Methods for endocrine disruptor screening were docked into our zfER homology models. Ideally, these in silico docking studies would be complemented with in vivo binding studies. To this end, fluorescently tagged estradiol was docked into zfERalpha and found to bind in the same manner as in human ERalpha, with fluorescein preferentially occupying a region between helices 11 and 12. Fluorescently tagged estradiol was synthesized and was found to localize along the path of primordial germ cell migration in the developing zebrafish embryo 3 d after fertilization, consistent with previous reports of 1) a role for estradiol in sex determination, and 2) the first appearance of ERs 2 d after fertilization. These data provide a foundation for future in silico and in vivo binding studies of estrogen agonists and antagonists with zebrafish ERs.

Systems-based design of bi-ligand inhibitors of oxidoreductases: filling the chemical proteomic toolbox.

Genomics-driven growth in the number of enzymes of unknown function has created a need for better strategies to characterize them. Since enzyme inhibitors have traditionally served this purpose, we present here an efficient systems-based inhibitor design strategy, enabled by bioinformatic and NMR structural developments. First, we parse the oxidoreductase gene family into structural subfamilies termed pharmacofamilies, which share pharmacophore features in their cofactor binding sites. Then we identify a ligand for this site and use NMR-based binding site mapping (NMR SOLVE) to determine where to extend a combinatorial library, such that diversity elements are directed into the adjacent substrate site. The cofactor mimic is reused in the library in a manner that parallels the reuse of cofactor domains in the oxidoreductase gene family. A library designed in this manner yielded specific inhibitors for multiple oxidoreductases.

Percutaneous penetration modifiers and formulation effects.

The enhancement/retardation of percutaneous permeation of diethyl-m-toluamide (DEET) in the presence of five percutaneous penetration modifiers (laurocapram, 3-dodecanoyloxazolidin-2-one (N-0915), S,S-dimethyl-N-(4-bromobenzoyl) iminosulfurane (DMBIS), S,S-dimethyl-N-(2-methoxycarbonylbenzenesulfonyl) iminosulfurane (DMMCBI) and tert-butyl 1-dodecyl-2-oxoazepan-3-yl-carbamate (TBDOC)) was investigated. These permeation modifiers were formulated in either water, propylene glycol (PG), ethanol or polyethylene glycol 400 (PEG 400). The permeation studies indicated that laurocapram enhanced DEET permeation in PG, but retarded in PEG 400. Likewise, N-0915 acted as a retardant with ethanol and PEG 400, but not with water. DMBIS decreased the permeation with ethanol as compared to permeation with water, PEG 400 or PG. Similarly, DMMCB acted as a retardant with ethanol and PEG 400, but not with water or PG. TBDOC formulations revealed its activity as a retardant with ethanol, but behaved as enhancer with water, PG and PEG 400. In addition, penetration modifier interactions with stratum corneum ceramide were investigated using chemical modeling. This investigation is significant since it confirms the role of pharmaceutical formulations and shows for the first time that an enhancer can become a retardant or vice versa depending upon the vehicle in which it is applied to the skin. Hence, we should be using the term "penetration modifiers" for all such compounds.

Polymer-drug interactions in tyrosine-derived triblock copolymer nanospheres: a computational modeling approach.

A combination of molecular dynamics (MD) simulations and docking calculations was employed to model and predict polymer-drug interactions in self-assembled nanoparticles consisting of ABA-type triblock copolymers, where A-blocks are poly(ethylene glycol) units and B-blocks are low molecular weight tyrosine-derived polyarylates. This new computational approach was tested on three representative model compounds: nutraceutical curcumin, anticancer drug paclitaxel and prehormone vitamin D3. Based on this methodology, the calculated binding energies of polymer-drug complexes can be correlated with maximum drug loading determined experimentally. Furthermore, the modeling results provide an enhanced understanding of polymer-drug interactions, revealing subtle structural features that can significantly affect the effectiveness of drug loading (as demonstrated for a fourth tested compound, anticancer drug camptothecin). The present study suggests that computational calculations of polymer-drug pairs hold the potential of becoming a powerful prescreening tool in the process of discovery, development and optimization of new drug delivery systems, reducing both the time and the cost of the process.

Chemical proteomic tool for ligand mapping of CYP antitargets: an NMR-compatible 3D QSAR descriptor in the Heme-Based Coordinate System.

Chemical proteomic strategies strive to probe and understand protein-ligand interactions across gene families. One gene family of particular interest in drug and xenobiotic metabolism are the cytochromes P450 (CYPs), the topic of this article. Although numerous tools exist to probe affinity of CYP-ligand interactions, fewer exist for the rapid experimental characterization of the structural nature of these interactions. As a complement to recent advances in X-ray crystallography, NMR methods are being developed that allow for fairly high throughput characterization of protein-ligand interactions. One especially promising NMR approach involves the use of paramagnetic induced relaxation effects to measure distances of ligand atoms from the heme iron in CYP enzymes. Distances obtained from these T(1) relaxation measurements can be used as a direct source of 1-dimensional structural information or to restrain a ligand docking to generate a 3-dimensional data set. To facilitate such studies, we introduce the concept of the Heme-Based Coordinate System and present how it can be used in combination with NMR T(1) relaxation data to derive 3D QSAR descriptors directly or in combination with in silico docking. These descriptors should have application in defining the binding preferences of CYP binding sites using 3D QSAR models. They are especially well-suited for the biasing of fragment assembly and combinatorial chemistry drug design strategies, to avoid fragment or reagent combinations with enhanced affinity for CYP antitargets.