The agonistic binding site at the histamine H2 receptor. I. Theoretical investigations of histamine binding to an oligopeptide mimicking a part of the fifth transmembrane alpha-helix

Molecular and cellular analysis of human histamine receptor subtypes

The human histamine receptors hH(1)R and hH(2)R constitute important drug targets, and hH(3)R and hH(4)R have substantial potential in this area. Considering the species-specificity of pharmacology of H(x)R orthologs, it is important to analyze hH(x)Rs. Here, we summarize current knowledge of hH(x)Rs endogenously expressed in human cells and hH(x)Rs recombinantly expressed in mammalian and insect cells. We present the advantages and disadvantages of the various systems. We also discuss problems associated with the use of hH(x)R antibodies, an issue of general relevance for G-protein-coupled receptors (GPCRs). There is much greater overlap in activity of 'selective' ligands for other hH(x)Rs than the cognate receptor subtype than generally appreciated. Studies with native and recombinant systems support the concept of ligand-specific receptor conformations, encompassing agonists and antagonists. It is emerging that for characterization of hH(x)R ligands, one cannot rely on a single test system and a single parameter. Rather, multiple systems and parameters have to be studied. Although such studies are time-consuming and expensive, ultimately, they will increase drug safety and efficacy.

Probability rule for chiral recognition

Molecular Chirality is of central interest in biological studies because enantiomeric compounds, while indistinguishable by most inanimate systems, show profoundly different properties in biochemical environments. Enantioselective separation methods, based on the differential recognition of two optical isomers by a chiral selector, have been amply documented. Also, great effort has been directed towards a theoretical understanding of the fundamental mechanisms underlying the chiral recognition process. Here we report a comprehensive data examination of enantio separation measurements for over 72000 chiral selector-select and pairs from the chiral selection compendium CHIRBASE. The distribution of alpha = k'(D)/k'(L) values was found to follow a power law, equivalent to an exponential decay for chiral differential free energies. This observation is experimentally relevant in terms of the number of different individual or combinatorial selectors that need to be screened in order to observe alpha values higher than a preset minimum. A string model for enantiorecognition (SMED) formalism is proposed to account for this observation on the basis of an extended Ogston three-point interaction model. Partially overlapping molecular interaction domains are analyzed in terms of a string complementarity model for ligand-receptor complementarity. The results suggest that chiral selection statistics may be interpreted in terms of more general concepts related to biomolecular recognition.

A qualitative model for the histamine H3 receptor explaining agonistic and antagonistic activity simultaneously

A pharmacophore model for histamine H3 ligands is derived that reveals the putative interaction of both H3 agonists and antagonists with an aspartate residue of the receptor. This interaction is determined by applying the density functional theory implemented in a program package adapted for parallel computers. The model reveals a molecular determinant explaining efficacy as the conformation of the aspartic acid residue differs according to whether it is binding to agonists or antagonists. The differences in structure-activity relationships (SAR) observed for the lipophilic tails of different classes of H3 antagonists are now explained, since the model reveals two distinct lipophilic pockets available for antagonist binding.

A pH-dependent model of the activation mechanism of the histamine H2 receptor

A pH-dependent model of agonist action for the histamine H2 receptor was developed by taking into account the different ionic states of the amino acid residues that constitute the agonist-binding pocket of the receptor. The model offers the possibility of examining diverse mechanistic pathways to yield the active form of the receptor according to the molecular structure of the ligand. The rationale is valid for either tautomeric or non-tautomeric agonists and provides new insight into the mechanism of receptor activation. The subsequent application of the operational model of agonism allows one to derive agonist concentration- effect relationships that may prove useful for both the simulation of agonist profiles under different physiological conditions and the estimation of the pharmacologic parameters of efficacy and potency. General principles involved in the formulation are expected to be valid for other G-protein-coupled receptors.

Quantum mechanical calculations on biological systems

Improvements in quantum chemical methods have led to increased applications to biological problems, including the development of potential energy functions for molecular mechanics and modeling of the reactive chemistry in enzyme active sites, with particularly interesting progress being made for metal-containing systems. An important direction is the development and application of hybrid quantum chemical-molecular mechanics methods.