M. H. Aprison

Comparison of binding mechanisms at cholinergic, serotonergic, glycinergic and GABAergic receptors

Employing computational methods and published data from molecular biological studies involving amino acid sequences in the polypeptide receptors, the authors studied and compared how two excitatory neurotransmitters, ACh and 5‐HT, and two inhibitory neurotransmitters, glycine and GABA, can bind to their respective recognition sites at CNS receptors. Models for each neurotransmitter interaction with specific amino acids are described and identified. Molecular mechanisms are identified that can explain how the binding process initiates ion flow through channels located within the postsynaptic membrane such that if the neurotransmitter is inhibitory, hyperpolarization occurs, and if excitatory, depolarization occurs. Although the theoretical work described indicates that there is a difference in molecular mechanisms operative at the anionic and cationic channels, and provides an explanation why the former is more specific, the molecular modeling data and the similarities of specific amino acids in the sequence in all four receptor polypeptides used to construct the four models support ACh, 5‐HT, glycine and GABA as being members of the same ligand‐gated ion channel superfamily.

Glycine and GABA receptors: Molecular mechanisms controlling chloride ion flux

We have been able to show that the three clearly identified atoms common to the inhibitory neurotransmitters glycine and GABA, that we previously hypothesized to serve as attachment points at the glycinergic and gabanergic receptor, can indeed interact through both electrostatic and hydrogen bonding to several amino acids, which have been identified in molecular biological investigations as both present and critical in the physiological functioning of key polypeptides common to these inhibitory receptors. In addition, amino acids also involved in stabilizing the interaction between the antagonists strychnine and R5135 at the glycinergic and gabanergic receptors, respectively, have been shown to fit our complex model. We identify in detail molecular mechanisms to explain how glycine and GABA initiate chloride ion movement from extraneuronal fluid in the synaptic cleft to intraneuronal volume. In addition, we also identify the molecular mechanisms involved in the blocking of chloride ion movement by strychnine at the glycinergic receptor and by R5135 at the gabanergic receptor. We also present two computer‐generated color prints, one for the glycine receptor and one for the GABA receptor, which show the quantum mechanically geometry optimized complex formed between receptor side chains, i.e., the part of the amino acids in the polypeptide that interacts with the zwitterionic inhibitory neurotransmitters. These computer‐generated color figures also show a) the important electrostatic and hydrogen bonding in these interactions, b) a van der Waals model of this complex to illustrate that no steric repulsions exist, and c) the molecular electrostatic potential energy map showing the electrostatic potentials of neurotransmitter bound to the receptor model. Finally, we show with computer calculations that the pseudo‐rings, formed between the positive quanidinium group in arginine and one of the oxygen atoms in the carboxyl group in both glycine or GABA, result in a positive planar region which appears to be involved in a charge‐transfer complex with aromatic benzene groups in amino acids such as phenylalanine and tryosine.

THE NICOTINIC CHOLINERGIC RECEPTOR : A THEORETICAL MODEL

Based on published affinity‐labeling and mutagenesis experiments describing the effect of changes in specific amino acids in molecular biological studies on the nicotinic acetylcholinergic receptor (nAChR), we have identified 12 amino acids which are important in functioning at the nicotinic cholinergic receptor. The work presented here provides an atomistic model of this important receptor based on our molecular modeling studies. We found five of these amino acids (TRP86, ASP89, TYR93, ASP138, and THR191) to be associated with the cationic end of acetylcholine (ACh), which is electron‐deficient. Three other amino acids (ARG209, TYR190, and TYR198) are associated with the ester end, where an enhanced electron density is present. After hydrogen bonding between the two oxygen atoms at the ester end, and two of the guanidinium hydrogen atoms in ARG209, ASP200 hydrogen bonds to the other two hydrogen atoms of the guanidinium group, thus forming a pseudo‐ring. Two aromatic amino acids (TRP149 and TYR151) then enhance the binding at the pseudo‐ring through additional hydrogen bonding and charge‐transfer complexation, with THR150 functioning to further stabilize this evolving charge‐transfer complex. We postulate that this latter process allows the ion channel to twist, thus opening it. From the published amino acid sequence in the polypeptides at the 5HT‐3, GABA, and glycine receptors (Maricq et al.: Science 254:432–437, 1991), we also speculate on which amino acids are involved in these three receptors.