3-fluoro-GABA enantiomers: exploring the conformation of GABA binding to GABAA receptors and GABA aminotransferase

Abstract

The neurotransmitter GABA is the main inhibitory neurotransmitter of the CNS. It is estimated that 20–50% of all neuronal synapses are GABAergic. Distinct types of GABA receptor have been identified (GABAA, GABAB and GABAAOr [GABAC]). The GABAA and GABAAOr variant found in the retina receptors (informally termed GABAC) are physiologically related in that they are both ligand (GABA)-gated chloride ion channels. The GABAB receptor mediates its response by modulating Ca2+ and K+ ion channels, which then trigger coupled G-proteins. These receptors have not succumbed to x-ray structural analysis at a suitable resolution to determine the nature of GABA binding. However, knowledge of how GABA binds is important for the design of novel agonists and antagonists, and the development of neuropharmaceutical agents. Comparison of the relative agonistic efficacy of the (R)- and (S)-enantiomers of 3-fluoro-GABA has been used as a method to probe the binding conformation of the neurotransmitter binding to target proteins. The analysis relies on the stereoelectronic influence of the C–F bond interacting with the ammonium group of GABA, an electrostatic interaction that favors solution conformers where the substituents come close (gauche rather than anti). This article describes how comparative analysis of these enantiomers has been used to assess the conformation of GABA binding to GABAA and the GABA-metabolizing enzyme, GABA transaminase.