Alcohol-Binding Sites in Distinct Brain Proteins: The Quest for Atomic Level Resolution

Abstract

Defining the sites of action of ethanol on brain proteins is a major prerequisite to understanding the molecular pharmacology of this drug. The main barrier to reaching an atomic-level understanding of alcohol action is the low potency of alcohols, ethanol in particular, which is a reflection of transient, low-affinity interactions with their targets. These mechanisms are difficult or impossible to study with traditional techniques such as radioligand binding or spectroscopy. However, there has been considerable recent progress in combining X-ray crystallography, structural modeling, and site-directed mutagenesis to define the sites and mechanisms of action of ethanol and related alcohols on key brain proteins. We review such insights for several diverse classes of proteins including inwardly rectifying potassium, transient receptor potential, and neurotransmit-ter-gated ion channels, as well as protein kinase C epsilon. Some common themes are beginning to emerge from these proteins, including hydrogen bonding of the hydroxyl group and van der Waals interactions of the methylene groups of ethanol with specific amino acid residues. The resulting binding energy is proposed to facilitate or stabilize low-energy state transitions in the bound proteins, allowing ethanol to act as a “molecular lubricant” for protein function. We discuss evidence for characteristic, discrete alcohol-binding sites on protein targets, as well as evidence that binding to some proteins is better characterized by an interaction region that can accommodate multiple molecules of ethanol.

Molecular mechanism underlying ethanol activation of G-protein–gated inwardly rectifying potassium channels

“…Formation of the disulfide bond alone, however, was insufficient for channel activation because MTS-Y modified L257C but did not activate the channel. The structural features and chemical forces observed for GIRK channels might apply to other alcohol pockets (37), such as with pentameric ligand-gated ion channels (16) and transient receptor potential channels (38). Recently, Sauguet et al (39) examined the structural basis of potentiation by alcohols and anesthetics in an ethanol-sensitized prokaryotic channel; they describe an intersubunit hydrophobic pocket in which 2-bromoethanol forms both polar (hydrogen bonds) and nonpolar interactions with the channel, the same basic elements observed in GIRK channels.…”

Structural basis for alcohol modulation of a pentameric ligand-gated ion channel

“…Alcohol has been proposed to occupy water-filled amphiphilic cavities in proteins (37). Furthermore, ion channel residues associated with alcohol modulation are proposed to neighbor regions critical to gating, thus converting the limited binding energy of one or more alcohol molecules into changes in the gating energy landscape as efficiently as possible (38). Therefore, we asked whether amino acid residues homologous to those that affect alcohol modulation in pLGICs might border solvent-accessible cavities and/or influence ion channel gating in GLIC.…”

Seeking Structural Specificity: Direct Modulation of Pentameric Ligand-Gated Ion Channels by Alcohols and General Anesthetics

“…Further optimization of structure/function techniques, including heterologous expression, electrophysiology, mutagenesis, chemical labeling, and spectroscopy (Forman and Miller, 2011) as well as molecular modeling and high-resolution structure techniques described later in this review, has facilitated the identification of multiple candidate sites for direct alcohol and anesthetic modulation and provided critical support for more specific protein theories of anesthesia and alcohol action (Howard et al, 2011b). Notable studies outlined below have elicited provocative suggestions regarding the presence of one or more conserved anesthetic binding sites among these receptors and the evolutionary rationale for their existence and persistence through the family and across species.…”