CELL-SURFACE RECEPTORS

The receptors with which the neurotransmitters interact are cell-surface receptors as opposed to the nuclear receptors, which are the target for the steroids. The cell-surface receptors comprise a family of transmembrane proteins. Chemical features involving ionic binding, hydrogen bonding and steric factors can contribute to the way in which a substance binds to these trans-membrane protein chains.

The structures of the amino acids that constitute the trans-membrane proteins play an important role in creating pockets that are favourable for binding. First, the folding of the protein chain creates a particular steric environment. Second, the hydroxyl groups of a serine or tyrosine or the amide of an asparagine provide hydrogen bonding groups. Third, the carboxyl groups of an aspartate or glutamate provide acidic groups to protonate bases while the basic groups of a lysine or a histidine can bind a carboxylic acid. Some charge transfer can take place to an aromatic amino acid such as phenylalanine while the alkyl side chains of leucine and valine provide hydrophobic interactions.