Covalent functionalization of G protein-coupled receptors by small molecular probes

Roughly one-third of all marketed drugs act by binding to one or more of the >800 human GPCRs, primarily through activation or inhibition via the orthosteric binding site. In addition, novel strategies to alter GPCR functioning are being developed, including allosteric, biased and covalently binding ligands. Molecular probes play an important role in verifying such drug molecules with new modes of action and providing information on all factors involved in GPCR signalling. Various types of molecular probes have been developed, ranging from small molecules to antibodies, each bearing its own advantages and disadvantages. In this mini-review, a closer look is taken at small molecular probes that functionalize GPCRs in a covalent manner, such as through the conjugation of reporter groups like fluorophores or biotin. Covalently bound reporter groups allow the investigation of GPCRs across an increasing range of biochemical assay types, yielding new insights into GPCR signalling pathways. Here, a broad range of recently developed 'functionalized covalent probes' is summarized. Furthermore, the use of these probes in biochemical assays and their applications in the field of GPCR research are discussed. Lastly, a view on possible future applications of these types of small molecular probes is provided.