Smith Research Group

Neuropeptide Receptors – Hirschmann/Nicolaou/Smith Collaboration (1987-2011)

In collaboration with Professor Ralph Hirschmann (also a member of this department) we have designed and synthesized glucopyransides and found them to be mimics of the cyclic hexapeptide L-363,301 in that they bind to the SRIF receptor, mimicking the β-turn of the latter. We also discovered that these two compounds bind also structurally diverse proteins which we attribute to the radial symmetry features of the glucose scaffold.

Subsequent studies have produced several noteworthy findings: (1) the glycoside inhibits GRF-induced growth hormone (GH) release by cultured rat anterior pituitary cells with an IC₅₀ of 3 micromolar (i.e. 1b is recognized at this endocrine receptor and it provides a sufficiently good fit to activate the receptor); (2) N-acetylation of the primary amino group generated an amide which was highly selective for the substance P (SP) receptor, with an IC₅₀ of 60 nM. That such minor chemical modification should produce such a dramatic switch in biological profile was a pleasing but unexpected result. It is worth noting that SRIF receptors, as well as the SP, and the β₂-adrenergic receptors, utilize G-protein-mediated signal transduction, suggesting that the binding of the glucosides to these three receptors may involve similar interactions within the conserved hydrophobic domains of the receptors.

Subsequently, analog synthesis generated an analog with an IC₅₀ at the human SRIF receptor subtype 4 of 53 nM and another congener with an IC₅₀ of 3 nM at the NK1 receptor. The glycosides have a binding profile at the SRIF receptor similar to analogously modified peptides. This demonstrates that the indole ethyl and 6-amino pentyl sidechains mimic the Trp and Lys residues, respectively, of the β-turn of L-363,301.

Unexpectedly a des-indole analog in which the C1 Trp-mimicking sidechain was replaced by a C1 methoxyl proved to be an SRIF agonist, a result which we see as but one example of the fact that the β-D-glucose scaffold possesses radial symmetry which confers other noteworthy properties on this scaffold. That the glucoside, but not L-363,301, binds the NK-1 receptor is also explained by the presence of radial symmetry in the former, but not in the c-hexapeptide. Interestingly, replacement of Lys⁹ of either L-363,301 or of D-Trp⁸-SRIF by p-F-Phe is required to change peptidal SRIF agonists into substance P antagonists. It is a consequence of the radial symmetry of the sugar scaffold that the same glucoside binds the SRIF receptor via the C-1 and C-6 sidechains but the NK-1 receptor via the C-1 and C-2 sidechains. Conversely, L-363,301 and its p-F-Phe⁹ analog both bind their respective receptors (SRIF and SP, respectively) via their i + 1 and i + 2 positions. In the course of this work we discovered that both indole and benzyl side chains, but not heterocycle side chains bind the Trp⁸ binding pocket. In collaboration with Professor E.R. Thornton we explained these results through correlation with the electrostatic potential of the aromatic rings.

Recently, we proposed that the small molecule binding domain of GPCRs ideally accommodates the sidechain projections of β-turns or other similarly oriented and exposed functionalities. From this perspective, the scaffolds become the counterpart to the privileged nature of their receptors.