R.H. Davies
School of Pharmacy, Cardiff University Wales

Abstract:

The hypothesis that a G protein ternary complex of the rhodopsin class can function as a  n efficient GTP synthase continues to gain support. A ternary complex model [1] based on crystal structures involving a b1-adrenoceptor adapted from bovine rhodopsin and of a Gas protein from the Gaib-1g2 form led to the identification of five potential acid-base-acid trans-membrane proton relays lying between a-helices I,II,III and VII, capable of delivering a proton some 56 Å towards the cytoplasm under the influence of a monocation [2]. The target residue for this proton signalling mechanism was a predicted phosphorylated ester in the G protein (Gas  Glu 203), release of the metaphosphate group in proximity to the GDP being achieved by delivery of the proton.

Insight into the mechanism of action of the ternary complex can be  obtained from thermodynamic data on partially stimulating ligands ‘in vitro’ on transposing  efficacy (e) and gross binding into a sum of agonist and antagonist binding components. The enthalpy associated with signal amplification in the agonist component using the phenoxypropanolamine partial agonist, prenalterol, is very low suggestive of a diffusion process or of ion channel opening [3,4]. The antagonist component has a very high favourable entropy component in agreement with that of other phenoxypropanolamine antagonists with  a 3000-fold contribution to the potency. The efficacy function e/(1-e) on the thermodynamic scale gives the  enthalpic difference between agonist and antagonist complexes (6.4 ± 1.1 kcal/mol)  consistent with ligand delivery of a proton to the aspartate anion on α-helix II  and resultant stabilisation to the neutral hydrogen bonded form. There is wider compatibility with  the ligand’s shuttle action  between aspartate residues on α-helices III and II  when driven by a monocation.

Prediction of thermodynamic binding  data  can be attempted by molecular dynamics, Monte Carlo, or direct integration methods. With the information now available on the specific proposed mechanism, the accurate prediction of partial agonism using  direct integration methods will be examined.


References:

[1]        Allen BPC, Nederkoorn PHJ, Timmermann H, Timms D, Broadley KJ, Davies RH. J Mol Str:THEOCHEM 859 (2008) 51.
[2]        a) Sakmann M, Noma A, Trautwein W. Nature 303 (1983) 250. b) Soejima M, Noma A, Pflueger Arch 400 (1984) 250.
[3]        Broadley KJ, Sykes SC, Davies RH. Biochem Pharmacol (2010), doi:10.1016/j.bcp.2010.07.044.
[4]        Broadley, KJ, Sykes SC, Davies RH, Adrenoceptors:Structure, Mechanisms, Function: Adv Pharmacol Sci 375, Birkhauser Verlag, Basel (1991).