Stepwise phosphorylation mechanisms and signal transmission within a ligand–receptor–Gαβγ-protein-complex

Abstract An extended α-helical structural model of a G-protein-coupled receptor has been combined with a G-protein structure to assess the ternary complex's potential to function as a GTP synthase. The establishment of the structural constraints between the G-protein and the receptor model has been based on the likelihood of a proton signalling mechanism existing within the ternary complex which can conduct a proton from the interior of the receptor to a phosphorylated histidine residue on the Gβ-subunit. The candidate histidine residue is β183. The earlier development of the receptor's action as a ligand-activated trans-membrane proton transducer is concluded to be potentially more extensive in the extended receptor structure, the receptor's proton signalling mechanism being based on a Tyr–Arg–Tyr–Arg–Tyr proton shuttle pentad. Further proton shuttle mechanisms can exist in the Gα,Gβ subunit interface. The final Tyr–Arg–Tyr shuttle triad exhibits an oxygen–oxygen interatomic distance of some 12.0 A in the G-protein crystal structure consistent with the distance predicted previously within a receptor proton shuttle triad. The arrival of a proton on the acid labile histidine phosphoramidate would enable release of the phosphate group and, through an expected S N 2 reaction, allow transfer to the neighbouring arginine residue α201 (Gαt numbering) with the formation of a high energy phosphono-arginine intermediate. Subsequent transport of the phosphate group to the GDP phosphorylation site involving the known movement of the Gα α2-helix at the Gα–Gβ subunit interface would then complete the synthesis through this stepwise phosphorylation. In the general context of cell signalling under more active conditions, the direct phosphorylation mechanism is consistent with the cell requiring more energy input than can be obtained by a GDP/GTP exchange process, the explicit catalytic phosphorylation mechanism being faster than the generally accepted exchange mechanism.