INTERACTIONS OF TYR(377) IN A LIGAND-ACTIVATION MODEL OF SIGNAL TRANSMISSION THROUGH BETA-1-ADRENOCEPTOR ALPHA-HELICES

An alignment of the alpha-helices of the beta1-adrenoceptor on the bacteriorhodopsin structure has produced a hypothesis that the receptor can act as a proton pump during excitation (through helices III, IV, V, VI, and VII) while at the same time, through a correlated gating mechanism, develop an intermittent ion channel (via helices I, II, III, and VII). Proton transfer is achieved through a proton shuttle consisting of Tyr377-Arg156-Tyr157 in which Arg156 is unprotonated in the resting state of the receptor. Activation of Tyr377 produces the initial transfer of a proton to Arg156, while the hydrogen-bond reorganization involved in this activation realigns two aspargine residues 369 and 373 that move toward the interface of helices III and VII. The beta-hydroxyl of the natural ligand via Ser145 together with the para-hydroxyl group of the natural ligand activate the proton transfer, displacing Trp183 and Asn373 from the phenolic oxygen atom of Tyr377 by stronger hydrogen-bond proton donor interactions. An estimate of the enthalpic changes produced by the full agonist, isoprenaline, at the CHARMM modeled hydrogen-bond distances of 2.1-2.2 angstrom where significant proton transfer is not yet expected to take place, gives a minimum value of 5.0-7.0 kcal for the hydrogen-bond reorganization based on 3-21G calculations and scaled values based on 6-31G** hydrogen-bond pairs. The enthalpic change approaches the 6-7 kcal observed in the polar bonding differences between agonist and antagonist components of the partial agonist, prenalterol, on the cardiac beta1-adrenoceptor and in that predicted from agonists and antagonists using a comparative ligand-receptor binding model for turkey erythrocyte beta-adrenoceptor data. In the CHARMM-based model, the loss of this activating energy brings the energetics of interaction close to alternative interactions of the natural ligand catechol moieties at higher levels within the membrane. The unusually high entropy of antagonist binding of phenoxypropanolamine beta1-adrenoceptor agent is qualitatively accounted for. In line with experimental prediction, it is concluded that agonist-ligand conformer-receptor interaction is a dominantly enthalpic process and major alpha-helical movement is not expected in agonist action.