GTP SYNTHASES - PROTON-PUMPING AND PHOSPHORYLATION IN LIGAND-RECEPTOR-G-BETA-ALPHA-PROTEIN COMPLEXES

A structural model for a ligand-receptor-G(s alpha)-protein complex to function as a GTP synthase is presented. The mechanism which is depen dent on the movement and rotation of the G(alpha)-protein alpha(2)-hel ix is seen to involve the delivery of, at least, one proton to the pho sphorylation site in the rotation of this helix. The cycle is driven b y a ligand-mediated proton pump through the alpha-helices of the recep tor, attachment of the conserved Tyr-Arg-Tyr receptor proton shuttle b eing made to an aspartate group on the G(s alpha)-protein terminal sid echain, which is itself linked to the Asn-Gln interaction known to con trol movement and rotation of the alpha(2)-helix between .GDP and .GTP structures. The energetics of proton transfer through the shuttle mec hanism and delivery of a proton to the aspartate group are shown to be sufficient to rupture this controlling interaction and its associated backbone bond. The complex leads to full spatial and energetic defini tion of the receptor proton shuttle mechanism, while there is a striki ng association of further Tyrosine and Arginine residues in the vicini ty of the G(s alpha)-protein Asn-Gln interaction. Calculations at the HF 6-31G* level confirm that a critical balance between ion pair and neutral forms of Tyr-Arg interactions under multiply hydrogen bonded c onditions in a hydrophobic environment controls proton transfer and re covery mechanisms. The intrinsic preference of the neutral Tyr-Arg for m over the ion-pair is 14.0 kcal/mol. Activation of the Tyrosine oxyge n atom in the neutral form by single -NH or -OH groups reduces this di fference by some 6.4-8.6 kcal/mol but the dominance of the neutral for m is maintained. The expected slight overestimates are consistent with the maximum activation enthalpy of 11.0-12.0 kcal/mol required to ini tiate proton transfer through the shuttle. The extended form of the sh uttle with die Arginine acting competitively between the two Tyrosine residues allows interpretation of observed enthalpic differences in li gand binding with and without the presence of GTP. The uniqueness of G s proteins among the G alpha-proteins is seen as their inability to tr ansfer a proton directly through the alpha(2)-helix switch Asn-Gln res idues. A possible proton pathway to the mid-point of the G(s alpha)-pr otein alpha(2) helix is outlined.