BETA-GAMMA SUBUNITS OF GUANINE NUCLEOTIDE-BINDING PROTEINS AND REGULATION OF SPONTANEOUS RECEPTOR ACTIVITY - THERMODYNAMIC MODEL FOR THE INTERACTION BETWEEN RECEPTORS AND GUANINE NUCLEOTIDE-BINDING PROTEIN SUBUNITS

被引:0
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作者
ONARAN, HO
COSTA, T
RODBARD, D
机构
[1] NICHHD, THEORET & PHYS BIOL LAB, BETHESDA, MD 20014 USA
[2] NIH, DIV COMP RES & TECHNOL, BETHESDA, MD 20014 USA
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中图分类号
R9 [药学];
学科分类号
1007 ;
摘要
We used a thermodynamic model to examine the interactions between receptor, guanine nucleotide-binding protein (G protein), and their ligands. The model describes the interactions as multiple equilibria occurring between three distinct protein species (receptor, G(alpha) subunit, and G(betagamma) complex) and two small ligands, i.e., agonist (which interacts with receptor) and guanine nucleotide (which binds to G(alpha)). The equilibrium distribution of free and complexed species is determined by the total concentration of the components, the affinities that govern the bimolecular reactions, and the allosteric interactions that ligands exert on each other when they are simultaneously bound to the same species. These allosteric factors are given in terms of free energy coupling. The model explains a number of experimental observations, as follows. (i) Both GTP and GDP can reduce agonist affinity, whereas the agonist enhances the net binding of GTP and diminishes that of GDP. (ii) G(betagamma), is more effective in reducing agonist-independent than agonist-dependent receptor activity. (iii) Removal of guanine nucleotides increases the ratio between agonist-independent and -dependent activation of G protein. The model leads to a number of interesting predictions. (i) Not only G(alpha) but also G(betagamma) has effects on hormone binding. (ii) As long as the distribution of protein species is [G(betagamma)] > [G(alpha)] > [receptor] (as often observed in the cell membrane), small changes in the concentration of G(betagamma), do not alter the overall response induced by agonist. (iii) Agonist activity examined at low concentrations of guanine nucleotide is inevitably different from that observed at high concentrations, typical of intact systems. (iv) Differences in potencies and maximal effects for various guanine nucleotide analogues may reflect differences in their coupling constants that are experimentally measurable. The present model suggests several experimentally testable hypotheses that could be important in elucidating the activation mechanism and regulatory flexibility of G protein-dependent transduction systems.
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页码:245 / 256
页数:12
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