Biostructural chemistry of magnesium ion: characterization of the weak binding sites on tRNA(Phe)(yeast). Implications for conformational change and activity

The thermodynamics and kinetics of magnesium binding to tRNA(Phe)(yeast) have been studied directly by 25Mg NMR. In 0.17 M Na+(aq), tRNA(Phe) exists in its native conformation and the number of strong binding sites (Ka greater than or equal to 10(4)) was estimated to be 3-4 by titration experiments, in agreement with X-ray structural data for crystalline tRNA(Phe) (Jack et al., 1977). The set of weakly bound ions were in slow exchange and 25Mg NMR resonances were in the near-extreme-narrowing limit. The line shapes of the exchange-broadened magnesium resonance were indistinguishable from Lorentzian form. The number of weak magnesium binding sites was determined to be 50 +/- 8 in the native conformation and a total line-shape analysis of the exchange-broadened 25 Mg2+ NMR resonance gave an association constant Ka of (2.2 +/- 0.2) X 10(2) M-1, a quadrupolar coupling constant (chi B) of 0.84 MHz, an activation free energy (delta G*) of 12.8 +/- 0.2 kcal mol-1, and an off-rate (koff) of (2.5 +/- 0.4) X 10(3) s-1. In the absence of background Na+(aq), up to 12 +/- 2 magnesium ions bind cooperatively, and 73 +/- 10 additional weak binding sites were determined. The binding parameters in the nonnative conformation were Ka = (2.5 +/- 0.2) X 10(2) M-1, chi B = 0.64 MHz, delta G* = 13.1 +/- 0.2 kcal mol-1, and koff = (1.6 +/- 0.4) X 10(3) s-1. In comparison to Mg2+ binding to proteins (chi B typically ca. 1.1-1.6 MHz) the lower chi B values suggest a higher degree of symmetry for the ligand environment of Mg2+ bound to tRNA. A small number of specific weakly bound Mg2+ appear to be important for the change from a nonnative to a native conformation. Implications for interactions with the ribosome are discussed.