Does the magnesium(II) ion interact with the alpha-phosphate of adenosine triphosphate? An investigation by oxygen-17 nuclear magnetic resonance.

The binding of Mg2+ with adenine nucleotides is an important problem in enzyme-catalyzed phosphoryl transfer reactions. The previously used 31P chemical shift method has been shown to be inadequate to define the chelation pattern of the Mg2+ complex with adenosine 5'-triphosphate (ATP) [Jaffe, E. K., & Cohn, M. (1978) Biochemistry 17, 652-657]. The center of controversy is whether the alpha-phosphate of ATP is involved in chelation. We have recently found that Mg2+ causes the 17O NMR signal of [gamma-17O3]ATP to broaden [Tsai, M.-D., Huang, S. L., Kozlowski, J. F., & Chang, C. C. (1980) Biochemistry 19, 3531-3536]. The 17O NMR method was then used to investigate the binding of Mg2+ with ATP and adenosine 5'-diphosphate (ADP). The results indicate that Mg2+ interacts with both the alpha- and beta-phosphate of ADP, and all the alpha-, beta-, and gamma-phosphates of ATP. The extent of alpha coordination in MgATP may be smaller than the beta and gamma coordination. These results establish the "macroscopic" structure of MgADP and MgATP but have not determined the "microscopic" structures. The site specificity of the line-broadening effect in 17O NMR was supported by the results from substitution-inert Co3+ complexes of ADP and ATP with known structures. The Co3+ coordination also causes a large upfield shift (180-200 ppm). Various experiments were performed to ensure that the nonspecific factors (factors unrelated to the quadrupolar effect of 17O) do not dominate the results, although a small contribution from them cannot be ruled out. The experimental problems and the detailed mechanism of the line-broadening effect in 17O NMR are discussed.