80680-28-0Relevant articles and documents
13C NMR line shapes of [2-13C]ATP in enzyme complexes and viscous solutions: Glycosidic rotation persists at high viscosities and is arrested in enzyme complexes
Nageswara Rao,Ray, Bruce D.
, p. 1566 - 1573 (2007/10/02)
13C NMR line shapes of [2-13C]ATP bound to a number of ATP-utilizing enzymes and free in solutions of varying viscosities have been investigated. The spin relaxation of this 13C nucleus is mediated by 13C-1H dipolar interaction with the attached proton and the 13C chemical shift ansiotropy (CSA). Since these are second-order tensor interactions, interference terms occur in their relaxation parameters. The line widths and other relaxation-matrix elements are derived, in Redfield notation, for a general CSA tensor and with molecular reorientation assumed isotropic. A measurement of the 13C CSA tensor of [2-13C]AMP was made to facilitate the analysis of the line width data. The experimentally observed line widths in the various enzyme complexes are well represented by the expressions derived, and the rotational correlation times deduced from the data correlate well with the molecular masses of the enzymes used in the study, viz., adenylate kinase, arginine kinase, 3-P-glycerate kinase, creatine kinase, pyruvate kinase, and methionyl tRNA synthetase. Adenylate kinase was also studied with [2-13C]AMP. In striking contrast with the line shapes in the enzyme complexes, the 13C line widths of [2-13C]ATP in viscous solutions of sucrose and glycerol were virtually independent of viscosity. For an increase in the solvent viscosity by a factor of 2300, the line width increased by a factor not more than about 5, although the estimated correlation times for overall rotation are similar to, if not longer than those for the enzymes studied. A similar behavior was observed for the 31P line widths. Addition of Mg(II) made no measurable difference to the viscosity insensitivity of 13C and 31P line widths. The results in viscous solutions are interpreted on the basis that the two large-amplitude internal motions with characteristic times in the nanosecond range, viz., the glycosidic rotation of the adenine base and the mobility of phosphate chain, persist in free ATP even at high viscosities. This reasoning is corroborated by the fact that the 13C line widths of [1′-13C]AMP are much larger in viscous solutions indicating that internal motions involving the ribose moiety are of lower amplitude. A corollary to this interpretation is that internal motions such as the glycosidic rotation and the phosphate chain motion in ATP, although not attenuated in viscous solutions, are nevertheless arrested in the enzyme complexes.
