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Abstract

The paramagnetic effects on the spin-relaxation rates of 31P nuclei in complexes of porcine muscle adenylate kinase with ATP, GTP, GDP, and AMP were measured in the presence of two dissimilar activating paramagnetic cations, Mn(II) and Co(II), to examine the structures of the enzyme-bound complexes. Experiments were performed exclusively on enzyme-bound complexes to limit contributions to observed
relaxation rates to two exchanging complexes (with and without cation). Measurements were made at three frequencies, 81, 121.5, and 190.2 MHz, and as a function of temperature in the range 5-30 °C to determine the effect of exchange on the observed relaxation rates. Relaxation rates in the E*MnATP, E-MnGTP, and E-MnGDP complexes were shown to be exchange-limited and therefore without structural information. Relaxation rates for the complexes E-CoATP, E-CoGTP, and E-CoGDP were shown to depend on Co(II)-31P distances. Inability to precisely estimate spectral densities arising from electronic relaxation of Co(II) restricts calculations of Co(II)-31P distances in these complexes to upper and lower limits. At the center of these limits, the Co(II)-31P distances of ß- and - in E-CoATP and E-CoGTP, and of ß- (E-CoGDP), are in the range 3.1-3.5 Á appropriate for the first coordination sphere. For all these complexes, the corresponding distance for -P is appreciably larger in the range 3.9-4.S Á. In the quaternary complex, E-MnGDP-AMP, while the 31P relaxation rates of -P and ß- (GDP) were exchange-limited, that for 31P in AMP was only partially exchange limited at 121.5 MHz [because of the longer Mn(II)-31P distance] as evidenced by its frequency dependence and an activation energy of 4 kcal/mol. The ability to measure structure-dependent 31P relaxation rates for AMP in the quaternary complex with both Mn(II) and Co(II) at all the frequencies allowed approximate estimates of the lifetime and correlation time for the Mn(II) complex. On this basis a value of 5.9 Á appears appropriate for the Mn(II)-P(AMP) distance in the complexes E-MnGDP-AMP and E-CoGDP-AMP.