31P NMR studies of the structure of cation-nucleotide complexes bound to porcine muscle adenylate kinase.

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 degrees 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 beta-P and gamma-P in E.CoATP and E.CoGTP, and of beta-P (E.CoGDP), are in the range 3.1-3.5 A appropriate for the first coordination sphere. For all these complexes, the corresponding distance for alpha-P is appreciably larger in the range 3.9-4.5 A.(ABSTRACT TRUNCATED AT 250 WORDS)