Insights into nucleotide binding in protein kinase A using fluorescent adenosine derivatives

The binding of the methylanthraniloyl derivatives of ATP (mant-ATP), ADP (m ant-ADP), 2'deoxyATP (mant2'deoxyATP), and 3'deoxyATP (mant-3'deoxyATP) to thr catalytic subunit of protein kinase A was studied to gain insights into the mechanism of nucleotide binding. The binding of the mant nucleotides l eads to a large increase in fluorescence energy transfer at 440 nm, allowin g direct measurements of nucleotide affinity. The dissociation constant of mant-ADP is identical to that for ADP, while that for mant-ATP is approxima tely threefold higher than that for ATP. The dissociation constant for mant -3'deoxyATP is approximately fivefold higher than that for 3'deoxyATP while derivatization of 2'deoxyATP does not affect affinity. The time-dependent binding of mant-ATP, mant-2'deoxyATP, and mant-ADP, measured using stopped- flow fluorescence spectroscopy, is best fit to three exponentials. The fast phase is ligand dependent, while the two slower phases are ligand independ ent. The slower phases are similar but not identical in rate, and have oppo site fluorescence amplitudes. Both isomers of mant-ATP are equivalent subst rates, as judged by reversed-phase chromatography, although the rate of pho sphorylation is approximately 20-fold lower than the natural nucleotide. Th e kinetic data are consistent with a three-step binding mechanism in which initial association of the nucleotide derivatives produces a highly fluores cent complex. Either one or two conformational changes can occur after the formation of this binary species, but one of the isomerized forms must have low fluorescence compared to the initial binary complex. These data soundl y attest to the structural plasticity within the kinase core that may be es sential for catalysis. Overall, the mant nucleotides present a useful repor ter system for gauging these conformational changes in light of the prevail ing three-dimensional models for the enzyme.