Advantages of 15N and Deuterium Spin Probes for Biomedical Electron Paramagnetic Resonance Investigations

Nucleotide binding to the heat-shock protein DnaK as studied by ESR spectroscopy

We employed ESR spectroscopy using spin-labeled adenine nucleotides to investigate nucleotide binding to the 70-kDa heat shock protein, DnaK, from Escherichia coli. Binding stoichiometries of 1 mol/ mol for both ATP and ADP to previously nucleotide-depleted protein in the presence of Mg2+ were determined directly and under equilibrium binding conditions. Of the spin-labeled adenine nucleotides available to us, only the derivatives with the spin label attached to the C8 position of the adenine moiety, 8-SL-AdoP3 and 8-SL-AdoP2 [8-(2,2,6,6-tetramethyl-piperidin-4-yl -1-oxyl-)amino-adenosine-5'-triphosphate or diphosphate], were bound sufficiently tightly by the heat-shock protein, resulting in ESR spectra typical for immobilized radicals. In the absence of Mg2+, only approximately 0.5 mol were bound. Subsequent addition of Mg2+, however, led to the previously observed maximum binding of 1 mol/mol. Both 8-SL-AdoP3 and 8-SL-AdoP2 were fully exchangeable upon addition of excess ATP or ADP suggesting that the analogs bound directly to the nucleotide binding sites within the protein. 8-SL-AdoP2 release was also observed in the presence of the co-chaperone GrpE, indicating that the spin-labeled analogs of adenine nucleotides function like the natural nucleotide-substrates of the heat-shock protein. Small differences in the ESR spectra of 8-SL-AdoP3 and 8-SL-AdoP2 in complex with DnaK were observed.

Binding of ATP to uncoupling protein of brown fat mitochondria as studied by means of spin-labeled ATP derivatives

ATP derivatives spin-labeled (SL) at C8, N6, C2' or C3' were employed in binding studies with the uncoupling protein of brown fat mitochondria. Substitution of the ribose strongly impaired binding, whereas labeling of the adenine moiety allowed for tight and functional complex formation. Detailed binding studies with C8-SL-ATP confirmed the known pH and Mg2+ dependence with a stoichiometry of one C8-SL-ATP bound per 66 kDa dimer. Corresponding studies of the uncoupling protein after modification with N-ethylmaleimide or diazobenzene-4-sulfonic acid revealed distinct differences in their effects on nucleotide binding and gating.