Fluorescent nucleotide derivatives as specific probes for the uncoupling protein: thermodynamics and kinetics of binding and the control by pH.

Fluorescent 2'-O-dansylated (DANS) purine nucleotides were synthesized. The fluorescence of the nucleotide derivatives is quenched in aqueous solutions but strongly enhanced on binding to the uncoupling protein (UCP) from brown adipose tissue mitochondria. The fluorescence enhancement was 30-, 10-, and 10-fold for DANSGTP, DANSATP, and DANSADP. One mole of DANS nucleotide binds to 1 mol of dimeric UCP. The binding affinity ranges from 10(5) to 10(8) M-1, similar to that of the unsubstituted nucleotides, while dansylation of AMP increases the affinity 50-fold. The pH dependence in the pKD/pH plots for the DANS nucleotides is basically similar to that for the unsubstituted nucleotides, i.e., for nucleoside diphosphates the slope delta pKD/delta pH < -1 at pH 5-6.5, = -1 at pH > 6.8, and only for triphosphates = -2 at pH > 7.2. Two different protonation sites with a pKH approximately 4 (Asp/Glu) and pKH approximately 7.2 (His), only for nucleoside triphosphates, are suggested to be involved in binding. The higher affinity of DANSGTP indicates additional participation in binding of the C-6 oxygen on the guanine. The binding as measured with the anion exchange method agrees with the fluorescence measurement for DANSGTP, whereas for the more loosely binding DANSATP it is 40% lower. This is interpreted in terms of tight/loose UCP-nucleotide complexes, 100% tight complex for DANSGTP (as well GTP or ATP) but 40% loose complex for DANSATP. By measuring the rapid kinetics using the fluorescence signal, the binding rate is found to be fast and fairly constant for the various nucleotides, whereas the dissociation is slow and strongly nucleotide dependent. The rates are pH dependent with delta pkon/delta pH = 1 for all the nucleotides and delta pkoff/delta pH = -1 for DANSNTP but more weakly with delta pkoff/delta pH < -0.5 for DANSADP and DAN-ATP. The pH dependence of the binding rate corresponds to a protonation at the carboxylate group (Glu/Asp). The high pH dependence of the dissociation rate only for DANSNTP is explained by deprotonation at the HisH+ which is involved only in nucleoside triphosphate binding. This is in line with the very strong pH dependence of nucleoside triphosphate affinity above pH 7 with a delta pKD/delta pH = -2 as an important regulatory mechanism for the H+ transport activity of UCP. The differences of the DANS nucleotides versus the DAN and unsubstituted nucleotides as well as the nucleoside tri- versus diphosphate are rationalized in a specific H+ dependent regulatory mechanism at the binding site.