Fluoresceinyl-ethylenediamine-ouabain detects an acidic environment in the cardiac glycoside binding site of Na+/K+-ATPase

A structural rearrangement of the Na+/K+-ATPase traps ouabain within the external ion permeation pathway

With the use of the energy of ATP hydrolysis, the Na+/K+-ATPase is able to transport across the cell membrane Na+ and K+ against their electrochemical gradients. The enzyme is strongly inhibited by ouabain and its derivatives, some that are therapeutically used for patients with heart failure (cardiotonic steroids). Using lanthanide resonance energy transfer, we trace here the conformational changes occurring on the external side of functional Na+/K+-ATPases induced by the binding of ouabain. Changes in donor/acceptor pair distances are mainly observed within the α subunit of the enzyme. To derive a structural model matching the experimental lanthanide resonance energy transfer distances measured with bound ouabain, we carried out molecular dynamics simulations with energy restraints applied simultaneously using a novel methodology with multiple non-interacting fragments. The restrained simulation, initiated from the X-ray structure of the E2(2K+) state, became strikingly similar to the X-ray structure of the sodium-bound state. The final model shows that ouabain is trapped within the external ion permeation pathway of the pump.

Ouabain binding site in a functioning Na+/K+ ATPase

The Na(+)/K(+) ATPase is an almost ubiquitous integral membrane protein within the animal kingdom. It is also the selective target for cardiotonic derivatives, widely prescribed inhibitors for patients with heart failure. Functional studies revealed that ouabain-sensitive residues distributed widely throughout the primary sequence of the protein. Recently, structural work has brought some consensus to the functional observations. Here, we use a spectroscopic approach to estimate distances between a fluorescent ouabain and a lanthanide binding tag (LBT), which was introduced at five different positions in the Na(+)/K(+) ATPase sequence. These five normally functional LBT-Na(+)/K(+) ATPase constructs were expressed in the cell membrane of Xenopus laevis oocytes, operating under physiological internal and external ion conditions. The spectroscopic data suggest two mutually exclusive distances between the LBT and the fluorescent ouabain. From the estimated distances and using homology models of the LBT-Na(+)/K(+) ATPase constructs, approximate ouabain positions could be determined. Our results suggest that ouabain binds at two sites along the ion permeation pathway of the Na(+)/K(+) ATPase. The external site (low apparent affinity) occupies the same region as previous structural findings. The high apparent affinity site is, however, slightly deeper toward the intracellular end of the protein. Interestingly, in both cases the lactone ring faces outward. We propose a sequential ouabain binding mechanism that is consistent with all functional and structural studies.

Convergent strategies for the attachment of fluorescing reporter groups to peptide nucleic acids in solution and on solid phase

The site-selective conjugation of peptide nucleic acids (PNA) with fluorescent reporter groups is essential for the construction of hybridisation probes that can report the presence of a particular DNA sequence. This paper describes convergent methods for the solution- and solid-phase synthesis of multiply labelled PNA oligomers. The solid-phase synthesis of protected PNA enabled the selective attachment of fluorescent labels at the C-terminal end (3' in DNA) which demonstrated that further manipulations on protected PNA fragments are feasible. For the conjugation to internal sites, a method is introduced that allows for the on-resin assembly of modified monomers thereby omitting the need to synthesise an entire monomer in solution. Furthermore, it is shown that the application of a highly orthogonal protecting group strategy in combination with chemoselective conjugation reactions provides access to a rapid and automatable solid-phase synthesis of dual labelled PNA probes. Real-time measurements of nucleic acid hybridisation were possible by taking advantage of the fluorescence resonance energy transfer (FRET) between suitably appended fluorophoric groups. Analogously to DNA-based molecular beacons, the dual labelled PNA probes were only weakly fluorescing in the single-stranded state. Hybridisation to a complementary oligonucleotide, however, induced a structural reorganisation and conferred a vivid fluorescence enhancement.

Effects of fluorescent pseudo-ATP and ATP-metal analogs on secondary structure of Na(+)/K(+)-ATPase

The secondary structure of Na(+)/K(+)-ATPase after modification of the ATP-binding sites was analyzed. Consistently with recent reports, we found in trypsin-treated Na(+)/K(+)-ATPase additionally to alpha-helix also beta-sheet structures in the transmembrane segments. However, binding of fluorescein 5'-isothiocyanate (FITC), the pseudo-ATP analog, to the ATP-binding site did not affect the secondary structure of undigested Na(+)/K(+)-ATPase. Consequently, fluorescence intensity changes of FITC-labeled Na(+)/K(+)-ATPase commonly used to observe conformational transitions of the enzyme reflect physiological changes of the native structure. The metal complex analogues of ATP, Cr(H(2)O)(4)ATP and Co(NH(3))(4)ATP, on the other hand, affected the secondary structure of Na(+)/K(+)-ATPase. We propose that these changes in the secondary structure are responsible for inhibition of backdoor phosphorylation.

Microenvironment of the high affinity ATP-binding site of Na+/K+-ATPase is slightly acidic

Fluorescein-5'-isothiocyanate (FITC) was used to study the high-affinity ATP-binding site of Na+/K+-ATPase. The molar ratio of specifically bound FITC per alpha-subunit of Na+/K+-ATPase was found to be 0.5 as followed from pretreatment experiments with another specific E1ATP-inhibitor Cr(H2O)4AdoPP[CH2]P. This indicated an existence of one high affinity ATP-binding site (E1ATP-binding site) in the native (alphabeta)2-diprotomer of Na+/K+-ATPase. Fluorescence dual-excitation ratio of specifically bound FITC revealed that at external pH 7.5, the pH value inside the E1ATP-binding site is 6.95 +/- 0.18. In addition, FITC fluorescence quenching by anti-fluorescein and by iodide choline indicated the limited access of water into the small pocket of the E1ATP-binding site.

Molecular distance measurements reveal an (alpha beta)2 dimeric structure of Na+/K+-ATPase. High affinity ATP binding site and K+-activated phosphatase reside on different alpha-subunits

ATP hydrolysis by Na+/K+-ATPase proceeds via the interaction of simultaneously existing and cooperating high (E1ATP) and low (E2ATP) substrate binding sites. It is unclear whether both ATP sites reside on the same or on different catalytic alpha-subunits. To answer this question, we looked for a fluorescent label for the E2ATP site that would be suitable for distance measurements by Förster energy transfer after affinity labeling of the E1ATP site by fluorescein 5'-isothiocyanate (FITC). Erythrosin 5'-isothiocyanate (ErITC) inactivated, in an E1ATP site-blocked enzyme (by FITC), the residual activity of the E2ATP site, namely K+-activated p-nitrophenylphosphatase in a concentration-dependent way that was ATP-protectable. The molar ratios of FITC/alpha-subunit of 0.6 and of ErITC/alpha-subunit of 0.48 indicate 2 ATP sites per (alpha beta)2 diprotomer. Measurements of Förster energy transfer between the FITC-labeled E1ATP and the ErITC-labeled or Co(NH3)4ATP-inactivated E2ATP sites gave a distance of 6.45 +/- 0.64 nm. This distance excludes 2 ATP sites per alpha-subunit since the diameter of alpha is 4-5 nm. Förster energy transfer between cardiac glycoside binding sites labeled with anthroylouabain and fluoresceinylethylenediamino ouabain gave a distance of 4.9 +/- 0.5 nm. Hence all data are consistent with the hypothesis that Na+/K+-ATPase in cellular membranes is an (alpha beta)2 diprotomer and works as a functional dimer (Thoenges, D., and Schoner, W. (1997) J. Biol. Chem. 272, 16315-16321).