Assignment of the protonated 13C resonances of apo-neocarzinostatin by 2D heteronuclear NMR spectroscopy at natural abundance

Solution NMR Structure Investigation for Releasing Mechanism of Neocarzinostatin Chromophore from the Holoprotein

Holo-neocarzinostatin (holo-NCS) is a complex protein carrying the anti-tumor active enediyne ring chromophore by a scaffold consisting of an immunoglobulin-like seven-stranded anti-parallel beta-barrel. Because of the labile chromophore reflecting its extremely strong DNA cleavage activity and complete stabilization in the complex, holo-NCS has attracted much attention in clinical use as well as for drug delivery systems. Despite many structural analyses for holo-NCS, the chromophore-releasing mechanism to trigger prompt attacks on the target DNA is still unclear. We determined the three-dimensional structure of the protein and the internal motion by multinuclear NMR to investigate the releasing mechanism. The internal motion studied by 13C NMR methine relaxation experiments showed that the complex has a rigid structure for its loops as well as the beta-barrel in aqueous solution. This agrees with the refined NMR solution structure, which has good convergence in the loop regions. We also showed that the chromophore displayed a similar internal motion as the protein moiety. The structural comparison between the refined solution structure and x-ray crystal structure indicated characteristic differences. Based on the findings, we proposed the chromophore-releasing mechanism by a three-state equilibrium, which sufficiently describes both the strong binding and the prompt releasing of the chromophore. We demonstrated that we could bridge the dynamic properties and the static structure features with simple kinetic assumptions to solve the biochemical function.

Release of the neocarzinostatin chromophore from the holoprotein does not require major conformational change of the tertiary and secondary structures induced by trifluoroethanol

Neocarzinostatin is a potent enediyne antitumor antibiotic complex in which a chromophore is noncovalently bound to a carrier protein. The protein regulates availability of the drug by proper release of the biologically active chromophore. To understand the physiological mechanism of the drug delivery system, we have examined the trifluoroethanol (TFE)-induced conformational changes of the protein with special emphasis on their relation to the release of the chromophore from holoneocarzinostatin. The effect of the alpha helix-inducing agent, TFE, on all the beta-sheet neocarzinostatin proteins was studied by circular dichroism, fluorescence, and (1)H NMR studies. By using binding of anilinonaphthalene sulfonic acid as a probe, we observed that the protein exists in a stable, partially structured intermediate state around 45-50% TFE, which is consistent with the results from tryptophan fluorescence and circular dichroism studies. The native state is stable until 20% TFE and is half-converted into the intermediate state at 30% TFE, which starts to collapse beyond 50%. High pressure liquid chromatographic analysis of the release of the chromophore caused by TFE treatment at 0 degrees C suggests that the release process, which occurs below 20% TFE, does not result from an observable conformational change in the protein. Kinetic measurements of the release of chromophore at 25 degrees C reveal that TFE does stimulate the rate of release, which increases sharply at 15% and reaches a maximum at 20% TFE, although no major secondary or tertiary structural change of the carrier protein is observed under these same conditions. Our data suggest that chromophore release results from a fluctuation of the protein structure that is stimulated by TFE. Complete release of the chromophore occurs at TFE concentrations where no overall observable unfolding of the apoprotein is seen. Thus, the results suggest that denaturation of the protein by TFE is not a necessary step for release of the tightly bound chromophore.

Assignment of 13C resonances and analysis of relaxation properties and internal dynamics of pike parvalbumin by 13C-NMR at natural abundance

Pike parvalbumin is an 11.5-kDa globular protein which binds Ca2+ through EF-hand structural motifs. Nearly complete assignment of the protonated 13C resonances has been achieved by means of heteronuclear two-dimensional experiments. The study shows that 13Ca chemical shifts can be very sensitive to localised conformational aspects. To characterise internal dynamics of pike parvalbumin, longitudinal-relaxation and transverse-relaxation rates and 1H-13C NOEs were measured for alpha-carbons at natural abundance by means of two-dimensional NMR spectroscopy. Relaxation data were obtained at a spectrometer frequency of 600 MHz for 69 residues with an even spread along the parvalbumin polypeptide chain. A double approach that included Lipari-Szabo analysis and direct mapping of spectral densities was used to interpret relaxation data in terms of internal dynamics. The former analysis provides valuable information about the overall rotational correlation time and S2 order parameters, while the mapping approach characterises the relative contributions of different motional frequencies. The results suggest that Ca(2+)-loaded pike parvalbumin has a rigid structure, even in the functional regions, i.e., the Ca(2+)-binding loops. The patterns of density-function values are more sensitive to the secondary structure than those of S2. Moreover, depending on the sampling frequency, these patterns reveal different aspects of structure-specific motions.