Two-dimensional NMR characterization of the deoxymyoglobin heme pocket

Traditionally, assigning the heme protein resonances has relied heavily on the comparison of spectra arising from protein reconstituted with specifically deuterated hemes and the native form. Such an approach can identify tentatively the broad, overlapping signals in the Fe(II) high-spin heme protein spectra. Although 2D NMR studies have reported alternative approaches to detect and assign paramagnetic signals, their effectiveness is limited primarily to Fe(III) low-spin systems and still depends upon isotopic labeling results to be definitive. For deoxymyoglobin, the reported 2D techniques have not produced any spin correlation maps. Nevertheless, our study demonstrates that the deoxymyoglobin spin correlations are indeed detectable and that a complete heme assignment, except for the meso protons, is achievable with only 2D NMR and saturation-transfer techniques. The 2D maps improve the spectral resolution dramatically and permit a comprehensive analysis of the deoxymyoglobin signals' temperature dependence, which supports the hypothesis that the electronic orbital ground state has contributions from both 5E and 5B2. The results also indicate a structural perturbation in the vicinity of the 2 vinyl group as the protein undergoes the transition from oxy- to deoxymyoglobin state and a significant contribution from zero field splitting. Moreover, saturation-transfer experiments show that NMR can observe directly oxygen binding kinetics.