Is apomyoglobin a molten globule? Structural characterization by NMR.

Multi-dimensional heteronuclear NMR spectroscopy has been used to obtain structural information on isotopically labeled recombinant sperm whale apomyoglobin in the native state at pH 6.1. Assignments for backbone resonances (1HN, 15N, and 13Calpha) have been made for a large fraction of the residues in the protein. The secondary structure indicated by the observed chemical shifts is nearly identical to that found in carbonmonoxy-holomyoglobin in all assigned regions. In addition the chemical shifts themselves are highly similar in both proteins. This suggests that the majority of the apomyoglobin polypeptide chain adopts a well defined structure which is very similar to that of holomyoglobin. However, backbone resonances from a contiguous region of the apoprotein, corresponding to the EF loop, the F helix, the FG loop, and the beginning of the G helix, are broadened beyond detection due to conformational fluctuations. We propose that the polypeptide in this region exchanges between a holoprotein-like conformation and one or more unfolded or partially folded states. Such a model can explain the current NMR data, the charge state distributions observed by mass spectrometry, and the effects of mutagenesis. Apomyoglobin possesses many of the characteristics of a native, globular protein and does not adhere to the classical description of a molten globule.