Eliezer D, Wright P E
Department of Molecular Biology, The Scripps Research Institute, La Jolla, CA 92037, USA.
J Mol Biol. 1996 Nov 8;263(4):531-8. doi: 10.1006/jmbi.1996.0596.
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.
多维异核核磁共振光谱已被用于获取在pH 6.1的天然状态下同位素标记的重组抹香鲸脱辅基肌红蛋白的结构信息。已对该蛋白质中大部分残基的主链共振（1HN、15N和13Calpha）进行了归属。在所有已归属区域中，观察到的化学位移所表明的二级结构与一氧化碳全肌红蛋白中的二级结构几乎相同。此外，两种蛋白质的化学位移本身也高度相似。这表明脱辅基肌红蛋白多肽链的大部分采用了一种定义明确的结构，该结构与全肌红蛋白的结构非常相似。然而，由于构象波动，脱辅基蛋白中对应于EF环、F螺旋、FG环和G螺旋起始部分的连续区域的主链共振变宽，无法检测到。我们提出该区域的多肽在类似全蛋白的构象与一种或多种未折叠或部分折叠状态之间交换。这样的模型可以解释当前的核磁共振数据、质谱观察到的电荷状态分布以及诱变的影响。脱辅基肌红蛋白具有许多天然球状蛋白的特征，并不符合熔球态的经典描述。
