Vallone Beatrice, Nienhaus Karin, Matthes Annemarie, Brunori Maurizio, Nienhaus G Ulrich
Department of Biochemical Sciences and Istituto Pasteur-Fondazione Cenci Bolognetti, University of Rome La Sapienza, Piazzale A. Moro 5, 00185 Rome, Italy.
Proc Natl Acad Sci U S A. 2004 Dec 14;101(50):17351-6. doi: 10.1073/pnas.0407633101. Epub 2004 Nov 17.
Neuroglobin (Ngb), a globular heme protein expressed in the brain of vertebrates, binds oxygen reversibly, with an affinity comparable to myoglobin (Mb). Despite low sequence identity, the overall 3D fold of Ngb and Mb is very similar. Unlike in Mb, in Ngb the sixth coordination position of the heme iron is occupied by the distal histidine, in the absence of an exogenous ligand. Endogenous ligation has been proposed as a unique mechanism for affinity regulation and ligand discrimination in heme proteins. This peculiarity might be related to the still-unknown physiological function of Ngb. Here, we present the x-ray structure of CO-bound ferrous murine Ngb at 1.7 A and a comparison with the 1.5-A structure of ferric bis-histidine Ngb. We have also used Fourier transform IR spectroscopy of WT and mutant CO-ligated Ngb to examine structural heterogeneity in the active site. Upon CO binding, the distal histidine retains (by and large) its position, whereas the heme group slides deeper into a preformed crevice, thereby reshaping the large cavity ( approximately 290 A(3)) connecting the distal and proximal heme sides with the bulk. The heme relocation is accompanied by a significant decrease of structural disorder, especially of the EF loop, which may be the signal whereby Ngb communicates hypoxic conditions. This unexpected structural change unveils a heme-sliding mechanism of affinity control that may be of significance to understanding Ngb's role in the pathophysiology of the brain.
神经球蛋白（Ngb）是一种在脊椎动物大脑中表达的球状血红素蛋白，它能可逆地结合氧气，其亲和力与肌红蛋白（Mb）相当。尽管Ngb和Mb的序列同一性较低，但它们的整体三维折叠结构非常相似。与Mb不同的是，在没有外源性配体的情况下，Ngb中血红素铁的第六个配位位置被远端组氨酸占据。内源性配位被认为是血红素蛋白中亲和力调节和配体识别的独特机制。这种特殊性可能与Ngb仍不清楚的生理功能有关。在这里，我们展示了一氧化碳结合的亚铁小鼠Ngb在1.7埃分辨率下的X射线结构，并与三价双组氨酸Ngb的1.5埃分辨率结构进行了比较。我们还利用野生型和突变型一氧化碳结合的Ngb的傅里叶变换红外光谱来研究活性位点的结构异质性。一氧化碳结合后，远端组氨酸（总体上）保持其位置，而血红素基团则更深地滑入一个预先形成的裂隙中，从而重塑了连接远端和近端血红素侧与主体的大腔（约290埃³）。血红素的重新定位伴随着结构无序性的显著降低，尤其是EF环的无序性，这可能是Ngb传递缺氧状态的信号。这种意想不到的结构变化揭示了一种亲和力控制的血红素滑动机制，这对于理解Ngb在大脑病理生理学中的作用可能具有重要意义。
