Kooter I M, Moguilevsky N, Bollen A, van der Veen L A, Otto C, Dekker H L, Wever R
E.C. Slater Institute, BioCentrum, University of Amsterdam, NL-1018 TV Amsterdam, The Netherlands.
J Biol Chem. 1999 Sep 17;274(38):26794-802. doi: 10.1074/jbc.274.38.26794.
The heme group of myeloperoxidase is covalently linked via two ester bonds to the protein and a unique sulfonium ion linkage involving Met(243). Mutation of Met(243) into Thr, Gln, and Val, which are the corresponding residues of eosinophil peroxidase, lactoperoxidase, and thyroid peroxidase, respectively, and into Cys was performed. The Soret band in the optical absorbance spectrum in the oxidized mutants is now found at approximately 411 nm. Both the pyridine hemochrome spectra and resonance Raman spectra of the mutants are affected by the mutation. In the Met(243) mutants the affinity for chloride has decreased 100-fold. All mutants have lost their chlorination activity, except for the M243T mutant, which still has 15% activity left. By Fourier transform infared difference spectroscopy it was possible to specifically detect the (13)CD(3)-labeled methionyl sulfonium ion linkage. We conclude that the sulfonium ion linkage serves two roles. First, it serves as an electron-withdrawing substituent via its positive charge, and, second, together with its neighboring residue Glu(242), it appears to be responsible for the lower symmetry of the heme group and distortion from the planar conformation normally seen in heme-containing proteins.
髓过氧化物酶的血红素基团通过两个酯键与蛋白质共价连接，并通过一个涉及甲硫氨酸（Met）243的独特锍离子键相连。分别将Met243突变为嗜酸性粒细胞过氧化物酶、乳过氧化物酶和甲状腺过氧化物酶相应的残基苏氨酸（Thr）、谷氨酰胺（Gln）、缬氨酸（Val）以及半胱氨酸（Cys）。现在发现氧化型突变体的光吸收光谱中的Soret带位于约411nm处。突变体的吡啶血色原光谱和共振拉曼光谱均受该突变影响。在Met243突变体中，对氯离子的亲和力降低了100倍。除M243T突变体仍保留15%的活性外，所有突变体均丧失了氯化活性。通过傅里叶变换红外差示光谱法能够特异性检测（13）CD（3）标记的甲硫氨酸锍离子键。我们得出结论，锍离子键发挥两种作用。第一，它通过其正电荷作为吸电子取代基；第二，它与相邻残基谷氨酸（Glu）242一起，似乎是造成血红素基团较低对称性以及偏离含血红素蛋白质中常见平面构象的原因。
