Lugo-Mas Priscilla, Dey Abhishek, Xu Liang, Davin Steven D, Benedict Jason, Kaminsky Werner, Hodgson Keith O, Hedman Britt, Solomon Edward I, Kovacs Julie A
Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, USA.
J Am Chem Soc. 2006 Aug 30;128(34):11211-21. doi: 10.1021/ja062706k.
Nitrile hydratase (NHase) is one of a growing number of enzymes shown to contain post-translationally modified cysteine sulfenic acids (Cys-SOH). Cysteine sulfenic acids have been shown to play diverse roles in cellular processes, including transcriptional regulation, signal transduction, and the regulation of oxygen metabolism and oxidative stress responses. The function of the cysteine sulfenic acid coordinated to the iron active site of NHase is unknown. Herein we report the first example of a sulfenate-ligated iron complex, Fe(III)(ADIT)(ADIT-O) (5), and compare its electronic and magnetic properties with those of structurally related complexes in which the sulfur oxidation state and protonation state have been systematically altered. Oxygen atom addition was found to decrease the unmodified thiolate Fe-S bond length and blue-shift the ligand-to-metal charge-transfer band (without loss of intensity). S K-edge X-ray absorption spectroscopy and density functional theory calculations show that, although the modified RS-O(-) fragment is incapable of forming a pi bond with the Fe(III) center, the unmodified thiolate compensates for this loss of pi bonding by increasing its covalent bond strength. The redox potential shifts only slightly (75 mV), and the magnetic properties are not affected (the S = (1)/(2) spin state is maintained). The coordinated sulfenate S-O bond is activated and fairly polarized (S(+)-O(-)). Addition of strong acids at low temperatures results in the reversible protonation of sulfenate-ligated 5. An X-ray structure demonstrates that Zn(2+) binds to the sulfenate oxygen to afford [Fe(III)(ADIT)(ADIT-O-ZnCl(3))] (6). The coordination of ZnCl(3)(-) to the RS-O(-) unit causes the covalent overlap with the unmodified thiolate to increase further. A possible catalytic role for the unmodified NHase thiolate, involving its ability to "tune" the electronics in response to protonation of the sulfenate (RS-O(-)) oxygen and/or substrate binding, is discussed.
腈水合酶（NHase）是越来越多被证明含有翻译后修饰的半胱氨酸亚磺酸（Cys-SOH）的酶之一。半胱氨酸亚磺酸已被证明在细胞过程中发挥多种作用，包括转录调控、信号转导以及氧代谢和氧化应激反应的调节。与NHase铁活性位点配位的半胱氨酸亚磺酸的功能尚不清楚。在此，我们报道了首例亚磺酸盐配位的铁配合物[Fe(III)(ADIT)(ADIT-O)]⁺（5），并将其电子和磁性性质与硫氧化态和质子化态已被系统改变的结构相关配合物的性质进行了比较。发现氧原子的添加会缩短未修饰的硫醇盐Fe-S键长度，并使配体到金属的电荷转移带发生蓝移（强度无损失）。S K边X射线吸收光谱和密度泛函理论计算表明，虽然修饰的RS-O⁻片段无法与Fe(III)中心形成π键，但未修饰的硫醇盐通过增加其共价键强度来补偿这种π键的损失。氧化还原电位仅略有变化（75 mV），磁性性质不受影响（保持S = 1/2自旋态）。配位的亚磺酸盐S-O键被激活且相当极化（S⁺-O⁻）。在低温下添加强酸会导致亚磺酸盐配位的5发生可逆质子化。X射线结构表明，Zn²⁺与亚磺酸盐氧结合形成[Fe(III)(ADIT)(ADIT-O-ZnCl₃)]（6）。ZnCl₃⁻与RS-O⁻单元的配位导致与未修饰硫醇盐的共价重叠进一步增加。讨论了未修饰的NHase硫醇盐可能的催化作用，包括其响应亚磺酸盐（RS-O⁻）氧的质子化和/或底物结合来“调节”电子性质的能力。
