Lindahl P A, Day E P, Kent T A, Orme-Johnson W H, Münck E
J Biol Chem. 1985 Sep 15;260(20):11160-73.
We have studied the Fe protein (Av2) of the Azotobacter vinelandii nitrogenase system with Mössbauer and EPR spectroscopies and magnetic susceptometry. In the oxidized state the protein exhibits Mössbauer spectra typical of diamagnetic [4Fe-4S]2+ clusters. Addition of Mg.ATP or Mg.ADP causes a pronounced decline in the quadrupole splitting of the Mössbauer spectra of the oxidized protein. Our studies show that reduced Av2 in the native state is heterogeneous. Approximately half of the molecules contain a [4Fe-4S]1+ cluster with electronic spin S = 1/2 and half contain a [4Fe-4S]1+ cluster with spin S = 3/2. The former yields the characteristic g = 1.94 EPR signal whereas the latter exhibits signals around g = 5. The magnetization of reduced Av2 is dominated by the spin S = 3/2 form of its [4Fe-4S]1+ clusters. These results explain a long standing puzzle, namely why the integrated spin intensity of the g = 1.94 EPR signal is substantially less than 1 spin/4 Fe atoms. In 50% ethylene glycol, 90% of the clusters are in the spin S = 1/2 form whereas, in 0.4 M urea, 85% are in the S = 3/2 form. In 0.4 M urea, the EPR spectrum of reduced Av2 exhibits well defined resonances at g = 5.8 and 5.15, which we assign to the S = 3/2 system. The EPR and Mössbauer studies yield a zero-field splitting of 2D approximately equal to -5 cm-1 for this S = 3/2 state.
我们利用穆斯堡尔谱、电子顺磁共振光谱和磁 susceptibility 测量技术研究了棕色固氮菌固氮酶系统的铁蛋白（Av2）。在氧化态下，该蛋白呈现出抗磁性[4Fe-4S]2+簇典型的穆斯堡尔谱。添加 Mg.ATP 或 Mg.ADP 会导致氧化态蛋白的穆斯堡尔谱的四极分裂显著下降。我们的研究表明，天然状态下还原的 Av2 是异质的。大约一半的分子含有电子自旋 S = 1/2 的[4Fe-4S]1+簇，另一半含有自旋 S = 3/2 的[4Fe-4S]1+簇。前者产生特征性的 g = 1.94 的电子顺磁共振信号，而后者在 g = 5 附近呈现信号。还原的 Av2 的磁化主要由其[4Fe-4S]1+簇的自旋 S = 3/2 形式主导。这些结果解释了一个长期存在的谜题，即为什么 g = 1.94 的电子顺磁共振信号的积分自旋强度远小于 1 自旋/4 个铁原子。在 50%的乙二醇中，90%的簇处于自旋 S = 1/2 形式，而在 0.4 M 的尿素中，85%处于 S = 3/2 形式。在 0.4 M 的尿素中，还原的 Av2 的电子顺磁共振谱在 g = 5.8 和 5.15 处呈现出明确的共振，我们将其归因于 S = 3/2 系统。对于这个 S = 3/2 态，电子顺磁共振和穆斯堡尔研究得出的零场分裂 2D 约等于 -5 cm-1。
