Krueger R J, Siegel L M
Biochemistry. 1982 Jun 8;21(12):2905-9. doi: 10.1021/bi00541a015.
Spinach ferredoxin-sulfite reductase (SiR) contains one siroheme and one Fe4S4 center per polypeptide subunit. The heme is entirely in the high-spin Fe3+ state in the oxidized enzyme. When SiR is photochemically reduced with ethylenediaminetetraacetate (EDTA)-deazaflavin, the free enzyme and its CN- and CO complexes show changes in absorption spectra associated with the heme even after the heme has been reduced from the Fe3+ to the Fe2+ state. With CO- or CN--SiR, these spectral changes are associated with the appearance of a classical "g = 1.94" type of EPR spectrum characteristic of reduced Fe4S4 centers. The line shapes and exact g values of the g = 1.94 EPR spectra vary with the nature of the ligand bound to the heme Fe. Photoreduction of free SiR results in production of a novel type of EPR signal, with g = 2.48, 2.34, and 2.08 in the fully reduced enzyme; this signal accounts for 0.6 spin per heme. (A small g = 1.94 type EPR signal, representing 0.2 spin per heme, is also found.) These data suggest the presence of a strong magnetic interaction between the siroheme and Fe4S4 centers in spinach SiR, this interaction giving rise to different EPR signals depending on the spin state of the heme Fe in the reduced enzyme.
菠菜铁氧化还原蛋白-亚硫酸盐还原酶（SiR）每个多肽亚基含有一个西罗血红素和一个Fe4S4中心。在氧化态的酶中，血红素完全处于高自旋Fe3+状态。当用乙二胺四乙酸（EDTA）-脱氮黄素对SiR进行光化学还原时，即使血红素已从Fe3+还原为Fe2+状态，游离酶及其CN-和CO复合物的吸收光谱仍会出现与血红素相关的变化。对于CO-或CN-SiR，这些光谱变化与还原型Fe4S4中心特有的经典“g = 1.94”型电子顺磁共振（EPR）光谱的出现有关。g = 1.94 EPR光谱的线形和精确g值随与血红素铁结合的配体性质而变化。游离SiR的光还原导致产生一种新型的EPR信号，在完全还原的酶中g值为2.48、2.34和2.08；该信号每个血红素占0.6个自旋。（还发现了一个小的g = 1.94型EPR信号，每个血红素代表0.2个自旋。）这些数据表明菠菜SiR中的西罗血红素和Fe4S4中心之间存在强磁相互作用，这种相互作用根据还原型酶中血红素铁的自旋状态产生不同的EPR信号。
