Schmidt P P, Andersson K K, Barra A L, Thelander L, Gräslund A
Department of Biophysics, Arrhenius Laboratories, Stockholm University, S-106 91 Stockholm, Sweden.
J Biol Chem. 1996 Sep 27;271(39):23615-8. doi: 10.1074/jbc.271.39.23615.
Ribonucleotide reductase catalyzes by free radical chemistry the reduction of ribonucleotides to deoxyribonucleotides. The R2 protein of a class 1 ribonucleotide reductase contains a stable tyrosyl radical of neutral phenoxy character, which is necessary for normal enzymatic activity. Here we present the EPR spectra from the tyrosyl free radical in the R2 protein from mouse at 9.62, 115, and 245 GHz. We show that the g-value anisotropy of the mouse R2 radical, when precisely determined from high field EPR spectra, is similar to that of the hydrogen bonded dark stable YD middle dot tyrosyl radical of photosystem II and different from that of the Escherichia coli R2 radical. Because the g-value anisotropy is an important indicator of the hydrogen bonding status of the tyrosyl radical, this result suggests that the mouse R2 radical has its tyrosylate oxygen hydrogen bonded with a D2O exchangeable proton, whereas this hydrogen bond is absent in the E. coli enzyme. It is suggested that the observed proton may be derived from the tyrosine that will become a tyrosyl radical.
核糖核苷酸还原酶通过自由基化学催化核糖核苷酸还原为脱氧核糖核苷酸。1类核糖核苷酸还原酶的R2蛋白含有一个具有中性苯氧基特征的稳定酪氨酸自由基，这是正常酶活性所必需的。在此，我们展示了来自小鼠R2蛋白中酪氨酸自由基在9.62、115和245 GHz下的电子顺磁共振（EPR）光谱。我们表明，当从高场EPR光谱精确测定时，小鼠R2自由基的g值各向异性与光系统II中氢键结合的暗稳定YD·酪氨酸自由基相似，与大肠杆菌R2自由基不同。由于g值各向异性是酪氨酸自由基氢键状态的重要指标，这一结果表明小鼠R2自由基的酪氨酸氧与一个可被重水交换的质子形成氢键，而在大肠杆菌酶中不存在这种氢键。有人认为，观察到的质子可能来自将成为酪氨酸自由基的酪氨酸。
