Voevodskaya N, Narvaez A-J, Domkin V, Torrents E, Thelander L, Gräslund A
Department of Biochemistry and Biophysics, Stockholm University, SE-106 91 Stockholm, Sweden.
Proc Natl Acad Sci U S A. 2006 Jun 27;103(26):9850-4. doi: 10.1073/pnas.0600603103. Epub 2006 Jun 15.
Ribonucleotide reductase (RNR) from Chlamydia trachomatis is a class I RNR composed of proteins R1 and R2. In protein R2, the tyrosine residue harboring the radical that is necessary for catalysis in other class I RNRs is replaced by a phenylalanine. Active C. trachomatis RNR instead uses the Fe(III)-Fe(IV) state of the iron cluster in R2 as an initiator of catalysis. The paramagnetic Fe(III)-Fe(IV) state, identified by (57)Fe substitution, becomes electron spin resonance detectable in samples that are frozen during conditions of ongoing catalysis. Its amount depends on the conditions for catalysis, such as incubation temperature and the R1/R2 ratio. The results link induction of the Fe(III)-Fe(IV) state with enzyme activity of chlamydial RNR. Based on these observations, a reaction scheme is proposed for the iron site. This scheme includes (i) an activation cycle involving reduction and an oxygen reaction in R2 and (ii) a catalysis cycle involving substrate binding and turnover in R1.
沙眼衣原体的核糖核苷酸还原酶（RNR）是一种由R1和R2蛋白组成的I类RNR。在R2蛋白中，其他I类RNR催化所需的带有自由基的酪氨酸残基被苯丙氨酸取代。活性沙眼衣原体RNR反而利用R2中铁簇的Fe(III)-Fe(IV)状态作为催化的起始点。通过（57）Fe取代鉴定的顺磁性Fe(III)-Fe(IV)状态，在正在进行催化的条件下冷冻的样品中可通过电子自旋共振检测到。其含量取决于催化条件，如孵育温度和R1/R2比例。这些结果将Fe(III)-Fe(IV)状态的诱导与衣原体RNR的酶活性联系起来。基于这些观察结果，提出了铁位点的反应方案。该方案包括（i）一个涉及R2中还原和氧反应的活化循环，以及（ii）一个涉及R1中底物结合和周转的催化循环。
