Mechanism of porphyrin ring cleavage in the heme oxygenase catalysis

血红素加氧酶（HO）通过三次连续氧化将血红素降解为胆绿素。虽然第三个氧合，开环的绿血红素，被认为是速率决定步骤，以调节HO酶的活性在体内，这一步骤已被最少的理解Ho催化。本研究的目的是通过详细的反应分析、晶体学和光谱学表征来描述HO的第三次氧化。在经典的HO反应中，我们发现HO通过O_2或H_2O_2的双途径降解绿血红素。连同抑制和突变的研究，我们提出了一个机制，涉及一个Fe-OOH verdoheme作为反应中间体。这一建议是有力支持的事实，即大环裂解与小烷基氢过氧化物（MeOH和EtOOH）纳入的烷基到一端的线性tetrapyrroles. Reactive过氧物种的晶体学特征首次在肌红蛋白，血红素酶的模型蛋白质。过氧肌红蛋白的晶体已成功地产生的氧肌红蛋白晶体在100 K的辐射还原与1.0 A同步辐射照射。用0.6A同步辐射收集衍射数据，不经过氧化反应，化学合成了新的血红素分解产物S-胆绿素，并通过LC-MS和NMR分析鉴定了其结构。在新的HO反应中，绿血红素的裂解通过不依赖于O2的方式进行，S-胆绿素与胆绿素的产物比在很大程度上取决于O2浓度。这些机制的变化预计会影响生理功能，即HO的O2传感特性。

Heme oxygenase (HO) degrades heme to biliverdin through three successive oxygenations. While the third oxygenation, ring opening of verdoheme, is considered as the rate-determining step to regulate HO enzyme activity in vivo, this step has been the least understood in the Ho catalysis. Specific aim of this research project is to delineate the third oxygenation by HO through detailed reaction analysis, crystallographic and spectroscopic characterization. We also have examined a new HO reaction that affords a novel heme catabolite under physiologically relevant condition.In the classical HO reaction, we have found that HO degrades verdoheme through a dual pathway using either O_2 or H_2O_2. Together with inhibitory and mutational studies, we have proposed a mechanism involving an Fe-OOH verdoheme as a reactive intermediate. This proposal is strongly supported by the fact that the macrocycle cleavage with small alkyl hydroperoxides (MeOOH and EtOOH) incorporated the alkyl group into one end of linear tetrapyrroles produced.The reactive peroxy species is crystallographically characterized for the first time in myoglobin, a model protein of heme enzymes. Crystals of the peroxy myoglobin have been successfully generated by radiolytic reduction of the oxy myoglobin crystals at 100 K by irradiation with 1.0 A synchrotron radiation. Diffraction data is collected by 0.6 A synchrotron radiation without further reaction of the peroxy species.The new heme catabolite, S-biliverdin, has been chemically synthesized and identified by LC-MS and NMR analysis. The verdoheme cleavage in the new HO reaction proceeds through an O2-independent manner and the product ratio of S-biliverdin over biliverdin largely depends on the O2 concentration. These mechanistic changes are expected to affect on the physiological functions, namely an O2 sensing property of HO.