Sugishima Masakazu, Sakamoto Hiroshi, Noguchi Masato, Fukuyama Keiichi
Department of Biology, Graduate School of Science, Osaka University, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan.
J Mol Biol. 2004 Jul 30;341(1):7-13. doi: 10.1016/j.jmb.2004.05.048.
Heme oxygenase (HO) catalyzes physiological heme degradation using O(2) and reducing equivalents to produce biliverdin, iron, and CO. Notably, the HO reaction proceeds without product inhibition by CO, which is generated in the conversion reaction of alpha-hydroxyheme to verdoheme, although CO is known to be a potent inhibitor of HO and other heme proteins. In order to probe how endogenous CO is released from the reaction site, we collected X-ray diffraction data from a crystal of the CO-bound form of the ferrous heme-HO complex in the dark and under illumination by a red laser at approximately 35 K. The difference Fourier map indicates that the CO ligand is partially photodissociated from the heme and that the photolyzed CO is trapped in a hydrophobic cavity adjacent to the heme pocket. This hydrophobic cavity was occupied also by xenon, which is similar to CO in terms of size and properties. Taking account of the affinity of CO for the ferrous verdoheme-HO complex being much weaker than that for the ferrous heme complex, the CO derived from alpha-hydroxyheme would be trapped preferentially in the hydrophobic cavity but not coordinated to the iron of verdoheme. This structural device would ensure the smooth progression of the subsequent reaction, from verdoheme to biliverdin, which requires O(2) binding to verdoheme.
血红素加氧酶（HO）利用氧气和还原当量催化生理性血红素降解，生成胆绿素、铁和一氧化碳。值得注意的是，HO反应在没有一氧化碳产物抑制的情况下进行，一氧化碳是在α-羟基血红素向胆绿血红素的转化反应中生成的，尽管一氧化碳是HO和其他血红素蛋白的有效抑制剂。为了探究内源性一氧化碳如何从反应位点释放，我们在黑暗中以及在约35K下用红色激光照射时，从亚铁血红素-HO复合物的一氧化碳结合形式的晶体收集了X射线衍射数据。差分傅里叶图表明，一氧化碳配体从血红素上部分光解离，光解的一氧化碳被困在与血红素口袋相邻的疏水腔中。这个疏水腔也被氙占据，氙在大小和性质上与一氧化碳相似。考虑到一氧化碳对亚铁胆绿血红素-HO复合物的亲和力远低于对亚铁血红素复合物的亲和力，来自α-羟基血红素的一氧化碳将优先被困在疏水腔中，而不会与胆绿血红素的铁配位。这种结构机制将确保后续反应从胆绿血红素顺利进行到胆绿素，这需要氧气与胆绿血红素结合。
