乳过氧化物酶(1.11.1.7,LPO)是哺乳动物血红素过氧化物酶,存在于哺乳动物的细胞外液,包括血浆,唾液,气道上皮衬里液,鼻衬里液,牛奶,眼泪,胃液,和肠粘膜。为了对入侵的微生物进行先天免疫,LPO利用过氧化氢(H2O2)将硫氰酸根(SCN-)和碘化物(I-)离子转化为氧化化合物次硫氰酸根(OSCN-)和次碘酸根(IO-)。先前确定的LPO与SCN-的配合物的结构,OSC-,和I-显示SCN-和I-在远端血红素腔中占据适当位置作为底物,而OSCN-在远端血红素腔中作为产物抑制剂结合。我们在这里报告了LPO与IO-的络合物的结构,这是LPO将碘化物转化为次碘酸的第一个结构证据。为了获得这个复合体,首先将LPO溶液与H2O2一起孵育,然后与碘化铵溶液混合,通过加入20%(wt/vol)的PEG-3350使络合物结晶。这些晶体用于X射线强度数据收集和结构分析。结构确定揭示了LPO的底物结合通道中存在四个次碘酸离子。除了这些,在不同的外部位置观察到其他六个次碘酸离子。我们推测,远端血红素腔中次碘酸离子的存在会阻止底物结合位点并抑制催化。通过比色底物的活性实验证实了这一点,ABTS(2,2'-氮杂-双(3-乙基苯并噻唑啉-磺酸)),在次碘酸和碘离子的存在。
Lactoperoxidase (1.11.1.7, LPO) is a mammalian heme peroxidase found in the extracellular fluids of mammals including plasma, saliva, airway epithelial lining fluids, nasal lining fluid, milk, tears, gastric juices, and intestinal mucosa. To perform its innate immune action against invading microbes, LPO utilizes hydrogen peroxide (H2 O2 ) to convert thiocyanate (SCN- ) and iodide (I- ) ions into the oxidizing compounds hypothiocyanite (OSCN- ) and hypoiodite (IO- ). Previously determined structures of the complexes of LPO with SCN- , OSCN- , and I- show that SCN- and I- occupy appropriate positions in the distal heme cavity as substrates while OSCN- binds in the distal heme cavity as a product inhibitor. We report here the structure of the complex of LPO with IO- as the first structural evidence of the conversion of iodide into hypoiodite by LPO. To obtain this complex, a solution of LPO was first incubated with H2 O2 , then mixed with ammonium iodide solution and the complex crystallized by the addition of PEG-3350, 20% (wt/vol). These crystals were used for X-ray intensity data collection and structure analysis. The structure determination revealed the presence of four hypoiodite ions in the substrate binding channel of LPO. In addition to these, six other hypoiodite ions were observed at different exterior sites. We surmise that the presence of hypoiodite ions in the distal heme cavity blocks the substrate binding site and inhibits catalysis. This was confirmed by activity experiments with the colorimetric substrate, ABTS (2,2\'-azino-bis(3-ethylbenzthiazoline-sulfonic acid)), in the presence of hypoiodite and iodide ions.