STRUCTURAL AND MECHANISTIC STUDY OF SIROHEME SYNTHASE

四聚吡咯辅助因子参与许多重要的细胞功能 如呼吸作用(血红素)、光合作用(叶绿素)、亚硫酸盐和 亚硝酸盐还原(亚铁血红素)和哺乳动物蛋氨酸合成(维生素 B12)。在人类中，维生素B12缺乏导致了这种疾病 恶性贫血，以及不适当的血红素前体堆积 会导致各种类型的皮肤和血液疾病，称为卟啉症。这个 这些辅因子的复杂生物合成途径始于共同的 前驱物质，并在不同的修改阶段分支，最长 和属于维生素BL2的最复杂的途径(Scott，1993；Battersby， (1994年)。一种多功能酶参与了 维生素BL2与四氢铁血红素生物合成、四氢铁血红素合成酶。这种酶是 负责三个不同的酶反应，卟啉环 甲基化、氢化和铁络合作用。三度空间 用X-射线衍射法解析铁血红素合成酶的结构 技巧。底物和辅因子复合体将提供详细的视图 配体结合所需的关键相互作用。催化剂 将通过定点突变来探索机制，以确定 结构中确定的残基的功能以保持关键 活动站点周围的位置。

Tetrapyrrolic cofactors participate in many critical cellular functions such as respiration (heme), photosynthesis (chlorophyll), sulfite and nitrite reduction (siroheme) and mammalian methionine synthesis (vitamin B12). In humans, a vitamin B12 deficiency contributes to the disease pernicious anemia, and the inappropriate build up of heme precursors causes various types of skin and blood disorders called porphyrias. The complex biosynthetic pathways of these cofactors start with common precursors, and branch at various stages of modification, with the longest and most complex pathway belonging to vitamin Bl2 (Scott, 1993; Battersby, 1994). One multifunctional enzyme participates at the crossroads of vitamin Bl2 and siroheme biosynthesis, siroheme synthase. This enzyme is responsible for three separate enzymatic reactions, porphyrin ring methylation, dyhdrogenation, and iron chelation. The three dimensional structure of siroheme synthase will be solved by X-ray diffraction techniques. Substrate and cofactor complexes will provide a detailed view of the critical interactions necessary for ligand binding. The catalytic mechanism will be probed by site directed mutagenesis to determine the function of residues identified in the structure to hold critical positions around the active sites.