Four proteins have been identified recently as diiron carboxylate proteins on the basis of conservation of six amino acids (four carboxylate residues and two histidines) constituting an iron-binding motif. Unlike previously identified proteins with this motif, biochemical studies indicate that each of these proteins is membrane bound, although homology modeling rules out a transmembrane mode of binding. Therefore, the predicted structure of each protein [the alternative oxidase (AOX), the plastid terminal oxidase (PTOX), the diiron 5-demethoxyquinone hydroxylase (DMQ hydroxylase), and the aerobic Mg-protoporphyrin IX monomethylester hydroxylase (MME hydroxylase)] is that of a protein bound monotopically to one leaflet of the membrane bilayer. Three of these enzymes utilize a quinol substrate, with two oxidizing the quinol (AOX and PTOX) and one hydroxylating it (DMQ hydroxylase). MME hydroxylase is involved in synthesis of the isocyclic ring of chlorophyll. Two enzymes are involved in respiration (AOX and, indirectly, the diiron DMQ hydroxylase through ubiquinone biosynthesis) and two in photosynthesis, through their roles in carotenoid and chlorophyll biosynthesis (PTOX and MME hydroxylase, respectively). We discuss what is known about each enzyme as well as our expectations based on their identification as interfacially bound proteins with a diiron carboxylate active site.

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膜结合二甲基羧化蛋白

基于构成铁结合基序的六个氨基酸（四个羧酸盐残基和两个组氨酸）的保守性，最近已将四种蛋白质鉴定为羧酸二铁蛋白。与先前鉴定的具有该基序的蛋白质不同，生化研究表明这些蛋白质中的每一种都是膜结合的，尽管同源性建模排除了跨膜结合模式。因此，每种蛋白质 [替代氧化酶 (AOX)、质体末端氧化酶 (PTOX)、二铁 5-去甲氧基醌羟化酶 (DMQ 羟化酶) 和需氧镁原卟啉 IX 单甲基酯羟化酶 (MME 羟化酶)] 的预测结构为一种蛋白质单局部结合到膜双层的一个小叶上。其中三种酶利用羟基喹啉底物，两个氧化醌醇（AOX 和 PTOX），一个羟基化它（DMQ 羟化酶）。MME羟化酶参与叶绿素同环的合成。两种酶参与呼吸（AOX 和通过泛醌生物合成间接参与的二铁 DMQ 羟化酶）和光合作用，通过它们在类胡萝卜素和叶绿素生物合成中的作用（分别为 PTOX 和 MME 羟化酶）。我们讨论了对每种酶的了解以及我们基于它们被鉴定为具有羧酸二铁活性位点的界面结合蛋白的预期。通过它们在类胡萝卜素和叶绿素生物合成（分别为 PTOX 和 MME 羟化酶）中的作用。我们讨论了对每种酶的了解以及我们基于它们被鉴定为具有羧酸二铁活性位点的界面结合蛋白的预期。通过它们在类胡萝卜素和叶绿素生物合成（分别为 PTOX 和 MME 羟化酶）中的作用。我们讨论了对每种酶的了解以及我们基于它们被鉴定为具有羧酸二铁活性位点的界面结合蛋白的预期。