Plant tetrapyrroles, including heme and bilins, are synthesized in plastids. Heme oxygenase (HO) catalyzes the oxidative cleavage of heme to the linear tetrapyrrole biliverdin as the initial step in bilin biosynthesis. Besides the canonical α-helical HO that is conserved from prokaryotes to human, a subfamily of non-canonical dimeric β-barrel HO has been found in bacteria. In this work, we discovered that the Arabidopsis locus AT3G03890 encodes a dimeric β-barrel protein that is structurally related to the putative non-canonical HO and is located in chloroplasts. The recombinant protein was able to bind and degrade heme in a manner different from known HO proteins. Crystal structure of the heme–protein complex reveals that the heme-binding site is in the interdimer interface and the heme iron is co-ordinated by a fixed water molecule. Our results identify a new protein that may function additionally in the tetrapyrrole biosynthetic pathway.

中文翻译：

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拟南芥基因座 AT3G03890 编码与血红素降解有关的二聚体 β-桶蛋白


植物四吡咯，包括血红素和胆素，在质体中合成。血红素加氧酶 (HO) 催化血红素氧化裂解为线性四吡咯胆绿素，这是胆素生物合成的第一步。除了从原核生物到人类保守的经典α-螺旋H2O之外，在细菌中还发现了非经典二聚体β-桶状H2O亚家族。在这项工作中，我们发现拟南芥基因座 AT3G03890 编码一种二聚体 β-桶蛋白，该蛋白在结构上与假定的非经典 HO 相关，并且位于叶绿体中。该重组蛋白能够以不同于已知 HO 蛋白的方式结合并降解血红素。血红素-蛋白质复合物的晶体结构表明，血红素结合位点位于中间二聚体界面，并且血红素铁由固定水分子协调。我们的结果确定了一种可能在四吡咯生物合成途径中额外发挥作用的新蛋白质。