Elucidation of molecular mechanism of bilin synthesis by ferredoxin-dependent bilin reductases

为了阐明PcyA的分子机制，我们测定了PcyA与反应中间体（18^1,18^2-二氢胆绿素）配合物、PcyA与合成底物（胆绿素XIIIα）配合物以及PcyA E76Q突变蛋白与BV配合物的晶体结构。这些结构研究表明，PcyA-BV中Glu76的羧基与d环乙烯基非常接近，表明OH的存在。这里是π氢键这个键是前面还原d环乙烯基的关键。当Val225被Asp取代时，该突变蛋白与BV复合物的吸收光谱与PcyA-BV的吸收光谱有很大的不同。在V225D-BV中，BV的取向发生倒转，BV结合后不发生诱导配合。这一观察结果提出了一个警告，即在没有确切结构的情况下，解释来自突变蛋白的功能数据。同时，我们表达、纯化和结晶了蓝藻胆绿素还原酶，一种从PcyA还原BV不同位点的酶，我们还制备并结晶了Se-Met形式的胆绿素还原酶，并对这些晶体进行了初步的x射线衍射分析。我们测定了叶绿素分解途径中的关键酶——红叶绿素分解代谢还原酶（RCCR）的晶体结构。其结构与PcyA基本相同，表明RCCR属于铁氧还蛋白依赖性的十亿蛋白还原酶家族。我们还测定了RCCR及其突变蛋白与底物复合物的晶体结构，提出了RCCR的反应机理和活性残基。

In order to elucidate the molecular mechanism of PcyA, we determined the crystal structures of PcyA in complex with the reaction intermediate (18^1,18^2-dihydrobiliverdin), PcyA in complex with the synthetic analog of substrate (biliverdin XIIIα), and PcyA mutant protein of E76Q in complex with BV. These structural studies showed that the carboxyl group of Glu76 in PcyA-BV was in close proximity to the D-ring vinyl group, suggesting the presence of OH...π hydrogen bond here and that this bond is a key for the preceding reduction of the D-ring vinyl group.When Val225 was replaced by Asp, the absorption spectrum of this mutant protein in complex with BV was very different from that of PcyA-BV. The orientation of BV in V225D-BV was inverted and the induced-fit did not occur upon BV binding. This observation presented a cautionary note about interpreting functional data derived from a mutated protein in the absence of its exact structure. In parallel we expressed, purified and crystallized cyanobacterial biliverdin reductase, an enzyme that reduces at different site of BV from PcyA, We also prepared and crystallized the Se-Met form of biliverdin reductase, and performed preliminary X-ray diffraction analyses for these crystals.We determined the crystal structure of red chlorophyll catabolite reductase (RCCR), a key enzyme in the chlorophyll breakdown pathway. The structure was essentially the same as PcyA, implicating that RCCR belongs to the ferredoxin-dependent bilin reductase family. We also determined the crystal structures of RCCR and its mutant protein in complex with substrate, proposing the reaction mechanism as well as the active residues of RCCR.