Crystal structure of a biliverdin IXalpha reductase enzyme-cofactor complex.
Crystal structure of a biliverdin IXalpha reductase enzyme-cofactor complex.
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DOI：
10.1016/s0022-2836(02)00383-2
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发表时间：
2002-06
影响因子：
5.6
通讯作者：
F. Whitby;J. Phillips;C. Hill;W. McCoubrey;M. Maines
中科院分区：
文献类型：
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作者：
F. Whitby;J. Phillips;C. Hill;W. McCoubrey;M. Maines
胆绿素还原酶（BVR）通过氧化β-烟酰胺腺嘌呤二核苷酸（NADH）或β-烟酰胺腺嘌呤二核苷酸磷酸（NADPH）辅因子，将开放四吡咯胆绿素IXα的γ-甲基桥还原为胆红素，从而催化血红素降解的最后一步。胆红素是哺乳动物主要的胆色素，具有抗氧化和抗补体活性。我们分别在1.2Å和1.5Å分辨率下测定了载脂蛋白BVR及其与NADH配合物的x射线晶体结构。与载脂蛋白酶的独立结构测定一致，BVR由一个n端二核苷酸结合结构域（罗斯曼折叠）和一个c端结构域组成，该结构域包含一个六链β-片，其一面被几个α-螺旋包围。c端和n端结构域广泛相互作用，在它们的界面处形成活性位点裂缝。本文报道的辅因子络合物结构表明，辅因子烟酰胺环延伸到活性位点间隙，在那里它与保守的氨基酸残基相邻，并且与已知的BVR催化反应的立体化学一致，环的si面可以进行氢化物转移。唯一可滴定的侧链，似乎是合适的定位，作为一般酸在催化作用是Tyr97。然而，由于Tyr97Phe突变蛋白保留50%的活性，该残基对催化作用并不是必需的。这一发现表明，辅助因子的氢化物转移可能在催化中起主导作用，这一过程可能是由不变残基Glu96、Glu123和Glu126靠近烟酰胺环而促进的。
Biliverdin reductase (BVR) catalyzes the last step in heme degradation by reducing the γ-methene bridge of the open tetrapyrrole, biliverdin IXα, to bilirubin with the concomitant oxidation of a β-nicotinamide adenine dinucleotide (NADH) or β-nicotinamide adenine dinucleotide phosphate (NADPH) cofactor. Bilirubin is the major bile pigment in mammals and has antioxidant and anticompliment activity. We have determined X-ray crystal structures of apo rat BVR and its complex with NADH at 1.2Å and 1.5Å resolution, respectively. In agreement with an independent structure determination of the apo-enzyme, BVR consists of an N-terminal dinucleotide-binding domain (Rossmann-fold) and a C-terminal domain that contains a six-stranded β-sheet that is flanked on one face by several α-helices. The C-terminal and N-terminal domains interact extensively, forming the active site cleft at their interface. The cofactor complex structure reported here reveals that the cofactor nicotinamide ring extends into the active site cleft, where it is adjacent to conserved amino acid residues and, consistent with the known stereochemistry of the reaction catalyzed by BVR, the si face of the ring is accessible for hydride transfer. The only titratable side-chain that appears to be suitably positioned to function as a general acid in catalysis is Tyr97. This residue, however, is not essential for catalysis, since the Tyr97Phe mutant protein retains 50% activity. This finding suggests that the dominant role in catalysis may be performed by hydride transfer from the cofactor, a process that may be promoted by proximity of the invariant residues Glu96, Glu123, and Glu126, to the nicotinamide ring.