Relation between the structure and the oxygen acitivation of heme oxygenase reaction

(1)HmuO的氯化血红素复合体是白喉棒杆菌中一种24 kDa的可溶性血红素降解酶，它与HmuO中近端组氨酸残基的中性咪唑发生轴向配位。对未结合的亚铁血红素-HmuO突变复合体的EPR分析表明，His20是HmuO中的近端亚铁血红素配体，这一点被自由亚铁血红素-H20A的共振拉曼光谱结果所证实。当与外源性咪唑结合时，His20A突变体恢复完全催化活性。因此，近端His配体的缺失是导致近端His突变体失活的原因。(2)我们发现大鼠HO-1的R183突变为E或D改变了HO催化的a区域选择性。结果表明，183位精氨酸与血红素丙酸基团的羧酸之间的氢键作用以及末端螺旋的空间位阻效应对α-区域选择性的重要性。(3)大鼠野生型HO-1酶在过氧化氢作用下将血红素降解为马鞭铁。然而，D140A血红素络合物与过氧化氢形成化合物II，不发生血红素降解。D140E突变株能正常降解血红素，但D140N的反应活性与D140A相似。这些结果表明，140位的羧酸盐是激活铁结合氧所必需的。(4)我们测定了大鼠胆绿素还原酶的晶体结构。该结构包含两个结构域：具有核苷酸结合折叠特征的N-末端结构域和C-末端结构域。我们提出了NADPH和胆绿素的结合模式。

(1)The hemin complex of HmuO, a 24-kDa soluble heme degradation enzyme in Corynebacterium diphtheriae, is coordinated axially to a neutral imidazole of a proximal histidine residue in HmuO. EPR of the NO-bound ferrous heme-HmuO mutant complexes reveals His20 as the proximal heme ligand in HmuO, and this is confirmed by resonance Raman result from the ligand free ferrous heme-H20A. When bound with exogenous imidazole, His20A mutant resumes full catalytic activity. Hence, it is the absence of the proximal His ligand, that is responsible for the inactivity of proximal His mutant.(2)We found that the mutation of R183 to E or D of rat HO-1 changes the a-regioselectivity of the HO catalysis. The result shows the importance of the hydrogen bonding interaction between the arginine at position 183 and the carboxylates of the heme propionate group, as well as steric effect of the distal helix, for the a-regioselectivity.(3)Wild-type enzyme of rat HO-1 degrades heme to verdoheme with hydrogen peroxide. However, D140A heme complex forms compound II with hydrogen oeroxide, and no heme degradation occurs. D140E mutant degrades heme normally, but D140N shows reactivity similar to that of D140A. These results indicate that the carboxylate at position 140 is essential to activate the iron-bound oxygen.(4)We determined the crystal structure of rat biliverdine reductase. The structure contains two domains: an N-terminal domain characteristic of a nucleotide binding fold and a C-terminal domain. We proposed modes of binding for NADPH and biliverdin.