Contrasting catalytic profiles of multiheme nitrite reductases containing CxxCK heme-binding motifs.
Contrasting catalytic profiles of multiheme nitrite reductases containing CxxCK heme-binding motifs.
复制标题
含有 CxxCK 血红素结合基序的多血红素亚硝酸还原酶的催化特性对比。
DOI：
10.1007/s00775-013-1011-7
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发表时间：
2013
期刊：
影响因子：
--
通讯作者：
Doyle RM
中科院分区：
文献类型：
--
作者：
Doyle RM
来自硝化还原硫碱弧菌（TvNiR）和大肠杆菌（EcNrfA）的多血红素细胞色素将亚硝酸盐还原为铵。两种酶都含有His/His连接的血红素以将电子传递到其活性位点，其中Lys连接的血红素具有含有His、Tyr和Arg残基的催化三联体的远端口袋。蛋白膜电化学揭示了这些酶的催化性能的显着差异。TvNiR，而不是EcNrfA，需要还原活化。光谱电化学暗示还原His/His-连接的血红素是该过程的关键，其限制了氢氧化物与活性位点血红素的铁形式结合的速率。描述亚硝酸盐还原EcNrfA的Km随pH值以S形方式变化，这与其通过pKa为7.6的残基的质子化（去质子化）进行的调制一致。该残基被认为是远端口袋中的催化His。与此相反，theKM的亚硝酸盐还原TvNiR的pH值的增加，使不同的功能的机制定义这个参数TvNiR的近似线性下降。在其他方面的催化性能的TvNiR和EcNrfA是相似的，即，pH值的依赖性ofVmax和亚硝酸盐的依赖性的催化电流-电位曲线通过循环伏安法解决，使得这些属性的决定因素似乎是保守的。
The multiheme cytochromes fromThioalkalivibrio nitratireducens(TvNiR) andEscherichia coli(EcNrfA) reduce nitrite to ammonium. Both enzymes contain His/His-ligated hemes to deliver electrons to their active sites, where a Lys-ligated heme has a distal pocket containing a catalytic triad of His, Tyr, and Arg residues. Protein-film electrochemistry reveals significant differences in the catalytic properties of these enzymes. TvNiR, but not EcNrfA, requires reductive activation. Spectroelectrochemistry implicates reduction of His/His-ligated heme(s) as being key to this process, which restricts the rate of hydroxide binding to the ferric form of the active-site heme. TheKMdescribing nitrite reduction by EcNrfA varies with pH in a sigmoidal manner that is consistent with its modulation by (de)protonation of a residue with pKa≈ 7.6. This residue is proposed to be the catalytic His in the distal pocket. By contrast, theKMfor nitrite reduction by TvNiR decreases approximately linearly with increase of pH such that different features of the mechanism define this parameter for TvNiR. In other regards the catalytic properties of TvNiR and EcNrfA are similar, namely, the pH dependence ofVmaxand the nitrite dependence of the catalytic current–potential profiles resolved by cyclic voltammetry, such that the determinants of these properties appear to be conserved.