The process of maturation of multiheme proteins is not yet well known, while that of monoheme ones has been relatively well investigated. Two kinds of partly unfolded tetraheme cytochrome c3 were obtained on overexpression in Shewanella oneidensis TSP-C. These proteins were characterized by circular dichroism and nuclear magnetic resonance spectroscopy. It turned out that the tetraheme architecture, and the fifth and sixth ligand coordination are almost mature, while some parts of the polypeptide are unfolded. The unfolded residues are mainly located in the helix-rich region including heme attachment and axial ligand sites. This suggests that the formation of the heme architecture, coordination of axial ligands and helix formation should be coupled with each other. While the former two can take place automatically, the helix formation would need help by a chaperone-like function in the cytochrome c maturation (Ccm) machinery. It must be working in sulphate-reducing bacteria. The Ccm machinery in S. oneidensis is likely insufficient to help the maturation of proteins with cyclic heme architectures. This is the first report providing an insight into the process of maturation of tetraheme cytochrome c3.

译文

:多血红素蛋白的成熟过程尚未广为人知,而单血红素蛋白的成熟过程已得到了比较充分的研究。过度表达在沙瓦氏假单胞菌TSP-C中获得了两种部分未折叠的四血红素细胞色素c3。这些蛋白质通过圆二色性和核磁共振光谱法表征。事实证明,在多肽的某些部分未折叠的情况下,四血红素的结构以及第五和第六配体的配位几乎已经成熟。未折叠的残基主要位于富含血红素的区域,包括血红素附着和轴向配体位点。这表明血红素结构的形成,轴向配体的配位和螺旋的形成应相互耦合。虽然前两个可以自动发生,但是螺旋的形成将需要细胞色素成熟(Ccm)机制中的类似伴侣分子的功能来帮助。它必须在减少硫酸盐的细菌中起作用。拟南芥中的Ccm机制可能不足以帮助具有环状血红素结构的蛋白质成熟。这是第一份报告,深入介绍了四血红素细胞色素c3的成熟过程。

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