Divergence in nonspecific hydrophobic packing interactions in the apo state, and its possible role in functional specialization of mitochondrial and microsomal cytochrome b5
Divergence in nonspecific hydrophobic packing interactions in the apo state, and its possible role in functional specialization of mitochondrial and microsomal cytochrome b5
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DOI：
10.1021/bi051337m
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发表时间：
2005-11-08
期刊：
影响因子：
2.9
通讯作者：
Benson, DR
中科院分区：
文献类型：
--
作者：
Cowley, AB;Sun, N;Benson, DR
哺乳动物细胞色素B（5）的外线粒体膜亚型（OM B（5））与微粒体亚型（Mc b 5）的区别在于其稳定性显著更高。相比之下，OM和Mc脱辅基细胞色素B（5）（apo-B（5））表现出相似的热力学稳定性。OM B（5）相对于Mc B（5）具有更高的稳定性的主要原因是在71位存在Leu。用相应的Mc B（5）残基（Ser）取代OM B（5）中的Leu-71不仅降低了全蛋白的稳定性，而且显著损害了脱辅基蛋白的稳定性。本文报道的研究旨在阐明Leu-71在相对于Mc b5稳定OM B（5）中所起的作用，其动机是稳定性可能与OM和Mc b5特性的其他差异有关，这些差异对其专门的亚细胞作用很重要。这些研究的结果表明，Leu-71在维持OM apo-b（5）（核心2）的血红素非依赖性折叠核心的结构完整性方面起着至关重要的作用，尽管它位于无序的空血红素结合口袋（核心1）中。Mc apo-B（5）S中核心2的构象完整性并不类似地依赖于71位疏水残基的存在，这为OM B（5）S中不存在的补偿结构特征的进化提供了新的证据。我们建议，Leu-71实现其对OM apo-b5核心2结构的影响，通过参与无序核心1的非特异性疏水性塌陷，模板化的β-片层中的残基的构象限制性侧链，分离两个核心。我们假设，这有额外的效果，保持核心1 OM apo-b（5）的状态比发生在Mc apo-b（5）的状态更紧凑，可能有助于更强的血红素结合，通过限制数量的非天然构象，空血红素结合口袋可以填充。
The outer mitochondrial membrane isoform of mammalian cytochrome b(5) (OM b(5)) is distinguished from the microsomal isoform (Mc b5) by its considerably greater stability. In contrast, OM and Mc apocytochrome b(5) (apo-b(5)) exhibit similar thermodynamic stability. Contributing substantially to the greater stability of OM b(5) relative to that of Mc b(5) is the presence of Leu at position 71. Replacing Leu-71 in OM b(5) with the corresponding Mc b(5) residue (Ser) not only diminishes holoprotein stability but also markedly compromises apoprotein stability. The studies reported herein were undertaken to clarify the role played by Leu-71 in stabilizing OM b(5)s relative to Mc b5s, and were motivated by the possibility that stability is related to other differences in OM and Mc b5 properties that are important for their specialized subcellular roles. The results of these studies show that Leu-71 plays an essential role in maintaining the structural integrity of the heme-independent folding core of OM apo-b(5) (core 2), despite its location in the disordered empty heme-binding pocket (core 1). The conformational integrity of core 2 in Mc apo-b(5)S is not similarly dependent on the presence of a hydrophobic residue at position 71, providing new evidence for evolution of compensating structural features not present in OM b(5)S. We propose that Leu-71 achieves its effect on OM apo-b5 core 2 structure by participating in a nonspecific hydrophobic collapse of disordered core 1, templated by more conformationally restricted side chains of residues in the beta-sheet that separates the two cores. We hypothesize that this has the added effect of maintaining core 1 of OM apo-b(5)s in a state more compact than that which occurs in Mc apo-b(5)s, possibly contributing to stronger heme binding by limiting the number of non-native conformations that the empty heme-binding pocket can populate.