Functional Implications of Heme-packing Motives in c-type Cytochromes

细胞色素是一种普遍存在的金属蛋白，作为氧化还原酶或电子载体在各种生物代谢途径中发挥着重要作用。其中，c亚型的特点是辅因子血红素共价附着在蛋白链上。细胞色素c通常含有大量与蛋白链相关的辅助因子，作为电子载体或氧化还原酶在细菌代谢途径中起着至关重要的作用。因此，辅助因子的排列显示血红素在不同蛋白质之间的包装的重复动机，即使在没有任何可检测的序列同源性。由于这些填充动机是进化保守的，因此具有明显的功能意义，但由于多个金属中心的强耦合，它们的性质难以用光谱方法研究。为了便于功能分析，我们已经开始从硫还原地杆菌或其近亲的富含细胞色素c的基因组中确定可能的血红素-血红素相互作用动机的二血红素模型系统，这些系统可用于突变研究并具有可解释的光谱特性。我们获得了第一个模型体系DHC2，确定了它的晶体结构并开始了光谱分析。在这里，我们建议进一步识别和表征合适的模型蛋白，并在系统的突变研究中分析它们的特性，相关光谱，电化学和晶体学分析。从简单的模型系统开始，我们希望对血红素相互作用有一个基本的了解，并随后将这些知识转移到更大更复杂的蛋白质上。

Cytochromes are ubiquitous metalloproteins that play an essential role as redox enzymes or electron carriers in various metabolic pathways of organisms from all kingdoms of life. Among them, the subtype c is characterized by a covalent attachment of the cofactor heme to the protein chain. Cytochromes c often contain a large number of cofactors relative to the protein chain and play crucial roles in bacterial metabolic pathways, either as electron carriers or as redox enzymes. Hereby the arrangement of cofactors shows recurring motives of heme packing among different proteins, even in the absence of any detectable sequence homology. Being evolutionarily con-served, these packing motives are obviously of functional significance, but due to the strong cou-pling of multiple metal centers their properties are difficult to investigate using spectroscopic methods. In order to facilitate functional analyses we have set out to identify possible dihemic model systems for heme-heme interaction motives from the cytochrome-c-rich genome of Geobacter sulfurreducens or its relatives which are accessible to mutational studies and have interpretable spectroscopic properties. We have obtained a first model system, DHC2, determined its crystal structure and initiated spectroscopic analyses. Here we propose to identify and characterize further suitable model proteins and analyze their properties in a systematic, mutational study correlating spectroscopy, electrochemistry and crystallographic analysis. From simple model systems we want to develop a fundamental understanding of heme interaction and subsequently transfer this knowl-edge to larger and more complex proteins.