Structures of the first intermediates of the visual cascade

视紫红质是脊椎动物眼睛中弱光视觉的光感受器，是一种所谓的G蛋白偶联受体或GPCR。视紫红质视觉级联的主要步骤是视紫红质的生色团视网膜的C11=C12双键从顺式异构化为反式，形成视紫红质中间体。视紫红质经过几个步骤转化为后视紫红质II，后者与G蛋白偶联，触发视觉级联反应。视紫红质是第一个解决其X射线结构的GPCRs。这种结构是在分子水平上理解视觉过程的第一步。然而，2.8ä的分辨率不足以详细分析发色团及其与蛋白质环境的相互作用。对于受体的选择性和反应性以及随后的相互转化来说，生色团几何构型的细微点仍然是值得怀疑的。该提案的主题是根据实验X射线结构计算视紫红质和视紫红质的真实模型。我们将对生色团使用严格的量子化学，对包括结合口袋在内的生色团使用基于密度泛函的紧结合(DFTB)，并对整个蛋白质使用量子力学/分子力学相结合的方法。对于视紫红质来说，重点将放在发色团和蛋白质之间的紧密结合上，这会稳定基态，并为光异构化做好准备。对于担视紫质，我们将集中在光激发后发色团的空间不匹配，它的强制几何结构如何影响结合口袋，并在其通往G蛋白激活的道路上为整个蛋白质做准备。

Rhodopsin, the photoreceptor for dim light vision in the vertebrate eye, is a so-called G-protein coupled receptor or GPCR. The primary step of the visual cascade of rhodopsin is the isomerization from cis to trans of the C11=C12 double bond of retinal, the chromophore of rhodopsin, to form the bathorhodopsin intermediate. Bathorhodopsin is transformed in several steps into metarhodopsin II which couples with the G-protein and triggers the visual cascade. Rhodopsin is the first GPCR of which the X-ray structure has been solved. This structure is a first step for understanding the visual process at the molecular level. However, the 2.8 Å resolution is not sufficient for a detailed analysis of the chromophore and its interaction with the protein environment. Fine points of the chromophore geometry, which are essential for the selectivity and reactivity of the receptor and the subsequent interconversions, remain in doubt. Subject of the proposal is the calculation of realistic models of rhodopsin and of bathorhodopsin based on the experimental X-ray structure. We will use rigorous quantum-chemistry for the chromophore, density functional-based tight-binding (DFTB) for the chromophore including the binding pocket, and combined quantum mechanical/molecular mechanics for the whole protein. The focus, for rhodopsin, will be on the tight fit between the chromophore and protein, which stabilizes the ground state and sets it up for photo-isomerization. For bathorhodopsin, we will concentrate on the steric non-fit of the chromophore after photoexcitation, how its enforced geometry affects the binding pocket and prepares the whole protein on its path towards G-protein activation.