Division of Hematology, Internal Medicine, University of Texas Medical School at Houston, Houston, Texas 77030, United States.
Biochemistry. 2012 Jan 10;51(1):172-86. doi: 10.1021/bi2015629. Epub 2011 Dec 13.
Selectivity among NO, CO, and O₂ is crucial for the physiological function of most heme proteins. Although there is a million-fold variation in equilibrium dissociation constants (K(D)), the ratios for NO:CO:O₂ binding stay roughly the same, 1:10(3):10(6), when the proximal ligand is a histidine and the distal site is apolar. For these proteins, there is a "sliding scale rule" for plots of log(K(D)) versus ligand type that allows predictions of K(D) values if one or two are missing. The predicted K(D) for binding of O₂to Ns H-NOX coincides with the value determined experimentally at high pressures. Active site hydrogen bond donors break the rule and selectively increase O₂ affinity with little effect on CO and NO binding. Strong field proximal ligands such as thiolate, tyrosinate, and imidazolate exert a "leveling" effect on ligand binding affinity. The reported picomolar K(D) for binding of NO to sGC deviates even more dramatically from the sliding scale rule, showing a NO:CO K(D) ratio of 1:~10(8). This deviation is explained by a complex, multistep process, in which an initial low-affinity hexacoordinate NO complex with a measured K(D) of ≈54 nM, matching that predicted from the sliding scale rule, is formed initially and then is converted to a high-affinity pentacoordinate complex. This multistep six-coordinate to five-coordinate mechanism appears to be common to all NO sensors that exclude O₂ binding to capture a lower level of cellular NO and prevent its consumption by dioxygenation.
对大多数血红素蛋白的生理功能而言，NO、CO 和 O₂ 的选择性至关重要。尽管平衡解离常数（K(D)）存在百万倍的差异，但当近端配体为组氨酸且远端位点为非极性时，NO:CO:O₂ 结合的比值大致保持不变，约为 1:10(3):10(6)。对于这些蛋白，存在一种“滑动比例规则”，可根据一个或两个缺失的配体类型来预测 K(D) 值。预测的 Ns H-NOX 与 O₂ 结合的 K(D) 值与在高压下实验确定的值相符。活性位点氢键供体打破了该规则，选择性地增加了 O₂ 的亲和力，而对 CO 和 NO 的结合影响很小。强场近端配体，如硫醇盐、酪氨酸盐和咪唑盐，对配体结合亲和力产生“平衡”效应。报道的 sGC 与 NO 结合的皮摩尔级 K(D) 值与滑动比例规则的偏差更大，NO:CO K(D) 比值为 1:~10(8)。这种偏差可以通过一个复杂的多步骤过程来解释，其中初始形成具有约 54 nM 测量 K(D) 值的初始低亲和力六配位 NO 复合物，与从滑动比例规则预测的值匹配，然后转化为高亲和力五配位复合物。这种多步六配位到五配位的机制似乎在所有排除 O₂ 结合以捕获更低水平细胞 NO 并防止其被氧化消耗的 NO 传感器中都很常见。
