Bradbury J H, Carver J A
Biochemistry. 1984 Oct 9;23(21):4905-13. doi: 10.1021/bi00316a013.
In paramagnetic metmyoglobin, cyanomyoglobin (CNMb), and deoxymyoglobin, His-36 has a high pK (approximately 8), and the NMR titration behavior of the H-2 resonance is perturbed, due to the presence at low pH of a hydrogen bond with Glu-38, which is broken at high pH. The His-36 H-4 resonance shows no shift with pK approximately 8 because of two opposing chemical shift effects but monitors the titration of nearby Glu-36 (pK = 5.6). In diamagnetic derivatives [(carbon monoxy)myoglobin (COMb) and oxymyoglobin (oxyMb)], the titration behavior of His-36 H-2 and H-4 resonances is normalized (pK approximately 6.8). The very slight alkaline Bohr effect in sperm whale myoglobin (Mb) is interpreted in terms of the pK change of His-36 from deoxyMb to oxyMb and compensating pK changes in the opposite direction of other unspecified groups. In sperm whale COMb at 40 degrees C, the distal histidine (His-64) and His-97 have pK values of 5.0 and 5.9. The meso proton resonances remote from these groups do not show a titration shift, but the nearby gamma-meso proton (pK = 5.3) responds to titration of both histidines, and the upfield Val-68 methyl at -2.3 ppm (pK = 4.7) witnesses the titration of nearby His-64. At 20 degrees C, the latter resonance is reduced in size, and a second resonance occurs at -2.8 ppm, which is insensitive to pH and, hence, more remote from His-64. Both resonances arise from two conformations of Val-68 in slow equilibrium. In oxyMb at 20 degrees C, only the latter resonance is observed, presumably because of the steric restrictions imposed by the hydrogen bond between ligand and His-64 in oxyMb, which is not present in COMb. In oxyMb the pK of His-97 (5.6) is similar to that of the meso proton resonances (5.5) and to the pK of other pH-dependent processes, including the very small acid Bohr effect. It is likely that these processes are controlled by the titration of His-97.(ABSTRACT TRUNCATED AT 250 WORDS)
在顺磁性高铁肌红蛋白、氰化肌红蛋白（CNMb）和脱氧肌红蛋白中，组氨酸-36具有较高的pK值（约为8），并且由于在低pH值下与谷氨酸-38存在氢键，该氢键在高pH值下断裂，H-2共振的核磁共振滴定行为受到干扰。组氨酸-36的H-4共振由于两种相反的化学位移效应，在pK约为8时没有位移，但可监测附近谷氨酸-36（pK = 5.6）的滴定。在抗磁性衍生物[（一氧化碳）肌红蛋白（COMb）和氧合肌红蛋白（oxyMb）]中，组氨酸-36的H-2和H-4共振的滴定行为正常化（pK约为6.8）。抹香鲸肌红蛋白（Mb）中非常轻微的碱性玻尔效应是根据组氨酸-36从脱氧肌红蛋白到氧合肌红蛋白的pK变化以及其他未指定基团在相反方向上的补偿性pK变化来解释的。在40℃的抹香鲸COMb中，远端组氨酸（组氨酸-64）和组氨酸-97的pK值分别为5.0和5.9。远离这些基团的中间质子共振没有显示滴定位移，但附近的γ-中间质子（pK = 5.3）对两个组氨酸的滴定有反应，并且在-2.3 ppm处的向场缬氨酸-68甲基（pK = 4.7）见证了附近组氨酸-64的滴定。在20℃时，后一种共振的大小减小，并且在-2.8 ppm处出现第二个共振，该共振对pH不敏感，因此离组氨酸-64更远。这两种共振均源于缬氨酸-68在缓慢平衡中的两种构象。在20℃的氧合肌红蛋白中，仅观察到后一种共振，推测是由于氧合肌红蛋白中配体与组氨酸-64之间的氢键所施加的空间限制，而COMb中不存在这种氢键。在氧合肌红蛋白中，组氨酸-97的pK值（5.6）与中间质子共振的pK值（5.5）以及其他pH依赖性过程的pK值相似，包括非常小的酸性玻尔效应。这些过程很可能是由组氨酸-97的滴定控制的。（摘要截断于250字）
