Tcherkasskaya O, Ptitsyn O B
Laboratory of Experimental and Computational Biology, National Cancer Institute, National Institutes of Health, Bethesda, MD 20892-5677, USA.
FEBS Lett. 1999 Jul 23;455(3):325-31. doi: 10.1016/s0014-5793(99)00792-9.
The molten globule state was shown to be the third thermodynamic state of protein molecules in addition to their native and unfolded states. On the other hand, it was reported that optical and hydrodynamic properties of pH-denatured apomyoglobin depend on the nature of anions added to the protein solution. This observation was used to conclude that there are many 'partly folded' intermediates between the native and unfolded states rather than one distinct molten globule state. However, little is known on the structures of pH-denatured apomyoglobin in the presence of different anions. Two tyrosine residues in horse apomyoglobin have been successively modified by the reaction with tetranitromethane. This approach was employed to measure the distances between tryptophans and modified tyrosines in different states of apomyoglobin by the method of direct energy transfer. Experimental data show that the distance between the middle of the A-helix and the beginning of the G-helix and/or the end of the H-helix in 'anion-induced' states are very close to those in the native holo- and apomyoglobins. This suggests that the AGH helical complex, being the most structured part of apomyoglobin in the molten globule state, exists also in pH-denatured apomyoglobin in the presence of different anions. Consequently, all non-native forms of apomyoglobin studied so far share the common important feature of its native structure.
除了天然态和去折叠态之外，熔球态被证明是蛋白质分子的第三种热力学状态。另一方面，有报道称，pH值变性的脱辅基肌红蛋白的光学和流体力学性质取决于添加到蛋白质溶液中的阴离子的性质。这一观察结果被用来推断，在天然态和去折叠态之间存在许多“部分折叠”的中间体，而不是一种独特的熔球态。然而，对于在不同阴离子存在下pH值变性的脱辅基肌红蛋白的结构却知之甚少。马脱辅基肌红蛋白中的两个酪氨酸残基已通过与四硝基甲烷反应而被相继修饰。采用这种方法，通过直接能量转移法来测量脱辅基肌红蛋白不同状态下色氨酸与修饰酪氨酸之间的距离。实验数据表明，在“阴离子诱导”状态下，A螺旋中部与G螺旋起始端和/或H螺旋末端之间的距离与天然全肌红蛋白和脱辅基肌红蛋白中的距离非常接近。这表明，作为熔球态脱辅基肌红蛋白中结构最紧密的部分，AGH螺旋复合体在不同阴离子存在下的pH值变性脱辅基肌红蛋白中也存在。因此，到目前为止所研究的脱辅基肌红蛋白的所有非天然形式都具有其天然结构的共同重要特征。
