MYOGLOBIN NO AT LOW PH - FREE 4-COORDINATED HEME IN THE PROTEIN POCKET
MYOGLOBIN NO AT LOW PH - FREE 4-COORDINATED HEME IN THE PROTEIN POCKET
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DOI：
10.1021/bi00008a030
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发表时间：
1995-02-28
期刊：
影响因子：
2.9
通讯作者：
MAGDE, D
中科院分区：
文献类型：
--
作者：
DUPRAT, AF;TRAYLOR, TG;MAGDE, D
在抹香鲸或马心肌红蛋白中，NO的结合和溶液pH值的降低共同作用，削弱并最终破坏铁和近端组氨酸之间的键。这让人想起在中性pH下观察到的鸟苷酸环化酶（催化GTP转化为cGMP的血红素酶）的反应。键断裂的特征在于从九线到三线ESR信号的光谱变化，并伴随着Soret带最大值的UV-可见光谱中从420 nm到387 nm的位移。分析的pH值依赖的光谱变化表明，它们是可逆的，至少在几个小时内，过渡是合作的，涉及六个质子在pH值降低，但只有两个，因为它是提高，和pK约为4.7。不同的蛋白质表现出不同的pK值，其通常低于“螯合的”血红素。pK差异反映了蛋白质结构提供的额外键稳定性。CO的热和光化学NO位移的调查表明，周围的配体的局部口袋，虽然显着改变（根据圆二色性调查），但仍然施加了障碍，对向外扩散的配体到溶剂中。纳秒和皮秒闪光光解表明，在低pH值的蛋白质中，通过单指数过程，配体与四配位物种发生了非常有效的成对重组。这比NO-螯合的血红素（其中不存在配体的远端屏障）的情况发生的程度显著更大。在中性pH下，当近端组氨酸键是完整的，成对重组NO需要更长的时间，并显示多指数动力学。总之，这些结果表明，即使远端效应可能也发挥了作用，近端效应作出了重要贡献，在调制配体-铁键的形成。
In either sperm whale or horse heart myoglobin, binding of NO and lowering of solution pH work together to weaken, and ultimately break, the bond between iron and the proximal histidine. This is reminiscent of the reaction observed at neutral pH in the case of guanylate cyclase, the heme enzyme that catalyzes the conversion of GTP to cGMP. Bond breaking is characterized by a spectral change from a nine-line to a three-line ESR signal and accompanied by a shift from 420 to 387 nm in the UV-vis spectrum of the Soret band maximum. Analysis of the pH-dependent spectral changes shows that they are reversible, at least within a few hours, that the transition is cooperative, involving six protons during pH lowering but only two as it is raised, and that the pK is about 4.7. Different proteins exhibit different pK values, which are generally lower than that for ''chelated'' protoheme. The pK differences reflect the extra bond stability afforded by the protein structure. Investigations of thermal and photochemical NO displacement by CO suggest that the local pocket around the ligand, although significantly altered (according to circular dichroism investigations), nonetheless still imposes a barrier against the outward diffusion of ligand into the solvent. Nanosecond and picosecond flash photolysis shows that in proteins at low pH there is an extremely efficient geminate recombination of the ligand with the four-coordinated species through a single-exponential process. This occurs to a significantly larger extent than for the case of NO-''chelated'' protoheme (where no distal barrier for ligand is present). At neutral pH, when the proximal histidine bond is intact, the geminate recombination for NO takes longer and displays multiexponential kinetics. Altogether, these results suggest that, even though distal effects probably also play a role, proximal effects make an important contribution in modulating ligand-iron bond formation.