ON THE PROTEIN (TYROSINE)-CHROMOPHORE (PROTONATED SCHIFF-BASE) COUPLING IN BACTERIORHODOPSIN
ON THE PROTEIN (TYROSINE)-CHROMOPHORE (PROTONATED SCHIFF-BASE) COUPLING IN BACTERIORHODOPSIN
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DOI：
10.1073/pnas.81.22.7083
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发表时间：
1984-01-01
期刊：
影响因子：
--
通讯作者：
ELSAYED, MA
中科院分区：
文献类型：
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作者：
HANAMOTO, JH;DUPUIS, P;ELSAYED, MA
在不同的pH值（7-11）同时研究了酪氨酸离子（从296 nm处的吸收）和去质子化的Schiff碱（M412）（M412）（M412）（M412）（M412）（M412）（M412）（M412）（M412）（M412）（M412）的形成动力学，以不同的pH值（7-11）同时研究和温度（5.级 - 25.级）。 M412与先前的观察结果一致，观察到了两个形成率。慢速在生理条件下占主导地位，并且比酪酸酯形成的速度稍快。这与酪氨酸形成是Schiff基地（M412）的先决条件的提议分歧。快速和缓慢的成分的幅度比pH敏感，在任何pH值下，都可以用来计算氨基酸PKA值为9.6。这是通过提出蛋白质中质子化的席夫碱的两个位点的存在来解释的。在一个地点中，Schiff底座接近中性形式的氨基酸残基，PKA值为9,6（产生缓慢的成分），而在另一个氨基酸值中，它靠近其共轭碱基。酪氨酸离子的形成以及M412的慢速成分的形成都具有与H-键能相当的激活能。建议一个模型来解释这一点，以及酪氨酸的可比较质子化速率和质子化席夫碱的缓慢成分。它涉及通过暴露于带正电荷物种的PKA来减少其PKA。
The kinetics of formation of both the tyrosinate ion (from its absorption at 296 nm) and the deprotonated Schiff base (M412) (from its absorption at 404 nm) [of Halobacterium halobium] are studied simultaneously at different pH values (7-11) and temperatures (5.degree.-25.degree. C). Two formation rates are obsered for M412 in agreement with previous observations. The slow one is dominant under physiological conditions and is slightly faster than that for the tyrosinate formation. This is in disagreement with the proposal that the tyrosine formation is a prerequisite to the deprotonation of the Schiff base (M412). The ratio of the amplitudes of the fast and slow components is sensitive to pH and, at any pH, it can be used to calculate an amino acid pKa value of 9.6. This explained by proposing the existence of 2 sites for the protonated Schiff base within the protein. In one site, the Schiff base is near the neutral form of an amino acid residue with pKa value of 9,6 (giving rise to the slow component), while in the other, it is near its conjugate base. The formation of the tyrosine ion as well as the formation of the slow and fast components of M412 all have activation energies that are comparable to H-bond energies. A model is suggested to account for this and the comparable deprotonation rates of tyrosine and the slow component of the protonated Schiff base. It involves the reduction of their pKa by their exposure to a positively charged species.