Whittaker M M, Whittaker J W
Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA.
Biochemistry. 1996 May 28;35(21):6762-70. doi: 10.1021/bi960088m.
We have observed thermochromism (temperature-dependent absorption) for anion complexes of manganese superoxide dismutase indicating a change in coordination number for the metal complex at low temperatures. The ligand field spectra for the Mn(III) ion, characteristic of five-coordination for the azide complex at 295 K, cleanly convert to spectra reflecting six-coordination at low temperature, with a midpoint for the transition near 200 K. The active site structure is temperature-dependent, a relatively rigid, distorted octahedral low-temperature Mn complex melting with dehydration (or displacement of one of the protein ligands) to form a five-coordinated complex under physiological conditions. Thermodynamic parameters for the transition estimated from van't Hoff analysis (delta HvH = 5 kcal/mol; delta SvH = 22 cal/mol K) are consistent with reduced chemical binding and increased fluxionality at room temperature. This thermochromism of MnSD demonstrates the existence of distinct isomeric forms of the active site metal complex, whose relative stability depends on the degree of vibrational excitation. The marginal destabilization of the six-coordinate anion complex under physiological conditions suggests that the enzyme may thermally control the stability of intermediates in a dissociative displacement mechanism for substrate binding and redox.
我们观察到锰超氧化物歧化酶的阴离子配合物具有热致变色现象（温度依赖性吸收），这表明低温下金属配合物的配位数发生了变化。Mn(III)离子的配体场光谱，在295K时叠氮配合物的特征是五配位，在低温下能清晰地转变为反映六配位的光谱，转变的中点接近200K。活性位点结构取决于温度，一种相对刚性、扭曲的八面体低温Mn配合物在生理条件下随着脱水（或蛋白质配体之一的取代）而熔化，形成五配位配合物。通过范特霍夫分析估计的转变热力学参数（ΔHvH = 5 kcal/mol；ΔSvH = 22 cal/mol K）与室温下化学结合减少和流动性增加一致。MnSD的这种热致变色现象证明了活性位点金属配合物存在不同的异构体形式，其相对稳定性取决于振动激发程度。生理条件下六配位阴离子配合物的轻微失稳表明，该酶可能通过热控制底物结合和氧化还原的解离取代机制中中间体的稳定性。
