The exchange of Fe3+ between acetohydroxamic acid and transferrin. Spectrophotometric evidence for a mixed ligand complex.
The exchange of Fe3+ between acetohydroxamic acid and transferrin. Spectrophotometric evidence for a mixed ligand complex.
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DOI：
10.1016/s0021-9258(18)34416-8
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发表时间：
1982-07
期刊：
影响因子：
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通讯作者：
R. Cowart;N. Kojima;G. Bates
中科院分区：
文献类型：
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作者：
R. Cowart;N. Kojima;G. Bates
转铁蛋白是一种血清铁转运蛋白，为研究生物金属离子交换反应提供了良好的模型。一个奇怪的问题是，虽然从理论上预计会出现一种混合配体-Fe3+-蛋白质，并得到动力学结果的支持，但到目前为止，还没有得到这种中间体的明确的分光光度证据。在乙酰异羟肟酸和转铁蛋白之间交换Fe3+的研究中，发现了这样的证据。停流分光光度法研究表明，Fe~(2+)-乙酰氧肟酸与载脂蛋白-CO_3(2-)的反应为明显的两相反应。第一阶段在大约4个S内完成，并导致形成一个瞬时物种，在432 nm处有明显的光谱最大值。第二阶段需要大约2分钟，并导致形成Fe3+-转铁蛋白-CO3(2-)。我们认为，过渡物种是一种混合配体络合物。Fe~(3+)-乙酰异羟肟酸的反应速率-浓度关系为线性，载脂蛋白-CO_3(2-)的反应速度-浓度关系为双曲线。这表明在载脂蛋白-CO3(2-)攻击之前，Fe3+-(乙酰异羟肟酸)3被限速稳定。乙酰异羟肟酸从Fe3+-转铁蛋白-CO3(2-)中除去Fe3+的反向反应不为中间体提供光谱证据。Fe~(3+)-转铁蛋白-CO_3(2-)的速度-浓度关系与乙酰异羟肟酸浓度呈双曲线关系，与Fe~(3+)-转铁蛋白-CO_3(2-)呈线性关系，表明Fe~(3+)-转铁蛋白-CO_3(2-)对Fe~(3+)的稳定是由构象变化引起的。
Transferrin, the serum iron transport protein, provides an excellent model for studying biological metal ion exchange reactions. A curious problem is that while a mixed ligand species of chelate-Fe3+-protein is anticipated from theoretical considerations and supported by kinetic results, no clear spectrophotometric evidence for such an intermediate has heretofore been obtained. In this study of the exchange of Fe3+ between acetohydroxamic acid and transferrin such evidence has been found. The reaction of Fe2+-acetohydroxamic acid with apotransferrin-CO3(2-) is distinctly biphasic when examined by stopped flow spectrophotometry. The first phase is complete within approximately 4 s and results in the formation of a transient species with a distinct spectral maximum at 432 nm. The second phase requires approximately 2 min and results in the formation of Fe3+-transferrin-CO3(2-). We suggest that the transient species is a mixed ligand complex. The reaction rate-concentration relationship for the formation of the intermediate is linear for Fe3+-acetohydroxamic acid and hyperbolic for apotransferrin-CO3(2-). This suggests a rate-limiting labilization of Fe3+-(acetohydroxamic acid)3 preceding attack by the apotransferrin-CO3(2-). The reverse reaction, the removal of Fe3+ from the Fe3+-transferrin-CO3(2-) by acetohydroxamic acid, does not provide spectral evidence for the intermediate. The velocity-concentration relationship shows a hyperbolic dependence on acetohydroxamic acid concentration and a linear dependence of Fe3+-transferrin-CO3(2-), suggesting a rate-limiting labilization of the Fe3+ of Fe3+-transferrin-CO3(2-) resulting from a conformational change.