Bhowmick Reshma, Jagannadham Medicherla V
Molecular Biology Unit, Institute of Medical Sciences, Banaras Hindu University, Varanasi 221005, India.
Protein J. 2006 Sep;25(6):399-410. doi: 10.1007/s10930-006-9026-3.
Structural and functional characteristics of jack bean urease (JBU), a hexameric enzyme having identical subunits, were investigated under neutral as well as acidic conditions by using CD, fluorescence, ANS binding and enzyme activity measurements. At low pH and low ionic strength, JBU exists in a partially unfolded state (U(A)-state), having predominantly beta structure and no tertiary interactions along with a strong ANS binding. Addition of salts like NaCl, KCl and Na(2)SO(4) to the U(A)-state induces refolding resulting in structural propensities similar to that of native hexamer. Moreover, at low concentrations, GuHCl behaves like an anion by inducing refolding of the U(A)-state. The anion-induced refolded state (I(A)-state) is more stable than U(A)-state and the stability is nearly equal to that of the native protein against chemical-induced and thermal denaturation. Overall, these observations support a model of protein folding for a multimeric protein where certain conformations (ensembles of substates) of low energy prevail and populated under non-native conditions with different stability.
刀豆脲酶（JBU）是一种具有相同亚基的六聚体酶，通过圆二色光谱（CD）、荧光、8-苯胺基-1-萘磺酸（ANS）结合及酶活性测定，对其在中性和酸性条件下的结构与功能特性进行了研究。在低pH值和低离子强度下，JBU以部分展开状态（U(A)态）存在，主要具有β结构，不存在三级相互作用，且与ANS有很强的结合。向U(A)态添加NaCl、KCl和Na₂SO₄等盐会诱导重折叠，产生与天然六聚体相似的结构倾向。此外，在低浓度下，盐酸胍（GuHCl）通过诱导U(A)态重折叠而表现得像阴离子。阴离子诱导的重折叠状态（I(A)态）比U(A)态更稳定，其稳定性在抗化学诱导和热变性方面几乎与天然蛋白质相当。总体而言，这些观察结果支持了一种多聚体蛋白质的蛋白质折叠模型，即在非天然条件下，某些低能量的构象（亚状态集合）占主导并具有不同的稳定性。
