Zhou Y, Hall C K, Karplus M
Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, USA.
Protein Sci. 1999 May;8(5):1064-74. doi: 10.1110/ps.8.5.1064.
The "calorimetric criterion" is one of the important experimental approaches for determining whether protein folding is an "all-or-none" two-state transition (i.e., whether intermediates are present at equilibrium). The calorimetric criterion states that the equivalence of the "measured" calorimetric enthalpy change and the effective two-state van't Hoff enthalpy change demonstrates that there is a two-state transition. This paper addresses the essential question of whether the calorimetric criterion is a necessary and sufficient condition for a two-state process and shows that it is necessary but not sufficient by means of specific examples. Analysis of simple models indicates that the heat capacity curve, regardless of whether it originates from a two-state process or not, can always be decomposed in such a way that the calorimetric criterion is satisfied. Exact results for a three-state model and a homopolymer tetramer demonstrate that the deviation from the calorimetric criterion is not simply related to the population of intermediate states. Analysis of a three-helix bundle protein model, which has a two-state folding from a random coil to ordered (molten) globule, shows that the calorimetric criterion may not be satisfied if the standard linear interpolation of baselines (weighted or unweighted) is employed. A specific example also suggests that the more recently introduced deconvolution method is not necessarily better than the simple calorimetric criterion for distinguishing a two-state transition from a three-state transition. Although the calorimetric criterion is not a sufficient condition for a two-state process, it is likely to continue to be of practical utility, particularly when its results are shown to be consistent with those from other experimental methods.
“量热法判据”是确定蛋白质折叠是否为“全或无”两态转变（即平衡时是否存在中间体）的重要实验方法之一。量热法判据表明，“测量的”量热焓变与有效的两态范特霍夫焓变相等，证明存在两态转变。本文探讨了量热法判据对于两态过程是否为充分必要条件这一关键问题，并通过具体实例表明它是必要条件但非充分条件。对简单模型的分析表明，无论热容曲线是否源自两态过程，总能以满足量热法判据的方式进行分解。对三态模型和均聚物四聚体的精确结果表明，与量热法判据的偏差并非简单地与中间态的数量相关。对一个三螺旋束蛋白模型的分析表明，该模型从无规卷曲到有序（熔融）球状体的折叠过程为两态，但如果采用基线的标准线性插值（加权或未加权），可能不满足量热法判据。一个具体例子还表明，最近引入的去卷积方法在区分两态转变和三态转变方面不一定比简单的量热法判据更好。尽管量热法判据对于两态过程不是充分条件，但它可能仍具有实际用途，特别是当其结果与其他实验方法的结果一致时。
