Ferreon Josephine C, Hilser Vincent J
Department of Human Biological Chemistry and Genetics, University of Texas Medical Branch at Galveston, Galveston, Texas 77555, USA.
Protein Sci. 2003 Mar;12(3):447-57. doi: 10.1110/ps.0237803.
Polyproline II (PPII) is reported to be a dominant conformation in the unfolded state of peptides, even when no prolines are present in the sequence. Here we use isothermal titration calorimetry (ITC) to investigate the PPII bias in the unfolded state by studying the binding of the SH3 domain of SEM-5 to variants of its putative PPII peptide ligand, Sos. The experimental system is unique in that it provides direct access to the conformational entropy change of the substituted amino acids. Results indicate that the denatured ensemble can be characterized by at least two thermodynamically distinct states, the PPII conformation and an unfolded state conforming to the previously held idea of the denatured state as a random collection of conformations determined largely by hard-sphere collision. The probability of the PPII conformation in the denatured states for Ala and Gly were found to be significant, approximately 30% and approximately 10%, respectively, resulting in a dramatic reduction in the conformational entropy of folding.
据报道，多聚脯氨酸II（PPII）是肽链未折叠状态下的主要构象，即使序列中不存在脯氨酸。在此，我们通过研究SEM-5的SH3结构域与其假定的PPII肽配体Sos变体的结合，利用等温滴定量热法（ITC）来研究未折叠状态下的PPII偏好性。该实验系统的独特之处在于它能直接获取取代氨基酸的构象熵变。结果表明，变性态系综至少可由两种热力学上不同的状态来表征，即PPII构象和一种符合先前关于变性态是主要由硬球碰撞决定的随机构象集合这一观点的未折叠状态。在Ala和Gly的变性态中，PPII构象的概率分别约为30%和约10%，这导致折叠的构象熵大幅降低。
