Bertrand P, Mbarki O, Asso M, Blanchard L, Guerlesquin F, Tegoni M
Université de Provence, Centre St Jérôme, Marseille, France.
Biochemistry. 1995 Sep 5;34(35):11071-9. doi: 10.1021/bi00035a012.
The enthalpic and entropic components of the redox free energy variation of cytochrome c553 from Desulfovibrio vulgaris Hildenborough and its mutant Y64V, flavocytochrome b2 from Saccharomyces cerevisiae, and the different hemes of cytochromes c3 from Desulfovibrio vulgaris Miyazaki and Desulfovibrio desulfuricans Norway have been determined in 0.1 M Tris-HCl pH 7.0 (7.6 for cytochromes c3) at 25 degrees C by using nonisothermal potentiometric titrations. The set of available experimental data demonstrates that the entropic component plays an important role in the control of the redox potential in c-type and b-type cytochromes. The variation of the entropic component within the class of cytochromes characterized by a positive value of E degrees ' is proposed to be mainly determined by the variation of the exposure of the heme propionates to the solvent. In the case of tetraheme cytochromes c3, the thermodynamic characteristics vary largely among the hemes belonging to the same molecule, which reflects the environmental peculiarities of each heme and also the heme-heme redox interactions. This study substantiates the existence of compensatory effects between large and opposite contributions to E degree ' predicted by all the current theoretical models which are based on electrostatic free energy calculations.
通过非等温电位滴定法，在25℃、0.1 M Tris-HCl pH 7.0（细胞色素c3为pH 7.6）条件下，测定了来自希登伯勒脱硫弧菌的细胞色素c553及其突变体Y64V、来自酿酒酵母的黄素细胞色素b2，以及来自宫崎脱硫弧菌和挪威脱硫脱硫弧菌的细胞色素c3不同血红素的氧化还原自由能变化的焓和熵成分。现有的一组实验数据表明，熵成分在控制c型和b型细胞色素的氧化还原电位中起重要作用。对于E°'为正值的细胞色素类别，熵成分的变化主要由血红素丙酸酯暴露于溶剂的变化决定。在四血红素细胞色素c3的情况下，同一分子中的血红素之间的热力学特征差异很大，这反映了每个血红素的环境特性以及血红素-血红素氧化还原相互作用。本研究证实了基于静电自由能计算的所有当前理论模型所预测的对E°'的大的相反贡献之间存在补偿效应。
