Narasimhulu S
Biochim Biophys Acta. 1978 Dec 1;544(2):381-93. doi: 10.1016/0304-4165(78)90106-x.
The effect of temperature on steroid C-21 hydroxylation and substrate-cytochrome P-450 binding reaction under turnover conditions (NADPH + O2 are investigated. The Arrhenius activity plot exhibited a single break, while the van 't Hoff plot of the substrate dissociation constant (Ks) exhibited four breaks between 10 and 40 degrees C which corresponded to the characteristic temperatures of the lipids' phase transitions. Unlike the case of the Ks value, the detergent Triton X-114 was without effect on the Arrhenius activity plot. This indicates that the single break in the case of the enzyme activity is distinct from but not necessarily independent of the multiple breaks inthe case of the Ks. At physiologic temperature and concentration of the substrate, the free energy (--9.5 kcal/mol) of the substrate-cytochrome binding reaction is more than sufficient to account for the apparent activation energy (6.6 kcal/mol) of the overall hydroxylation. This suggests that the substrate-cytochrome P-450 binding reaction has the potential of being a source of energy for the overall reaction.
研究了在周转条件下（NADPH + O2）温度对甾体C-21羟化作用及底物与细胞色素P-450结合反应的影响。阿累尼乌斯活性图呈现出一个断点，而底物解离常数（Ks）的范特霍夫图在10至40摄氏度之间呈现出四个断点，这与脂质相变的特征温度相对应。与Ks值的情况不同，去污剂Triton X-114对阿累尼乌斯活性图没有影响。这表明酶活性情况下的单个断点与Ks情况下的多个断点不同，但不一定相互独立。在生理温度和底物浓度下，底物与细胞色素结合反应的自由能（-9.5千卡/摩尔）足以解释整个羟化反应的表观活化能（6.6千卡/摩尔）。这表明底物与细胞色素P-450的结合反应有可能成为整个反应的能量来源。
