Iijima S, Oh M J, Saiki T, Beppu T
J Biochem. 1986 Jun;99(6):1667-72. doi: 10.1093/oxfordjournals.jbchem.a135641.
Heat-stable malate dehydrogenase isolated from Thermus flavus AT62 was completely inactivated by treatment with diethylpyrocarbonate. The inactivation was accompanied by the loss of 1.2 histidine residues per subunit of the enzyme. The enzyme was protected from inactivation by NADH. The enzyme was also inactivated by dye-sensitized photooxidation. Methionine residues, in addition to histidine residues, were destroyed in the inactivated enzyme. Kinetic analyses of the inactivation indicated that the pK value of the residue involved in the inactivation was 8.20 at 25.0 degrees C and 7.52 at 60.0 degrees C. From the pK values and the heat of ionization calculated from the van't Hoff plot of pKs, a histidine residue was identified to be primarily involved in the inactivation. The effect of temperature on the pK value of the essential group in this enzyme from a thermophilic organism is discussed.
从嗜热栖热菌AT62中分离出的热稳定苹果酸脱氢酶经焦碳酸二乙酯处理后完全失活。失活伴随着每个酶亚基损失1.2个组氨酸残基。NADH可保护该酶不被失活。该酶也会因染料敏化光氧化而失活。除组氨酸残基外，失活酶中的甲硫氨酸残基也被破坏。失活的动力学分析表明，参与失活的残基在25.0℃时的pK值为8.20，在60.0℃时为7.52。根据pK值和从pK的范特霍夫图计算出的电离热，确定一个组氨酸残基主要参与失活。讨论了温度对这种嗜热生物酶中必需基团pK值的影响。
