Department of Biochemistry, University of Nebraska, 68583-0718, Lincoln, Nebraska, USA.
Photosynth Res. 1992 Feb;31(2):157-64. doi: 10.1007/BF00028792.
Temperature, activating metal ions, and amino-acid substitutions are known to influence the CO2/O2 specificity of the chloroplast enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase. However, an understanding of the physical basis for enzyme specificity has been elusive. We have shown that the temperature dependence of CO2/O2 specificity can be attributed to a difference between the free energies of activation for the carboxylation and oxygenation partial reactions. The reaction between the 2,3-enediolate of ribulose 1,5-bisphosphate and O2 has a higher free energy of activation than the corresponding reaction of this substrate with CO2. Thus, oxygenation is more responsive to temperature than carboxylation. We have proposed possible transition-state structures for the carboxylation and oxygenation partial reactions based upon the chemical natures of these two reactions within the active site. Electrostatic forces that stabilize the transition state of the carboxylation reaction will also inevitably stabilize the transition state of the oxygenation reaction, indicating that oxygenase activity may be unavoidable. Furthermore, the reduction in CO2/O2 specificity that is observed when activator Mg(2+) is replaced by Mn(2+) may be due to Mg(2+) being more effective in neutralizing the negative charge of the carboxylation transition state, whereas Mn(2+) is a transition-metal ion that can overcome the triplet character of O2 to promote the oxygenation reaction.
温度、激活金属离子和氨基酸取代已知会影响叶绿体酶核酮糖-1,5-二磷酸羧化酶/加氧酶的 CO2/O2 特异性。然而，对于酶特异性的物理基础的理解一直难以捉摸。我们已经表明，CO2/O2 特异性的温度依赖性可归因于羧化和加氧部分反应的活化自由能之间的差异。2,3-烯二醇核酮糖 1,5-二磷酸与 O2 的反应具有比该底物与 CO2 的相应反应更高的活化自由能。因此，加氧比羧化对温度更敏感。我们根据这两种反应在活性部位的化学性质，提出了羧化和加氧部分反应的可能过渡态结构。稳定羧化反应过渡态的静电力也不可避免地稳定加氧反应的过渡态，表明加氧酶活性可能是不可避免的。此外，当激活剂 Mg(2+)被 Mn(2+)取代时观察到的 CO2/O2 特异性降低可能是由于 Mg(2+)更有效地中和羧化过渡态的负电荷，而 Mn(2+)是一种过渡金属离子，可以克服 O2 的三重态特征，促进加氧反应。
