Koenig S H, Brown R D
Ann N Y Acad Sci. 1984;429:99-108. doi: 10.1111/j.1749-6632.1984.tb12319.x.
We emphasize that the interconversion of CO2 and HCO-3 catalyzed by carbonic anhydrase must be regarded as two-substrate, two-product reactions, involving CO2, H2O and HCO-3, H+. Questions then arise regarding the interactions of substrates and products with enzyme during catalysis. Hydration of CO2 has been described by others as a pair of half reactions in which HCO-3 product is released from enzyme, followed by (usually buffer-catalyzed) release of H+. This "ping-pong" view is extended here to a "ping-ping-pong" sequence; based on analyses of specific models for the several intermediate complexes, we conclude that the second "ping" relates to the rate of ligand exchange of H2O from a pentacoordinate enzyme-H2O-HCO-3 complex, about which little is known at present and that is difficult to investigate. We discuss the types of experiments used to measure enzymatic activity and note that (1) they generally measure the rate of the combined "ping-ping" sequence rather than either independently, and (2) the pathway for exchange of substrate H2O is unrelated to the exchange of H2O measured by proton magnetic relaxation and by loss of isotopically labeled oxygen from the CO2-HCO-3 system. The "ping-ping-pong" view, while giving new insights and raising new questions, is consistent with OH- as the ligand of the high-pH form of isozymes I and II, as well as with the results of magnetic relaxation, isotope mixing and loss, 13C NMR linewidths of CO2 and HCO-3 at equilibrium, and stopped-flow kinetic measurements.
我们强调，由碳酸酐酶催化的二氧化碳与碳酸氢根之间的相互转化必须被视为双底物、双产物反应，涉及二氧化碳、水以及碳酸氢根、氢离子。于是就产生了关于催化过程中底物和产物与酶相互作用的问题。其他人已将二氧化碳的水合作用描述为一对半反应，其中碳酸氢根产物从酶中释放出来，随后（通常由缓冲液催化）氢离子释放出来。在此，这种“乒乓”观点被扩展为“乒-乒-乓”序列；基于对几种中间复合物特定模型的分析，我们得出结论，第二个“乒”与来自五配位酶-水-碳酸氢根复合物的水的配体交换速率有关，目前对此知之甚少且难以研究。我们讨论了用于测量酶活性的实验类型，并指出：（1）它们通常测量的是组合的“乒-乒”序列的速率，而非单独测量其中任何一个的速率；（2）底物水的交换途径与通过质子磁共振弛豫以及从二氧化碳-碳酸氢根系统中损失同位素标记氧所测量的水的交换无关。“乒-乒-乓”观点虽然提供了新的见解并提出了新的问题，但与同工酶I和II高pH形式的氢氧根作为配体的情况一致，也与磁共振弛豫、同位素混合与损失、平衡时二氧化碳和碳酸氢根的13C NMR线宽以及停流动力学测量结果一致。
