Russell W K, Lindahl P A
Department of Chemistry, Texas A&M University, College Station 77843, USA.
Biochemistry. 1998 Jul 14;37(28):10016-26. doi: 10.1021/bi980149b.
Acetogenic carbon monoxide dehydrogenases catalyze the reversible oxidation of CO to CO2 and the synthesis of acetyl-coenzyme A, utilizing two novel Ni-Fe-S active sites (the C- and A-clusters, respectively) and an [Fe4S4]2+/1+ cluster (the B-cluster) that serves to transfer electrons. Enzyme samples were titrated under equilibrium conditions using various partial pressures of CO in Ar and CO2 atmospheres. EPR signal intensities from each cluster were analyzed as a function of potential using the Nernst equation. The presence of CO2 raised the reduction potentials of the A-, B-, and C-clusters, and it appeared to increase the strength of CO (substrate for acetyl-CoA synthesis) binding to the reduced A-cluster. Carbon dioxide also appeared to stabilize an intermediate EPR-silent state of the C-cluster and alter the saturation/relaxation properties of the reduced B-cluster. Simulations assuming n values (number of e- involved in reduction) larger than appropriate for the individual reactions generally fit better to the titration data than those which assumed the appropriate n, indicating positive redox cooperativity. Carbon dioxide did not inhibit 1,10-phenanthroline from removing the labile Ni from the A-cluster, but it did inhibit the CO/acetyl-coenzyme A exchange activity, probably by causing CO to bind more tightly to the A-cluster. Taken together, these results indicate a significant CO2-dependent conformational change affecting the properties of all three clusters and both subunits. Since the enzyme operates in vivo in a CO2 environment, the CO2-induced conformation may be mechanistically important.
产乙酸一氧化碳脱氢酶催化一氧化碳可逆氧化为二氧化碳以及乙酰辅酶A的合成，利用两个新型镍 - 铁 - 硫活性位点（分别为C簇和A簇）和一个用于传递电子的[Fe4S4]2+/1+簇（B簇）。在平衡条件下，使用氩气和二氧化碳气氛中不同分压的一氧化碳对酶样品进行滴定。利用能斯特方程分析每个簇的电子顺磁共振（EPR）信号强度作为电位的函数。二氧化碳的存在提高了A簇、B簇和C簇的还原电位，并且似乎增加了一氧化碳（乙酰辅酶A合成的底物）与还原态A簇的结合强度。二氧化碳还似乎稳定了C簇的一种中间EPR沉默状态，并改变了还原态B簇的饱和/弛豫特性。假设n值（参与还原的电子数）大于单个反应合适值的模拟通常比假设合适n值的模拟更符合滴定数据，表明存在正的氧化还原协同性。二氧化碳并不抑制1,10 - 菲咯啉从A簇去除不稳定的镍，但它确实抑制了一氧化碳/乙酰辅酶A交换活性，可能是通过使一氧化碳更紧密地结合到A簇。综上所述，这些结果表明存在显著的依赖二氧化碳的构象变化，影响所有三个簇和两个亚基的性质。由于该酶在体内的二氧化碳环境中起作用，二氧化碳诱导的构象在机制上可能很重要。
