Celis H, Romero I
J Bioenerg Biomembr. 1987 Jun;19(3):255-72. doi: 10.1007/BF00762416.
The relation that exist between the Pi-PPi exchange reaction and pyrophosphate hydrolysis by the membrane-bound pyrophosphatase of chromatophores of Rhodospirillum rubrum was studied. The two reactions have a markedly different requirement for pH. The optimal pH for hydrolysis was 6.5 Mg2+ or Pi for the enzyme; Mn2+ and Co2+ support the Pi-PPi exchange reaction partially (50%), but the reaction is slower than with Mg2+; other divalent cations like Zn2+ or Ca2+ do not support the exchange reaction. In the hydrolytic reaction, Zn2+, at low concentration, substitutes for Mg2+ as substrate, and Co2+ also substitutes in limited amount (50%). Other cations (Ca2+, Cu2+, Fe2+, etc.) do not act as substrates in complex with PPi. The Zn2+ at high concentrations inhibited the hydrolytic reaction, probably due to uncomplexed free Zn2+. In the presence of high concentration of substrate for the hydrolysis (Mg-PPi) the divalent cations are inhibitory in the following order: Zn2+ greater than Mn2+ greater than Ca2+ greater than or equal to Co2+ greater than Fe2+ greater than Cu2+ greater than Mg2+. The data in this work suggest that H+ and divalent cations in their free form induced changes in the kinetic properties of the enzyme.
研究了红螺菌载色体膜结合焦磷酸酶催化的Pi-PPi交换反应与焦磷酸水解反应之间的关系。这两个反应对pH的要求明显不同。水解反应的最佳pH值为6.5，该酶需要Mg2+或Pi；Mn2+和Co2+部分支持Pi-PPi交换反应（50%），但反应比Mg2+存在时慢；其他二价阳离子如Zn2+或Ca2+不支持交换反应。在水解反应中，低浓度的Zn2+可替代Mg2+作为底物，Co2+也能有限量地替代（50%）。其他阳离子（Ca2+、Cu2+、Fe2+等）与PPi形成复合物时不作为底物。高浓度的Zn2+抑制水解反应，可能是由于未络合的游离Zn2+。在存在高浓度水解底物（Mg-PPi）时，二价阳离子的抑制作用顺序为：Zn2+＞Mn2+＞Ca2+≥Co2+＞Fe2+＞Cu2+＞Mg2+。这项工作的数据表明，游离形式的H+和二价阳离子会引起该酶动力学性质的变化。
