Leigh R A, Pope A J, Jennings I R, Sanders D
Biochemistry and Physiology Department, Agricultural and Food Research Council Institute of Arable Crops Research, Rothamsted Experimental Station, Harpenden, Hertfordshire AL5 2JQ, United Kingdom.
Plant Physiol. 1992 Dec;100(4):1698-705. doi: 10.1104/pp.100.4.1698.
The responses of the vacuolar membrane (tonoplast) proton-pumping inorganic pyrophosphatase (H(+)-PPase) from oat (Avena sativa L.) roots to changes in Mg(2+) and pyrophosphate (PPi) concentrations have been characterized. The kinetics were complex, and reaction kinetic models were used to determine which of the various PPi complexes were responsible for the observed responses. The results indicate that the substrate for the oat root vacuolar H(+)-PPase is Mg(2)PPi and that this complex is also a non-competitive inhibitor. In addition, the enzyme is activated by free Mg(2+) and competitively inhibited by free PPi. This conclusion differs from that reached in previous studies, in which it was proposed that MgPPi is the substrate for plant vacuolar H(+)-PPases. However, models incorporating MgPPi as a substrate were unable to describe the kinetics of the oat H(+)-PPase. It is demonstrated that models incorporating Mg(2)PPi as the substrate can describe some of the published kinetics of the Kalanchoë daigremontiana vacuolar H(+)-PPase. Calculations of the likely concentrations of Mg(2)PPi in plant cytoplasm suggest that the substrate binding site of the oat vacuolar H(+)-PPase would be about 70% saturated in vivo.
对燕麦（ Avena sativa L.）根液泡膜（液泡包被膜）质子泵无机焦磷酸酶（H(+)-PPase）对Mg(2+)和焦磷酸（PPi）浓度变化的响应进行了表征。动力学很复杂，使用反应动力学模型来确定各种PPi复合物中哪些是观察到的响应的原因。结果表明，燕麦根液泡H(+)-PPase的底物是Mg(2)PPi，并且该复合物也是一种非竞争性抑制剂。此外，该酶被游离Mg(2+)激活，并被游离PPi竞争性抑制。这一结论与先前研究得出的结论不同，先前研究提出MgPPi是植物液泡H(+)-PPases的底物。然而，将MgPPi作为底物的模型无法描述燕麦H(+)-PPase的动力学。结果表明，将Mg(2)PPi作为底物的模型可以描述落地生根液泡H(+)-PPase的一些已发表的动力学。对植物细胞质中Mg(2)PPi可能浓度的计算表明，燕麦液泡H(+)-PPase的底物结合位点在体内大约70%饱和。
