Stellwagen E, Thompson S T
Biochim Biophys Acta. 1979 Jul 11;569(1):6-12. doi: 10.1016/0005-2744(79)90075-5.
The presence of the monovalent cations Tl+, NH+4, K+, Rb+ or Cs+, in decreasing order of potency, produce a marked equivalent increase in the specific enzyme activity of phosphofructokinase (ATP:D-fructose-6-phosphate 1-phosphotransferase, EC 2.7.1.11) purified from extreme thermophile, Thermus X-1. By contrast, the monovalent cations Li+, Na+ or CH3NH+3 produce no detectable catalyitic activation at concentrations up to 100 mM. The relative potency of these cations suggests that each polypeptide chain in the tetrameric enzyme possesses a cationbinding site having tetragonal symmetry and that the protein ligands are principally hydroxyl or carboxylate oxygens. Only the enzyme-cation complex and not the enzyme by itself exhibits cooperativity with respect to the dependence of catalytic rate on the concentration of the substrate, fructose 6-phosphate. In the presence of subsaturating but not saturating concentrations of substrate, the catalytic activation produced by monovalent cations is also cooperative. Exclusion chromatographic measurements indicate that the enzyme remains tetrameric at catalytic concentrations in the presence or absence of an activating monovalent cation.
单价阳离子Tl⁺、NH₄⁺、K⁺、Rb⁺或Cs⁺按效力递减顺序存在时，会使从嗜热栖热菌（Thermus X-1）纯化得到的磷酸果糖激酶（ATP：D-果糖-6-磷酸1-磷酸转移酶，EC 2.7.1.11）的比酶活性显著等量增加。相比之下，单价阳离子Li⁺、Na⁺或CH₃NH₃⁺在浓度高达100 mM时未产生可检测到的催化激活作用。这些阳离子的相对效力表明，四聚体酶中的每条多肽链都拥有一个具有四方对称性的阳离子结合位点，并且蛋白质配体主要是羟基或羧酸盐氧。只有酶-阳离子复合物而非酶本身在催化速率对底物6-磷酸果糖浓度的依赖性方面表现出协同性。在底物浓度不饱和但非饱和的情况下，单价阳离子产生的催化激活作用也是协同性的。排阻色谱测量表明，在存在或不存在激活单价阳离子的情况下，该酶在催化浓度下仍保持四聚体状态。
