A proteolyzed derivative of Escherichia coli phosphofructokinase is no longer sensitive to allosteric effectors and still shows cooperativity in substrate binding.
A proteolyzed derivative of Escherichia coli phosphofructokinase is no longer sensitive to allosteric effectors and still shows cooperativity in substrate binding.
复制标题
大肠杆菌磷酸果糖激酶的蛋白水解衍生物不再对变构效应子敏感，并且仍然显示出底物结合的协同性。
作者：
G. Le Bras;J. Garel
枯草杆菌蛋白酶对大肠杆菌磷酸果糖激酶的有限水解产生了一种同质的衍生物。这种蛋白水解的蛋白质由四条多肽链组成，分子量为32 000，而原始酶的分子量为37 000。每条链上约5千道尔顿的去除保持酶活性，并且不改变对底物ATP和果糖6-磷酸的表观亲和力。然而，有限的蛋白水解，影响果糖6-磷酸结合的协同性：希尔系数从几乎4在天然酶中减少到只有2在其蛋白水解衍生物。此外，蛋白水解的蛋白质不再对变构效应物、激活剂或抑制剂敏感。蛋白水解后调节特性的这些变化显然是由于效应物结合位点的破坏。变构效应GDP保护磷酸果糖激酶免受蛋白水解和不可逆的热失活;然而，GDP在保护蛋白水解的蛋白免受热变性方面是低效的。这些结果表明，磷酸果糖激酶可能作为二聚体的二聚体，其中同向性和异向性变构效应不介导的相同的季相互作用。
Limited proteolysis of Escherichia coli phosphofructokinase by subtilisin yields a homogeneous derivative. This proteolyzed protein is composed of four polypeptide chains, with a molecular weight of 32 000 as compared to 37 000 for the original enzyme. Removal on each chain of about 5 kdaltons maintains the enzymatic activity and does not change the apparent affinity for the substrates ATP and fructose 6-phosphate. Limited proteolysis, however, affects the cooperativity of fructose 6-phosphate binding: the Hill coefficient is reduced from almost 4 in the native enzyme to only 2 in its proteolyzed derivative. Also, the proteolyzed protein is no longer sensitive to allosteric effectors, activator, or inhibitor. These changes in regulatory properties upon proteolysis are apparently due to the destruction of the effector binding site. The allosteric effector GDP protects phospho-fructokinase against proteolysis and irreversible thermal inactivation; GDP is, however, inefficient in protecting the proteolyzed protein against thermal denaturation. These results suggest that phosphofructokinase may function as a dimer of dimers, in which homotropic and heterotropic allosteric effects are not mediated by the same sets of quaternary interactions.