Murakami K, Yoshino M
Biochim Biophys Acta. 1980;613(1):153-9. doi: 10.1016/0005-2744(80)90201-6.
The effect of divalent cations on the purified AMP nucleosidase (AMP phosphoribohydrolase, EC 3.2.2.4) from Azotobacter vinelandii was investigated. All alkaline earth metal-ATP complexes were essential activators of the enzyme, and free alkaline earths also activated the enzyme in an allosteric manner: the apparent Ka for ATP and nH values (Hill interaction coefficient) decreased from 0.45 to 0.05 mM, and from 4 to 2, respectively, with increase in Mg2+ concentration. Transition metal-ATP complex also activated AMP nucleosidase, but a potent activation of the enzyme was followed by a progressive decrease in activity as the concentrations of transition metals increase. The enzyme fully activated in the presence of Mg2+ was inhibited by the higher concentrations of transition metals with the identical I0.5 values when Mg2+ was present. These results suggest the presence of two classes of binding sites for divalent cations. One is the activating site for ATP-metal complex, which is suggested to be commonly occupied by alkaline earths and transition metals. The other sites are those for free metal binding, the sites for free alkaline earths and free transition metals are activating and inhibitory sites, respectively.
研究了二价阳离子对从棕色固氮菌中纯化得到的AMP核苷酶（AMP磷酸核糖水解酶，EC 3.2.2.4）的影响。所有碱土金属-ATP复合物都是该酶的必需激活剂，游离的碱土金属也以别构方式激活该酶：随着Mg2+浓度的增加，ATP的表观Ka值和nH值（希尔相互作用系数）分别从0.45 mM降至0.05 mM，从4降至2。过渡金属-ATP复合物也激活AMP核苷酶，但随着过渡金属浓度的增加，在该酶被有效激活后，其活性会逐渐降低。在Mg2+存在的情况下，当过渡金属浓度较高时，在Mg2+存在下完全激活的酶会受到抑制，且抑制常数I0.5值相同。这些结果表明存在两类二价阳离子结合位点。一类是ATP-金属复合物的激活位点，推测碱土金属和过渡金属通常都会占据该位点。另一类位点是游离金属结合位点，游离碱土金属和游离过渡金属的位点分别是激活位点和抑制位点。
