The AMP-activated protein kinase (AMPK) is a sensor of cellular energy status. When activated by increases in ADP:ATP and/or AMP:ATP ratios (signalling energy deficit), AMPK acts to restore energy balance. Binding of AMP to one or more of three CBS repeats (CBS1, CBS3, CBS4) on the AMPK-γ subunit activates the kinase complex by three complementary mechanisms: (i) promoting α-subunit Thr172 phosphorylation by the upstream kinase LKB1; (ii) protecting against Thr172 dephosphorylation; (iii) allosteric activation. Surprisingly, binding of ADP has been reported to mimic the first two effects, but not the third. We now show that at physiologically relevant concentrations of Mg.ATP2− (above those used in the standard assay) ADP binding does cause allosteric activation. However, ADP causes only a modest activation because (unlike AMP), at concentrations just above those where activation becomes evident, ADP starts to cause competitive inhibition at the catalytic site. Our results cast doubt on the physiological relevance of the effects of ADP and suggest that AMP is the primary activator in vivo. We have also made mutations to hydrophobic residues involved in binding adenine nucleotides at each of the three γ subunit CBS repeats of the human α2β2γ1 complex and examined their effects on regulation by AMP and ADP. Mutation of the CBS3 site has the largest effects on all three mechanisms of AMP activation, especially at lower ATP concentrations, while mutation of CBS4 reduces the sensitivity to AMP. All three sites appear to be required for allosteric activation by ADP.

中文翻译：

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AMP 激活的蛋白激酶可以被 ADP 变构激活，但 AMP 仍然是关键的激活配体

AMP 激活蛋白激酶 (AMPK) 是细胞能量状态的传感器。当 ADP:ATP 和/或 AMP:ATP 比率增加（信号能量不足）而被激活时，AMPK 会发挥恢复能量平衡的作用。 AMP 与 AMPK-γ 亚基上的三个 CBS 重复序列（CBS1、CBS3、CBS4）中的一个或多个结合，通过三种互补机制激活激酶复合物：（i）通过上游激酶 LKB1 促进 α-亚基 Thr172 磷酸化； (ii)防止Thr172去磷酸化； (iii)变构激活。令人惊讶的是，据报道 ADP 的结合可以模拟前两种效应，但不能模拟第三种效应。我们现在表明，在生理相关浓度的 Mg.ATP2−（高于标准测定中使用的浓度）下，ADP 结合确实会引起变构激活。然而，ADP 仅引起适度的激活，因为（与 AMP 不同），在浓度略高于激活变得明显的浓度时，ADP 开始在催化位点引起竞争性抑制。我们的结果对 ADP 作用的生理相关性提出了质疑，并表明 AMP 是体内的主要激活剂。我们还在人类 α2β2γ1 复合物的三个 γ 亚基 CBS 重复序列中的每一个上对参与结合腺嘌呤核苷酸的疏水残基进行了突变，并检查了它们对 AMP 和 ADP 调节的影响。 CBS3 位点的突变对 AMP 激活的所有三种机制影响最大，尤其是在较低的 ATP 浓度下，而 CBS4 的突变会降低对 AMP 的敏感性。所有三个位点似乎都是 ADP 变构激活所必需的。